heavy_chevy29

i put a 750 double pumper with mechanical secondaries on my 409 sbc. it started to run like crap and i found out that i had a bad cat. so for right now i have a straight pipe(just for testing purposes). that made no difference in the way the engine ran. i can get the motor to idle good with around 15 psi of vaccum. when i take it out for a ride, it drives good until i step on it. then it starts to back fire (through the exhaust and sometimes through the carb). it also feels like it has no power when you jump on it. and if the car is sitting at an idle you can rev it to about 1500 and it will respond verywell. if i try to rev it from and idle to around 2000rpm it hesitates and sometimes backfires. when its hot it is hard to start. when its could it will start up pretty easy. i have no clue what to do. me and my father have spent all day trying to get it to run right. the timing is at 8*after. i put new plugs and wires on it. please help me out. thanks alot

Chickenman35

Did it run properly before?

One thing I see wrong right away is your timing. 8% AFTER Top Dead Center?? Is that correct or a misprint? Timing is way, way retarded.

Try about 12% before with vacuum advance disconnected while setting idle. Then reconnect vacuum advance.

Then go to this post and read my reply to " Street Avenger stumbles " That would be a good starting point for you:

Holley Tuning Tips

heavy_chevy29

im sorry the timing is 8* before

Chickenman35

You can safely add another 4 deg on timing, although I do not believe that is your major problem. Make sure that the vacuum advance is disconnected when setting idle. Set the timing at as low a speed as possible, less than 750rpm if possible to get an accurate reading. Try 12deg static advance and reconnect the vacuum advance ( you are running one I hope ) to manifold vaccuum. Then reset you idle speed.

Check the float level when car is running. You want 1\16" below the bottom of the sight hole on the Primary side and a tad lower on the Secondary side.

Did you adjust shooter sizes and cams as per the link? What shooter size and what pump cams are in the thing now. Most important...did it run OK with this carb before, or did the problem start when you put this carb on? That will help us with a diagnosis

Make sure there is no slop in the linkage at idle. You must have .015" free-play at WOT. You need a good hard pump shot from both sets of shooters when you crack the throttle wide open suddenly. Check the pump shot with the motor off. Ypu should have and immediate shot from the primaries with any movement of the throttle. If the shot is weak or delayed then you have to find out why.

heavy_chevy29

the car ran great when i first put the carb on. it always had the small hesitation as soon as you touch the gas, but it wasn't bad. it just started getting really bad. this weekend i will check all the things that you are telling to check. i read up on the pump nozzles and cam, but i don't know were to start with that stuff. should i get bigger nozzles or smaller ones. if i really hit the gas hard it back fire through the carb. it actually shot a large flame through the carb. i also checked the float levels. the directions with the carb tell you to adjust them so that the fuel just starts to trickle out of the sight holes. also what do you guys have you idle mixture screws set at. mine are at 1 1/2 turns out from full lean. i tried messing with them and got no results. thanks alot for all your help.

Chickenman35

You generally want to go with bigger nozzles...but not knowing what you have makes it difficult to make a logical ( Vulcan speak ) suggestion. Pull the shooters, post back with the sizes for primary and secondary. Look at the pump cams. What color are they? Better yet...remove the pump cams and look at the number stamped on them. Note which hole the attaching screw is in..#1 or #2. post back with the numbers. Then we can go from there.

Idle screws: 1 1\2 turns out from fully closed is about right. Curious that you said they had no effect. No effect on idle? You should be able to kill the engine by turning them all the way in.

You may have a vaccuum leak some where. If it ran fine before but is running crappy now that could be a cause. If you can kill the engine by turning in the idle mix screws then that pretty much eliminates vacuum leaks as a problem. But, if you cannot kill the engine with the idle mix screws then start checking for vacuum leaks. Carb base gskts, vacuum hoses, carb base gskts, intake gskts ( check the manifold torque ) EGR valves and Vacuum Advance cans are all possible sources of vacuum leaks. A spray bottle of water while the engine is running is a good way to check items.

BTW, a bad Vaccuum Advance can cause your problem...and can easily be over looked. Your static advance is a bit on the low side.... 8 deg BTDC. Now the vaccuum advance usually adds about 15 to 20 degrees to that at idle. IF the Vaccuum advance ( assuming you're running one. Let us know!! ) is not working that would produce a low advance condition at idle and when you quickly " wing " the carb you could get the backfire. It takes a second or so for the vacuum in the canister to bleed off.

Check the mechanical advance mechanism as well. Make sure that it is not sticking or frozen. Take the Dist cap off. Turn the rotor with your hand. It will turn one way only. Movement should be free with not binding or sloppiness. When released the rotor should " snap " back to the norma l position. A weak return or not returning all the way is an obvious problem. Usually a small shot of WD40 or penetrating fluid will clear things up.

Lots of things to check. Test things one at a time and don't forget to write settings down. Let us know what happens. Don't be surprised if you find more than one problem.

heavy_chevy29

thanks i'll check everything tommorrow night. i do have a vacuum advance distributor. it is brand new so i don't think that is faulty. i'll check it later. and about the timing, at 8* the engine sometimes has a slow crank when i try to start it. should i still try it at 12*. also i was talking to one of the guys at my work and he said that a 750cfm carb is way to much fuel and that i should get a jetting kit and jet it to a 650. what do you think about that. thanks alot for your help.

EvilCartman

iTrader: (1)

Jetting down to 650 jets will probably make it run too lean. I found that out messing around with my 750. Noticed the temp gauge was higher than normal, jetted up and temps went back to normal and performance went up. A jet kit is a must have in my opinion. On another note, I got a bad distributor right from the parts house so anything is possible.

Chickenman35

Don't go muckin' about with the jet's......yet. A 650 cfm carb and a 750cfm carb have totally different fuel curves. The fuel curve depends on a whole pkg not just the jets. It would be nice to know what you have for jets ...but if it ran fine before....well, jets just don't get bigger all by themselves. The backfires indicate a lean mixture...not an overly rich mixture.

I run a 780cfm VS Holley on my 355ci SBC. It's tuned to the nuts...but will pull from idle all the way to 7,200 rpm and I can pull 20mpg on the hwy. A 750 will just be fine on your 409.

Tuning carbs is a bit of a black art. I've been doing it for 25 years on Holley's Q-jets, Webers, Mikuni Solexes and SU's. It takes time and patience and a step by step approach.

MrDude_1

i have to disagree with your black art statement...

a step by step logical approch can help you properly jet the carb.. no art about it.

ive only been doing it since i was 15, and mostly on dirtbikes and small engines, but everything i learned has transfered over to cars just fine.

the real trick is making sure everything is alright with the rest of the engine before you go mucking with the carb.

nick harmon

i dont know much about the carb in question, to know if it has a power valve or not, but could that be the culprit if it has on? you know what im sure someone would have already said that if it had one, so never mind.

heavy_chevy29

ok i got the numbers you asked for.

accelerator pump nozzle:
primary-28
secondary-31

the cam is pink and in the number on hole on both sides.

i don't know my exact secondary jet but i know that they are in the 80's and the primaries are 71.

i also have my vacuum advance hooked up to the carb, not the manifold. in the directions for the carb it says that this vacuum port has no vacuum at idle. im going to switch this and see it i get any results.let me know what you guys think.

and the power valve os fine. that was the first thing that i checked

Chickenman35

Ok ...now we're getting somewhere.

I have checked my various Holley books for the cam profiles. Dave Emanuel's book gives volumes but mor important...amount of cam lift at degree's of throttle. The Pink cam is a "lazy" cam. It pumps a lot of volume but spreads the shot out over a long period of time or degree's of throttle movement. Small block Chevy's like a quick, fat initial shot.

For example. A blue cam ( #427 ) has a maximum cam lift of .1425" at 30 degrees of throttle travel when placed in the number 2 hole. A Pink cam in the #1 hole takes until 50 degrees of throttle to reach the same amount of lift. A Pink cam ( #330 ) has even slower lift when placed in any other hole.

The #28 shooter size on the Primary is also too small.

Try these setups.

1) Buy a accelerator pump cam tuning kit Part # 20-12. Install a Blue Cam in hole #2 on the Primary side and Secondary side. Adjust AcCam lever so that there is no freeplay at idle. Don't go too far.. just turn the adjsuster untill the slack is just taken out. Remember that you must stiil have .015" of pump travel left at wide open throttle to prevent pump damage.

2) Shooter size on Primary too small. If you have a dual plane manifold such as a Performer or Performer RPM I would recommend starting out with #31 Primary shooters and #31 secondary shooters. If you have a Single Plane manifold or a Performer RPM Airgap model I would suggest going to a #35 Primary shooter and a #31 Secondary shooter. Single plane manifolds require more pump shot and the Perf RPM Airgap requires more pump shot than the regular Performer and Perf RPM because of the cooling effect of the air flowing under the runners. Fuel does not vaporise as well. Note: Buy the tube type shooters

3. Change that vacuum advance to manifold vacuum. You need more initial advance. Try and get twelve degrees initial in there. If the car gets hard to start...then buy a gear reduction starter. The stock metric starter is just way, way too wimpy.

Note: If you have Vortec heads, Fast-Burn or heads with a Chamber size less than 60cc's then go no more than 10 degrees initial. Please let me know what heads you have. Do you know what your total mechanical advance is?


I think this will make a dramatic improvement. Do some " tip in " tests, only opening the Primaries . Any stumble means you need more a slightly larger nozzle.

If this seems fine do some Secondary rollins. 3rd gear best for this. You've got to be able to feel where the Secondaries open. With a DP when the secondaries start to open the throttle pedal gets a tad harder to push...'cause you're now working against the Secondary return springs.

If you feel any stumble when the secondaries open then go to a larger pump shot. Note: Bigger is not always better. I'm suggesting starting out with #31's. Don't go buying some #40's and thinking that will be better. Just like overcamming an engine. You can have too much!!

The cam change will really wake things up. Put the Blue cams in then play with the Nozzles. A tall gear ratio will require more pump shot in general ( 3.27's as oppossed to 3.73's for example ) as will an Auto Box over a manual.

I think when you make these changes that SOB will take off like a scalded cat. Keep me informed.

Edit: Leave the jets alone. They are fine.

heavy_chevy29

thanks alot. i'll order the parts today. i won't get the parts until wednesday so i won't really have any time to put the parts on until next weekend. i will let you know as soon as possible.

i do have the performer intake and 373s. could you also reccomend a new starter for me, so i can bump up the timing.

you also mentioned something about the holley books that you have. were could i pick up a good holley book.

again, thanks alot. you are really helping me big time.

Chickenman35

Just a Standard Performer Intake, or an RPM model? The RPM model outperforms the Std Performer in every way....I've talked at length with Edelbock Tech about this. Performer has slghtly more torque off idle to 1,500 rpm but the RPM model matches it step by step from there. Expect a 10hp gain going from a Performer to an RPM , plus extended RPM range.

For a starter, Summit carries all sorts. They have their own generic gear reduction starter, Powermaster makes a nice gear Reduction starter #9502.

Summit Online Catalog


Summit's Starter page

Books: Dave Emanuel's book " Holley Carburators " by SA Design is one of the best. ISBN # 0-931472-08-3.

Go's into much more detail with more advanced modifications and information than other books. IE: A lot of books give you how much the various pump cams deliver in total volume over ten strokes....but the do not give a detailed chart of pump cam lift at various degrees of throttle opening. This is critical.

Emanuel's book cover's all the basic's as well of course. I'd recommend it as a #1 choice. Any decent book store should be able to get it, or you can order it online through Chapters or Amazon.

The HP Books: " Holley Carburators and Manifolds" is not bad...for the basics if you can't get Emanuel's books. Might be worth reading as well. ISBN # 0-912656-48-4

BTW, the pump cam kit may not have two blue cams in the kit. Some do...some don't. I've found Holley varies the packaging.

Don't worry... use the green cam on the #1 hole on the hole. Will give an almost identical profile as the blue cam on the #2 hole. Use the green cam if necessary on the Secondary side. BTW, this is something you could only figure out by having a detailed chart of cam lift at various degrees of throttle opening....something that Emanuel's book gives you.

Glad to help.

carzykev

where can a guy find that book on Holley Carbs?

Kevin

Chickenman35

Amazon.com carries them. Here's my search for books by Dave Emanuel: Holley Book's by Dave Emanuel

Dave has a newer book called " Super Tuning and Modifying Holley Carburetors (S-A Design) " . The ISBN number is 1884089283 . It's a revised version of the Book I mentioned.

heavy_chevy29

i found a tuning kit that summit sells. it comes with various nozzles and two of every cam. this way i can play around with it a little more. i'm sure that i will still need you assistance so ill let you know how things go. i probably won't be able to do anything until next week end. i very busy between work and school.

i also read on the holley web site that if the car lets out black some you need to get a smaller nozzle. i for got to tell you that my car does have black smoke coming out the exhaust. this is why i got the kit for summit. thanks again

Chickenman35

Yep I know about the black. The black is caused by the TOTAL amount of fuel. The Pink cams actually deliver MORE total fuel than the Blue Cams ....BUT they deliver it far too late in reference to throttle opening.

Nozzle size does not affect the amount of fuel delivered. Nozzle size affects the timing of the fuel delivery in combination with the pump cam. A Small nozzle with the Pink cam gives a looooong slow delivery of the fuel. The engine doesn't want that. A larger nozzle with the early and agressive timing of a blue cam gives a hard, fat shot early in the cycle. That's what the engine wants. Pink cams with a#28 shooter can still be injecting fuel when then engine is pulling nearly 3,000 rpm's. By then you're well into the main circuit.

BTW, the Pink cam is a bit of a specialty cam. Pink cams are normally used for Hard Core manual tranny or Trans Brake Drag cars that may be launching over 4,000 rpm. The throttle is already open probably 20 degrees...so that's why they have a lazy initial ramp.... to not use up the pump shot.... then they have 90% of the shot in the final 70 deg of throttle movement so the car will launch hard. Most Holley Competition DP's are delivered this way, since I guess, they figure that the majority of Holley Competition Carbs are used for Drag Racing. For the street or any non-Drag Racing competition you need just the opposite . The Blue cam with proper nozzle size gives you this.

You'll normally always get a puff of black out the back when you fully mat it. Depends on too many variables. An absoutely stock engine with a small carb and a light car may not much pump shot. A heavier car, bigger carb, medium or high duration cam, different intake with no crossover heat etc, etc will require a MUCH bigger shot of fuel.

When you get rid of the backfires and stumble...THEN you can work backwords to reduce pump shot IF necessary.

Trust me on this.

sqzbox

Dude! I don't want to contradict you but if you hook the vacuum adv. up to manifold vacuum, your going to be pulling constant vacuum on the vac. adv. assy. Being you have a stick shift I would trash the vac.adv. dist. and get a mec. adv. dist.

Chickenman35

Ok to disagree, but you should know why I recommend using Manifold Vacuum ( Taken from a manifold vacuum tube on the carb preferably ). Find out a little bit more how Vacuum Advances and Engine Timing works. I would suggest reading David Vizards book " Performance with Economy ".

Manifold vacuum is NOT constant. It varies with throttle opening. Punch it and manifold vacuum drops to near zero...just as ported vacuum does.

The main difference is that ( with manifold vacuum ) at idle and just off idle, you will have full vacuum advance, whereas with "ported " vacuum you will not. Full manifold vacuum is desirable over ported vacuum.

At idle a SBC will run best with 25 to 30 deg of total advance. That includes 10 degrees of initial timing and 20 deg in the Vacuum advance. Or 15 deg intial and 15 degrees total. Any way you cut it you want about 25 to 30 degrees total advance at idle.

Note that the mechanical advance has not even kicked in yet. At cruise speeds you can have as much as 40 to 45 degrees total timing which includes initial timing, vacuum advance and mechanical advance. Because there is no load on the engine it can tolerate much leaner mixtures. Leaner mixtures take more time to burn....thius the need for lots of advance. A vacuum advance is necessary for good fuel economy and emissions.

Now lets say boot it. The manifold vacuum immediately drops off to near Zero... and all of the vacuum advance drops out. The engine is under high load , so it can't tolerate as much vacuum and the mixture is enriched by the Power Valve circuitry so it doesn't need as much advance. Ported Vacuum is only used because it helps the mfg's meet emission specs. Other than at idle and tip in, a Ported vacuum source will pull almost identical vacuum figures to manifold vaccuum. The extra advance off idle with a manifold vacuum source really helps throttle response.

Recommending a Mechanical Advance only distributor for a street driven car is just plain wrong. Another "Myth" passed on by poor understanding of how things work. On a full out race car, that only sees the Drag Strip or Road Course.... then yes, a mechanical advance only distributor is the way to go. Mainly because Vacuum at idle is virtually non-existant. But for any street driven, carburated car you will see better performance, increased fuel milage, lowered emissions and you won't foul your plugs with a Vacuum Advance distributor.

Recommended reading:

David Vizard: " How to Build Horsepower in any Engine " Volume 1

ISBN 0-931472-24-5

David Vizard: " How to Build Horsepower " Volume 2 Carburators and Intake Manifolds

ISBN 1-884089-14-3

David Vizard : " Performance with Economy "

ISBN 0-931472-09-1

Dave Emanual: " Small Block Chevy Performance "

ISBN 1-55788-027-1

All of these books are an absolute MUST HAVE for any one who seriously wants to learn about Performance engines.

I've got a library full of books on Engines, Transmissions, Suspensions and brakes. Reading is the best way to gain accurate knowledge on anything Technical.

BTW, if you haven't heard of David Vizard...do yourself a favour. Do a " Google " search on his name. Smokey Yunick and Vizard are two of the same. If you don't know who " Smokey " was....well...sorry there's no hope for you

sqzbox

I've read so many books I can't remember half of what I used to. I can only remember my exp. building and racing countless autos.
AMC, Mopar, and Pontiacs my favorite. Every one of them had to run on the street (driveability). When you open the throttle, ported vacuum increases to pull adv. on the adv. diaphram. Full manifold vac. to the diaphram negates this process and pulls all vac. adv. on dist. minus mech. adv. 25 to 30* at idle? How would you get the rpm down? Why not just disconnect the vacuum and set timing to 25or 30*? That is why they hook vacuum adv. to ported vac. on the carb from the factory. As soon as you step on the peddal you've got your 25 or 30* back. I think if you hook up a vacuum gague to the metered port you will find the vac. increases as the throttle opens up, not drop off. and a smooth idle. Constant vac. to the diaphram will end up damaging it. All this might not mean any thing to a full race car, but I had to drive mine to the track on weekends and work every day. You don't have to get red on me! I'm not discounting your learnings but if I'm wrong I can handle it. 34 yrs. of working on cars is a long time to be wrong on something!! But I'm 53 now and still learning stuff. Ill have to get me a copy of those books you mentioned ! thanks!

sqzbox

Whow man you really lashed me pretty good!!

F-BIRD'88

I'll go along with most of whats recommened to cure the problems. Except that this car probabily has a vacuum leak or an
ignition problem, possibily just fouled plugs.
Any time a motor backfires when you wing the throttle there is more wrong than minor carb dialing in or wether you have manifold vacuum advance or ported advance.

Check the seconday side of the ignition system (plugs rotor cap wires) and for a manifold or carb base vacuum leak before you blame the carb.
After that the carb can use some minor tweeking of the accelerator pump(s) to sharpen it up.

I disagree with hooking up vacuum advance to full manifold vacuum. It does not modulate the vacuum advance system according to the engines needs as does the ported vacuum method.

Then I'd go through the distributor checking for proper mechanical advance curve and timing.
A mechanical curve of 10 to 12 initial advanceing to 34 to 36 deg at 3500rpm is a good curve for this type of street motor.
Combine in 15 to 20deg of vacuum advance controled by ported vacuum source works well.

The jetting on this carb is just about right as is.

Chickenman35

Reading through the whole thread you will note that I already suggested checking for Vacuum leaks.

Sorry... we are going to have to disagree on manifold vacuum vs ported vacuum. With my Road Racing back ground, Power Valve actuation and Power valve channel restriction tuning is something that is critical to us. I have done many, many runs with vacuum guages hooked up while running the car on a load cell chassis Dyno and before that on a nice two mile long hill that is perfect for doing power pulls I've made runs with guages reading Manifold vacuum and Ported vacuum at the same time to check for exactly that reason. Manifold vacuum certainly does meet the engines needs as compared to Ported vacuum. Always remember..." There is more than one way to skin a cat " .

BTW...there is no problem what so ever in getting the idle down. In fact running Manifold vacuum usually lets you reduce the primary butterfly opening so that the butterflies are in the correct position relative to the transition slot. Cars with larger street cams really respond to manifold vaccuum for smoothing out the idle. Give it a try I think you'll be pleasantly surprised. Or don't ...doesn't really matter to me . I know it works.

Now...I'm fairly new here, so I've still got to earn people's respect. But I've been around the block enough times, built enough race engines, Tuned enough Race cars and Won enough National and Regional events to know my stuff. Not bragging or dissin' anyone...but I am a fairly well known " Tuner " in these parts . ( As far as Road Racing go's. I don't Drag Race ). BTW...I'm no spring Chicken either. 49 and still learning stuff....


Why don't we wait until next week and see how Heavy Chevy does? I've got a virtual $5 bill saying that if Heavy Chevy follows my instructions to the " T "...then his problem will be gone.....hopefully

Chickenman35

You might find this interesting.

My Carb

F-BIRD'88

Here is one example of when using full manifold vacuum to modulate a vacuum advance system, it does the wrong job.

When comeing off the throttle at mid to high rpm the throttle closes and manifold vacuum suddenly goes from low / mid vacuum to very high. (+21")
The vacuum advance will suddenly jump to full advance because of the full manifold vacuum.

Where as with ported vacuum the vacuum advance would stay low because when the throttle is closed, Ported vacuum is low.

Imagine the action of the vacuum advance under full manifold vac control : during a full throttle gear shift on a manual trans car.

The advance would suddenly flash high during the shift because of the quick closing and reopening of the throttle and could put the motor into dangerous preignition/detonation because of the momentary overadvanced timing.

Where as with ported vacuum control the advance would stay stable during the shift.

With full manifold vacuum control, anytime the manifold vacuum is high, advance will be high reguardless of the throttle position and reguardless wether the motor is accelerating or decelerating,- load is high or load is low.
Ported vacuum sences airflow through the venturi (therfore rpm)
as well as load and unloading when the throttle suddenly closes.

The amount and timing of the vacuum advance may need to be dialed in to best match the individual engine you're dialing in but ported vacuum as a signal source is still the best overall.

I don't know of any engine or driving mode of operation where it is a dissadvantage once properly dialed in, in combination with a matching proper mechanical advance curve.

Can you tell me a specific time or mode where using ported vacuum advance is a direct disadvantage as compared to full manifold vacuum?

Chickenman35

Not quite sure by what " Lashing you pretty good " means..... I had a Girlfriend who used to do that.... was quite enjoyable as I recall

F-BIRD'88

" In fact running Manifold vacuum usually lets you reduce the primary butterfly opening so that the butterflies are in the correct position relative to the transition slot. Cars with larger street cams really respond to manifold vaccuum for smoothing out the idle. Give it a try I think you'll be pleasantly surprised. "

I have...


Recurving the distributor for quite a bit more initial advance at idle
works much better.

Usually when you get up to a big rough idle cam with say 250+ deg @.050" and a tight lobe separation there is so much low rpm egr action because of the long overlap of the cam that a locked advance curve with 36deg timing at both idle and full throttle is better. (when combined wiith a properly matched vacuum advance curve useing ported vacuum)

Try it ....

Other cammed motors that fall in the middle will require a advance curve that also falls somewhere right in the middle but still like a lot of initial timing.

Allows you to properly position the throttle blades during idle too.

Chickenman35

That statement is simply incorrect. Manifold vacuum does not remain high regardless of of throttle position and regardless of whether the motor is accelerating or decelerating. Put a vacuum gauge in the car.... connect it to Manifold vacuum at the carb and do some driving. Both systems will pull nearly identical values at cruise. Both will drop to near Zero at WOT. Manifold vacuum WILL give better throttle response off idle than ported advance ...and it does not cause any detrimental affects like you have suggested. I've been running all sorts of cars for close to 30 years set up like this and have not had one,,,not one single instance of any damage or detonation.

An interesting arguement...but that does not appear to happen in the real world. I've never encountered detonation under the circumstances you describe..... and I've been HillClimbing and Autocrossing cars for over 30 years...running carbed cars with manifold vacuum. We are always up and down on the throttle and shifting. Has never been a problem.

What happens is that during a quick shift the manifold Vacuum does of course go from zero to very high... but it takes time for the vacuum can to reacte Don't forget Vacuum cans have an air bleed in them and vacuum restriction built in to slow down the reaction. They are actually quite slow operating in response time as compared to throttle opening.

So you do your wide open shift, the manifold vacuum goes high...but before the vacuum canister can actually respond, you complete the shift and manifold vacuum again drops to near zero. Net affect on timing is zero.

I've already posted what the advantages of Manifold Vacuum are to Ported vacuum. A better throttle reponse off idle. No draw backs that I have ever encountered.

BTW...I should have mentioned this earlier. I originally started out racing Import cars such as Datsun 510's and 1200's. These cars and many other Import cars used Manifold Vacuum for distributor advance straight from the factory. Only when emission laws got tighter did they switch to Ported vacuum...which I believe I mentioned. I worked as a Nissan\Honda Dealer Partsman for 23 years. I was also the West Coast Canadian Distributor for Nissan Competition Parts.

A good thread this. Has got some minds thinking. I'm just passing on some advice that I've tried and tested over many,many years of racing. Whether you believe me or not...it really doesn't matter. I know what works for me and the cars\engines that I build. There are a thousand and one different ways to tune a car. Some of those ways actually work

Sleepy now...going to bed. Gonna be running Karl Hunter' car tomorrow at an Autocross.

F-BIRD'88

In your quest to get more throttle response off idle , when u use fullmanifold vacuum you tend to get too much vac and too much timing which tends to cause detonation when rolling on the throttle
Ported vacuum doesn't do this.

As you roll onto the throttle using your method, vac falling (timing is falling from high to low but with a lag. Like you said the (some)vac cans takes time to react.
Using ported vac combined with more initial timing is better.
(on engines that need that.)

Another time the vacuum advance would be doing the wrong job when hooked to full manifold vacuum is when an engine is decelerating( throttle closed) during a long decel from high epeed.

Ever had a problem with pesky blown head gaskets?
The main cause is detonation you never hear.
Setting up a motor like you suggest could be the very reason and you won't hear the detonation. But you will pay for it.
During deceleration the timing would definatly be over advanced.
Get back on the throttle suddenly especially on a hot motor and you're going to get trouble.

I've tried your method and seen others try it too but it never really works too well. And is not what the origional poster needs
to do.

Interesting but we're getting way off the origional topic.

Chickenman35

Yep...but now you're talking a pretty serious drag motor...which is pretty far removed from Heavy Chevies motor. Not saying that your way doesn't work...it obviously does. But a 250deg+ cam at .050" is hardly a street cam.

Remember I don't Drag Race...I Autocross and Hillclimb. I have tried Ported Vacuum. Manifold vacuum in every case has given me better throttle response. Works very well on the street too.

We've obviously both come across combos that work. No use bashing our heads against the wall. Heavy Chevy can do back to back tets and see what works best for him. I usually recommend throwing out the Stock HEI Vacuum can and Replacing it with a Crane Adjustable usnit. Most of the Stock HEI cans spec out at around 20deg of advance. That's a bit too much. The cran unit allows a maximum of about 15 deg. Crank in a bit more initial and Bob's your Uncle.

Lets agree to disagree. I'm tired and we're going in circles ....ZZZZzzzz

F-BIRD'88

Well I think we found common ground then cause I agree 100%
with going to a Crane adjustable vacuum can.

Chickenman35

Yikes!!! It's 1:47 AM Pacific Time What the heck am I doing staying up? What the heck are you doing in Ontario!!!!

F-BIRD'88

Well it's 5.00AM here in Niagara Falls got 60tv channels and nothings on....

Should be asleep but got nothing to do tomorrow anyways.

tend to be a night person..

EvilCartman

iTrader: (1)

Hehe, interesting read and I'm a nightowl too :/ Did go to Amazon and buy that book tonight about Holley carbs, should make for some good reading. I got some different things to try on my car now too concerning the ported vacuum vs manifold. Right now I have no vacuum advance. Total timing is 38, I put a recurve kit in a long time ago to have full advance come in at 2500 I believe. It's been doing fine so far and I get 20-22 mpg on the highway.

sqzbox

Why not trash the vacuum adv. dist. and go with a mech. adv. dist. You can dial everything in (init. adv. , rate of adv. and total deg. of adv.) and don't have to mess with a vac. can resivor or vacuum lines. being your racing autocross ( I do too) and hill climb, your your throttle position is either WOT or closed most of the time right? . I don't understand why you need vac. adv. Mallory makes a dam good mech. dist. that when set up good (custom to your application) will out perform the vac. unit. by helping you decelerate (decompression) going into the corners.
I would love to get together with you and experiment! with this therory. I believe we have alot in common racing. I have raced cars since 1968 domestic to the U. S. and others (Opel and Fiat) in autocross, drag, rally, and yes , even on the street. Turned wrenches in dealerships, garages, gas stations, and dark places by feel. In 1965-66 my two uncles and I (the gofer) had the fastest stock 442 in america with the help of dealership money!
Didn't use no vac. adv. dist. all mech. with the 4 speed. I realize different racing events use different applications desined for all types of events. hill, drag, auto-x, road, and even boats. Poor ol' heavy chev is just trying to drive it around town, not the winners circle. He needs driveabilty on the street to get to work and cruise around town. ( and pick up on all those beutiful ladies right?) Heavy chev said his carb was working good till it screwed up. Something has changed over time. backfiring through the carb and exaust could be something other than the carb like worn timing chain, jumped timing, or bad dist. gear or ign. module. all that backfiring can't be good on the power valves. they have a check valve nowadays to keep that from happening. He said he checked the power valves allready. I've always thought that backfiring thru the carb was to lean and thru the exaust was to rich. His is doing both I'd be looking around the ignition system first seems his spark timing is a little eratic to be doing both. I know it's hard to diagnose a problem on a keyboard but hey, were just trying to give him ideas to help right? Sometimes I can get so fustrated I can overlook the obvious and it makes me so mad I could kick my own *** if I could reach it!! see you sleepy heads later. this is the most exiting thread I've been on since I started posting on this site. Maybe we can all learn something!!:rockon:

heavy_chevy29

wow. thats alot of different opinions.thanks alot for the help. looks like i have my work cut out for me when the parts get here.

i switched the vacuum advance to manifold vacuum and it worked a little better for off idle acceration. when i rev it up the rpms come down slower and sometimes stick for a second or so.i still have some hestitation and backfiring. what do you guy mean by the vacuum canister. is that the shiny peice that is connected to the distributer were you put the vacuum line. the distributer is not factory, it is a brand new summit vacuum advance distributer. i don't think that anything is wrong with that. the rest of the ignition sytem is new. new wires, coil, distibuter. i put new plugs when i put the motor in, which was about 7 months ago. i checked them and the were fouled up. i cleaned, checked the gap and reinstalled them. i already mention that anything over 8* initial timing, the motor will be hard to start. i really don't have the money to go out and get a new starter right now. but i am trying to save money for this reason. i trying to use everything that i have and then when i get money i can add on.

as for any of the engine components being worn out, i don't think thats the problem because the motor is only about 7 months old. i used all new parts to built it.

as someone mentioned this car is not a race car. the motor is very mild. i don't drive it every day. i used to cruize around town and do some street racing if i have to. i would also like to be able to bring to the race rack and drag race it every now and then.

i also forgot to mention that the car lets out alot of black smoke at an idle. when you step on it there is a huge cloud of black smoke. that means its to rich, right. im also not to clear on the advance curve(i think that what it was called). is that when you change the springs and the weights in the distibuter. if so what sould mine be set at.

the car only backfires through the carb not the exhaust. sorry abou that

sqzbox

What was the last thing you put on the engine? the carb? or the dist? If it was the dist., and you put it in right out of the box, you might not have the correct rate of adv. for your cam. I'm not familiar with summit dist. weather it's internally adj. or weights and springs like GM's . Either way you can try (buy trial and error) to speed up the rate of adv.(faster) if that makes it worse try slowing it down from where you have it now and see if it's any better. they have weight and spring kits with weights that have a more of perf. angle on them and three sets of springs light, (fast rate of adv) medium, (normal rate) and heavy (slow rate). Just another idea to try and help! If you try this, I would recomend you put your vacuum back to metered port during during this process. If you try this on man. vac. it will all be for nothing.

sqzbox

Oh boy! here we go again!!!!:rockon: What am I thinking?
carb. ran good
now it doesn't
something went wrong
look again for something you missed

read holley book real good!
find the problem
fix it
then start the changes
you have to have something to work with before you start making changes or you might make things worse. You said it was running rich, I'd be looking for whats causing that first! Good luck man , that's about all I can think of from here.

heavy_chevy29

the distibuter did come with extra springs. this is the chart that was inckuded with the distributer,



Black/black 800 3200
black/natural 800 2800
black/gold 800 2600
natural/natural 600 2200
natural/gold 500 1800
gold/gold 500 1600

natural/natural is the way it comes. does this sound ok.

black=heavy natural=medium gold=light


i checked the plugs today and they were fouled up again so i cleaned them and put them back it. what can i do to lean it up a little. i always thought that when you get a backfire through the carb then you are running to lean. how is it that it is running rich with a lean backfire

bluegrassz

iTrader: (3)

I had the same problem. I converted from TPI to carb. I added a holley 750 vs to the stock L98. It backfired through the carb and the plugs were black and sooty.

I found that I had a intake leak. I fixed the intake leak and now it does not backfire and the plugs run clean now It is still not 100%, but I am getting closer. I still have some tunning to do.

Good luck on getting it fixed.

heavy_chevy29

is there any way you can check for an intake leak.

sqzbox

Adv. starts at 600 rpm all in at 2200. would work good at the track with WOT launch! For the street you might start working your way up the blacks. not so early start and all in later. you might be pulling to much, too fast and catching an intake valve open before it closes all the way with your loping cam. It would have to be a trial and error thing to see if it helps. To early or late of a spark will cause incomplete combustion resulting in carbon build-up on the valve face, plugs, chambers, pistons, and the rest of your exaust system. They probably installed Nat./Nat to work good with the stock cam in your motor. But!! you have a larger cam in there and the valves open earlier and stay open longer.
Spark timing is critical to performance even more with a larger cam
stock cams are more forgiving. Who knows who installed the cam and if it was degreed in properly. The backfire through the carb could mean the spark is catching an intake valve before it closes. (to early spark adv) are you sure the valve lash is set properly? Too tight and the valves open to early. should be 0 lash or a few .001- gap on the rocker to stem clearance. I would sure keep on checking those power valves in the carb and make sure their not blown. also check the base plate on the carb and make sure it's not warped or a cracked base plate ear where the man. bolt holes go thru. I've cracked those before. If you can turn the air fuel ratio screws on the metering plate all the way in and the engine continues to run, You have a leak in the idle circuit or the throttle blades are open to far taking the idle circuit out of play alltogether. More stuff to check!!

Chickenman35

Use a spray bottle of water. I mentioned that way back in my second post....

I also answered why you can have a seemingly rich mixture and yet a lean condition all at the same time. The Pink cam is part of the problem. Delivers too much fuel...but at the wrong time.


You may have more than one problem. Go with a step by step logical approach. Write each change down and it's affects if you have to.

Chickenman35

Richness at idle. Idle speed must be below 1,00 rpm to begin with. Hook up a vacuum gauge to manifold vacuum. Turn the idle mixture in 1\4 turn at a time till the engine just starts to slow. Watch the Vacuum guage. Just as the engine starts to slow the vacuum reading should drop. Note that exact point. Turn the idle screws in and out minutely to confirm this point ( Maximum vacuum reading and best idle rpm ). This point is " Best idle "

Now turn the idle mix in 1/8th turn . Vacuum reading should drop a bit and idle should slow a bit, but engine should still idle smoothly and the idle speed should still be easily adjustable with the idle speed screw. This is called " Lean Best Idle ". Should get rid of the black at idle.

Conversly 1\8th turn richer on the mixture scre would be " Rich Best Idle ". You can go slighter leaner than " Lean Best Idle ", but you should never go richer than " Rich Best Idle ". That just wastes fuel and fouls plugs. We can deal with some possible jetting changes later....but for now lets solve one thing at a time.

The Pump Cam changes that I mentioned and the nozzle changes should fix your backfire problem And eliminate most of the black smoke when you punch it. Read the specifications on the Pink cam that I posted again so you understand what is happening.

Do triple check for a vacuum leak as mentioned several times. Use the water sprayer when the engine is running. Pay particular attention to hoses; intake gskt ) just spray water directly at top of Intake manifold where it bolts to the head and the base gskt area.

F-BIRD'88

You cannot "clean fouled plugs"
Once a plug is carbon fouled it is finished. It will always refoul no matter what.

Install new spark plugs. Look for the problem that caused the plugs to foul in the first place.

Again Ignition problem is the most likey source next is a flooding carb.

There is nothing wrong with your jetting or accelerator pump
or cams or shooters.

You have a basic fault problem not a minor tuning problem.

Ignition or vacuum leak.. Replace the spark plugs with new ones and start looking.

Pull the whole distribtor and install a different one. Even an old one just to eliminate the possibility.


Did I mention CHANGE THE PLUGS?????

Chickenman35

Sorry...you are just plain wrong about the pump nozzles and accelerator cams. The combo on the car as stated will not work and will cause the problem stated. No if ands or buts about it.

Ignition system on H.C's car is brand new throughout as he has already mentioned. Changing the plugs is a good idea...but is not the cause of his origianl problem...Which was....The car Backfires when he steps on it hard. That is a combination of Pump cam timing ( mainly ) and perhaps not enough initial timing.

He's gonna check for Vacuum leaks as we all have suggested.

Jetting on the Primary side will also have to be brought down a notch. Probably to 68' or 68's on the Primary side. That and the Pink accelerator cam are the main problem of the plug fouling. Re-Read the specs on the Pink Cams. Get Emanual's book. Pink cams have a huge pump volume. But it's all towards the end of Throttle opening.

I'm not going to waste any more time arguing back and forth on this. I'll take this to PM with HC if he wants. Too many Cooks in this kitchen .... no flame intended.....but as the English say...you're losing the Plot.

Chickenman35

Actually, cleaning Spark Plugs is a perfectly acceptable practice. As long as it is done with a proper air\abrasive cleaner.

Champions Racing technical books still recommend cleaning as an acceptable practice for plugs that have low milage and are just fuel fouled, Oil fouled or have light Carbon Deposits.

New Spark Plugs are certainly preferred...no arguement there. But your statement : "Once a plug is carbon fouled it is finished. It will always refoul no matter what " is incorrect.

sqzbox

I noticed in one of your earlier posts you didn't know your secondary jet size and that you had checked your power valve and it was ok. this must mean that you haven't pulled the secondary fuel bowl yet. My point is, ( I could be wrong on this Chickenman!) I thought double pumper's had a secondary power valve also? Not sure though, but if it does, and you haven't checked it, need I say more? What the hell,,,, even if it don't, you can still justify pulling the bowl, and finding out the secondary jet size. It's been a long time since I messed with a double pumper (Litteraly!!!)