It was suggested to me 9 months ago, and to this day I'm still having fuel delivery problems. It just seems like I have to use too much of the idle speed screw to get a decent idle. The engine doesn't respond to idle mixture screws and my gas milage is still bad.

65/68 jets shouldn't be over feeding. Should I drill?

 

sounds like powervalve

 

You do not have near enough of a radical set up to ever require that u drill the throttle plates for extra air flow.
This is not your problem.

Your problem lies somewhere else.

Did you hook up the PCV system and reference the throttle plates to the transfer slots on both the primary and secondaries?
remove the carb to do this.

Your initial timing should be 16 to 25degrees BTC at idle
with that cam. If the timing at idle is lower than this, you will want to open the throttles more at idle to get enough idle speed. Then the throttles are open past the optimum spot on the transfer slot making for a over rich idle and a off idle flat spot. Correct the base timing and timing curve so the car will idle with the throttles open the designed amount. (.030" of transfer slot exposure)

 

I'm at 15* at idle.

Transfer slots looked good (I had the carb off the other day)

PCV is hooked up.

Floats are set exactly to holley instructions.

 

Does the engine still run if you turn the idle mixture screws all the way in?

Test the power valve. May be leaking. Fuel is getting into the engine from other than though the idle circuit.
Warped metering block surface. (gasket)
What about the transfer slot exposure on the secondary side?
Try more initial timing. It will allow you to close the throttles more at idle and maintain the same idle speed.

 

may need the holley heat shield to keep the fuel blowl temps in check.

 

Transfer slot was exposed triple it's width. So I guess that would be about .05?

Is running this much intial timing bad?

 

Thats what I'm talking about. The transfer slot is exposed too much at idle.
Remove tha carb. Reset the throttle opening at the proper amount and reinstall the carb. Do not touch the idle speed screw. start the car and adjust the ignition timing to what ever it wants to achieve a good idle at 800rpm. let the motor fully warm up and adjust the idle mixture screws for best idle quality (vacuum) at 800rpm.
Re adjust the timing at idle. Note what it wants for timing and recurve the distributor to achieve that base timing with a total timing of 34-36deg.
Do all this without vacuum advance.

There is nothing wrong with running this amount of timing at idle with a cam that is large than stock.
The extra duration and overlap of the cam causes the airfuel mixture to be diluted with exhaust gas which slows the fuelburn rate at idle thus you have to run more initial timing at idle to correct the fuel burn timing.
The more overlap, the more initial timing required.
The practial limit is wether your starter will crank the motor over well enough at that timing setting.
I have a larger comp XE284h cam. I run a full 36degrees at idle (advance locked out) purrs like a kitten at 800rpm
No drilled throttle blades required.
I have a Accel multi ignition retard that retards the timing while cranking.
You should be fine at 20-26 deg initial.

 

Sounds good, I've got a really beastly aftermarket starter....so that isn't a problem.

Could the transfer slot be whats causing my fuel mileage problems? I don't even know what that slot does.

Once again I appreciate all the info Fbird.

 

Your bad fuel mileage is mostly caused by improper ignition timing.
get the idle transfer slot exposure right and your carb idle mix screw will respond to adjustment again.
Your exessive compression ratio (detonation) will likely limit you running a optimum timing curve without higher octane gas.
So you may not be able to get the best out of it.
engine knock is going to be the limiting factor.

Secondly by the camshaft, (its too big for "mileage")
thirdly, carb jetting.

After all this you may want to consider adding some manifold heat to help fuel vapourization at criuise.
It will improve midleage and drivability.
Edelbrock performers for vortec heads have a provision of running hot water under the manifold plenum.
look for the two pipe plugs on each side of the carb on the manifold.
You want a cool carb and a warm manifold plenum for best mileage.

 

....and if you can't get enough idle speed with the Primary transfer slot in the proper position relative don't forget you have a Secondary idle adjsutment. That can be used to increase idle speed while not opening the Primary butterflies too much.


As for timing, 12 to 14 static with vaccuum advance disconnected....16 if you can. 34 to 38 degree total ( not including vaccuum ) for heads with combustion chambers from 64CC to 70CC, 30 to 32 degree total for small cahmber heads..58cc or less.

You are running a vacuum advance aren't you? If you're not then that will definately contribute to poor gas milage.

Use a Crane vacuum can adjusted to 15 deg maximum vacuum advance and run very light spring tension. I am an advocate of running full manifold vacuum on SBC's. It increases your vacuum reading at idle, makes the idle more stable ( particularily with mild to large cam motors ) and gives a bit better throttle response at tip in.

 

Chickenman, F-Bird88

I like reading both of your posts because you give such good information and explain it well. However your opinions seem to differ on something's, one being part/full time vaccum for the distibuter, and another is intial timing, that's atleast what I gathered from this thread.

F-Bird you make a very good point about the overlap of the cam and the poor buring in the combustion chamber and needing more igntion timing. I personally think that I have fallen victim to that on my car. My car seems to run rich alot, I put in a 2.5 power valve and that helped considerably. I have had problems with leaky intakes, sucking oil in the intake and I have finally gotten those things taken care of. However. My car still smokes at WOT, it's a blueish/greyish color. Last week I checked my plugs and they were burning 100% clean, they looked like they had a little ash color to them, so i backed my intial timing down from 16 to 12, I drove it a week, now my plugs are black, and actually had to change the plugs because one had started to miss. After reading your post, it made sense to me that I should move my timing up, crusing around at low rpms, there's a unburnt fuel smell, and a slight girgle in the motor, after WOT and I clear it out, it goes away, I went to a hotter plug, AC Delco 44T to try and help combat this, I checked one plug last night, and it looked better, but not like it should.

Chickenman,
Why do you only recomend 16 degree's intial compared to F-Bird's 20-25? I am sure you have good reasons, I just would be interested in knowing what they are. I am not trying to start a flame way by any means, I just want to hear more from the both of you on this subject, I like to hear as many opinons, with technical backing, as I can to better help me understand things, especially carbs.

I have to go back to work, thanks for both of your time.

Jason


I am not trying to hijack the thread, I will repost on my old thread. Thanks

 

Well I'm never using Holley sight plugs again, they are complete junk. I've got 2 brand new moroso plugs in there, and both are set perfect now (thought they were perfect before, but apparently you can't see anything through Holley's so called "sight plug") I can see the fuel levels perfectly clear now.

I bumped my initial up to 18*, running nice. I think I might go to a different power valve. Now the holley site says:

"In either case, the power valve selected should have a vacuum
opening point about 2" Hg below the intake manifold vacuum reading taken."

So if I have 14" W.C. @ idle in gear, I divide that by 2, whichs gives me 7. Meaning I would want a 4.5 or 5.5 power valve, not the 6.5 that came in my carb. Is this correct?

My car is somewhat hesitant under 1200RPM. Idle in gear is around 700RPM. Between 700 and 1200 its slower to rev, and response is not good. Once it hits 1200 it is much more responsive and much smoother. Someone mentioned power valve being the culprit.

 

Jason: One of the hardest things to determine (without a O2 sensor) is wether your car is "running rich" or not.
The fuel smell you are smelling means unburned fuel is coming out the exhaust. More likely the fuel is burning late and or missfireing. (incomplete combustion) This can also be caused by a lean mixture ( usually when the motor is cold). Yours is likely burning late. More initial advance will help. How much, will depend on the size of your cam: (overlap and duration) and your starters ability to turn the motor over fast enough when hot.
Get the motor good and hot and try advancing the initial timing and see if it will start without the starter labouring. Then recurve your distributor to allow this new increased initial advance with the same total advance (32-36 deg). Then tune in the vacuum advance amount and rate for efficient part throttle cruising.
what cam are u using?

I'm running a Comp XE284H-10 Hyd in my 406. Seems to run real nice with the timing locked out at 36deg.
You may not like 36deg at idle but if you get the initial up in the 20's+ a big cammed motor will run much better at low speed/idle.

Your car may be smoking at WOT because of old carbon
buildup in the exhaust system or combustion chambers.
It is also a *strong indication* of an ignition problem.
Usually the secondary side {cap coil rotor wires}
It takes a very rich carb jetting to make the car actually smoke at WOT. Some times extra fuel actually flows from the accelerator pump shooters at WOT. check this for an anti siphon check valve under the accel shooters. The engine may also be still getting some oil from somewhere. (PCV valve cover, intake manifold gaskets, piston rings)

Althou some advocate using full manifold vacuum for a vacuum advance source, I like running more initial.
When you open the throttle or load the motor vacuum falls= timing falls too. can cause the motor to hunt at idle when the vacuum advacne is hooked to full manifold vacuum. I find ported vacuum works better overall.

 

F-bird,

Thank you for the reply. I will reread it later when I get home later, I only have a few minutes right now. To answer your question, my cam is as follows.

288/294 adv 236/242 @.050 .570/590 lift (approx) 110 LSA

 

Should be a pretty responsive motor. I'd try increasing the initial to around 20 or so and recurve the distributor.

 

I'm real happy with my cam and setup all around for that matter. I don't understand why my plugs sometime look good and other times they don't. I need to keep track of exactly what I do to the car. I will check the timing friday when I go to the track and see if I can recurve it. A friend of mine has a Car Craft that shows every cam/weight GM ever made and has the numbers for them, and shows what combonation will yeild what timing curves and total advance. He looked through the junkyards and has found all of the weights. I am reposting in my old carb thread later tonight to bring it back to the top. After I get the timing lined out it's time to tune the carb, I know there has to be more in this engine somewhere, I haven't done any tuning as of yet.

Thanks again.

Jason

 

i have been reading both these guys post for a while now and have learned from both of them. I have finally figured out the answer to your question as to why they both recommens different initial settings and different ports for dist. vac.

However the final result isn't much different, unless i see this wrong.

for instance.
F-Bird likes 20-25 initial on a cammed car ( hooked to ported vac. ) but he runs his at 36 deg at idle.


chickenman recommends 12-16 ( 16 if you can) plus the 15 deg manifold vac , which you get at idle EQUALLING 27-31 initial.

see, not much different in the numbers when the car is ideling.

The difference is with chickenman's lower initial, you won't have to have the killer starter, but some cars may not like to drop that low in initial when WOT from idle ??

Now you say what about at cruise , we'll F-Birds using ported so you'll have the 36 plus the 15 vac. at cruise and Chickenman recommends 32-36 total and the 15 deg of vac is present too.

I hope this doesn't offend either one of you, but i've been thinking of this for quite a while.


I fully agree, there both very imformative.

 

I'd check for an ignition problem. onmy car even though the battery is well grounded to the engine block the ignition will miss fire when i rev it, IF there is not a solid ground conection between the body and the motor.
Doesn;t like to ground through the motor mounts at all.
A pesky ignition fault is much more likely to cause your mysterious flouled plugs than the carb being a few jets off.

 

If I am to understand it correctly, a bigger cam will have more problems with full manifold (due to advance dropping out) then a stock cam would.

Fbird: It seems like my motor wants more initial advance. I have it at 18* right now, and I'm on the border for my starter. Where it is now, I get a nice clean start, no struggle to fire. But it still smells like I'm not getting a complete burn.

I've got the 65 primary jet and 68 secondary. I'm going to lbuy 63 and 61 jets and see if leaning it out will help.

The thing is, I don't really think my jets could be that oversized. Is it that I need even more initial? My starter starts struggling around 22* initial. What should I do? Another thing is, as initial timing increases, the mechanical timing decreases to offset the intial. That means mechanical timing may only be 12-16*. How on earth can I get a curve to get the 12-16* to come in later, as the spring kit I bought doesn't have a kit that will bring 12-16* late enough. Do I have to change the weights? Also, as initial increases, do I run even less VA since both are fully present at cruise?

 

but the advance doesn't drop out...any faster than ported vacuum. The trick with big cammed motors is that you must use a Crane adjustable vacuum advance. I've always recommended this.

Not only is the total vacuum advance adjustable but so is the rate at which it comes in. IE: The spring tension. Typically a stock vacuum can has a fairly strong diaphram spring.....usually start pulling in at 12" or more of vacuum. Now this won't work with a big cam. vaccum advance will dance in and out. Really noticable with an auto car.

With a Crane unit, you can loosen the tension on the spring so that you can get it to advance as low as 6" or so. Then you have no problem with the vacuum dropping off at idle. Big cammed motors love the extra advance.So you can set a reasonable base timing 14 to 16 degrees and add another 15 from the vacuum advance at idle. Will really stabilise a mild to big cammed motor. Will also run cooler when idling in traffic or stop and go.

Once you are above the transfer slots, ported and full manifold vacuum ( taken from the carb ) values are exactly the same.

 

Are you talking about full manifold vacuum? My distributor allows for changes in both rate and total vacuum advance given.

So what your saying if I read correctly:

You set the VA canister to give full advance (when using full manifold) at 6", so when you accelerate the timing does not drop off? What about WOT? What then? It will still drop out.

I had really bad problems running full manifold, if I was at a dead stop, and I floored it, the engine just shut off. You would have though I just cut the ignition. I also had bad response. Moving to ported and running more advance solved this. However I think I still need more initial. My cam isn't "that big" 224/230 but it seems like it wants more initial. I don't really understand, because my buddy runs a 350 with the exact same cam, only difference is he runs 462 double humpers and I run protopline vortecs, he runs 9.7:1 and I run 10.7:1. His car idles nicely at 8* initial on ported vacuum. Full manifold did give me a nice idle, but everywhere else it was worse. If I could get 24*+ of initial at idle while still running ported it would be nice, but that is probably going to kill my starter. I have to drop my exhaust to replace the starter (already been done twice), so I'd rather not kill it.

 

To dial in the spring tension on the Crane cam, I loosen the spring tension as far as possible. Then I adjust my initial to approx 14 to 16 degree BTDC ( with no vaccuum advance ). With engine warm, connect the vaccuum advance and adjust your idle speed. Then take a vacuum reading from the manifold. With auto cars take the reading with the car in gear.

Now ...let's say you have 12 in of vacuum, in gear, with the vaccuum advance connected. Shut things off. Get a hand held vacuum pump ( Mighty-Vac ) and adjust the spring tension of the vacuum advance so that you have the diaphram pulled in all the way approx 2" hg below your idle reading. IE: 10" hg. That should be a good baseline setting. You may have to play with things a bit get it "spot" on...but that should give you a good starting point.

At WOT the vacuum will totally drop off...that is the way it is designed to work, whether ported or full manifold vacuum...both react the same regardless. At WOT it is not uncommon to have vacuum readings as low as .5"hg. Anything higher than 1.5" to 2" HG at WOT usually indicates a carburator that is too small for the engine ( or Secondaries not opening all the way )

At cruise or light\med throttle settings, an engine can tolerate, and requires , more ignition advance because the volumetric efficiency is reduced and the engine is under light load. The throttle plate, being partially closed, reduces the mixture density ( dynamic compression of the engine is greatly reduced ). This takes longer to burn...so you have to light it off sooner. That is the purpose of a vaccuum advance.

Aluminium heads can tolerate a bit more timing and compression. Head chamber size makes a difference in timing curve.

Assuming a Combustion Chamber size of 64cc to 70cc's, I've tuned similiar engines such as yours and I've had very good success with the following setup for street\strip cars.

1: Initial timing 14 to 16 degrees BTDC ( Vacuum advance not connected ). 20 degrees in the Mechanical advance to give 34 to 36 degrees total. Small chamber heads, such as Vortec's require less timing. Both initial and total. 32 max with small chamber heads ( less than 60cc )

2: Crane adj vacuum advance ( rate setup as noted ) installed with limiter set to 15 deg of advance. This will give you 29 to 31 degrees BTDC at idle. The engine should be very happy with this.

3: Mechanical advance curve. Initial timing ( vac advance disconnected) should be set with engine rpm no higher than 700 - 750 rpm. The mechanical advance should not start before 1,000 rpm. If it does there is a danger of the engine " hunting" at idle. Put in slightly stiffer springs to correct this.

Full mechanical advance should be "all" in by 3,500 to 4,000 rpm...but no sooner than 3,200 rpm.

LilJayV10 has resurrected an old thread HERE


Some good info there.

And as with every thing in life...there is always more than one way to " Skin a cat". I've had great success tuning cars using full manifold vacuum . Others have had equal success using Ported vacuum. The distributor curve will be different of course, depending on which method you use, and must be carefully tailored to that particular setup.

 

Snip: Sounds like you had more than just an advance curve issue. Could have also been the engine wanted a bit more Pump shot.

No way should the vacuum advance dropping out when you floor it have killed the engine. Something else was going on there.

The fact that it responded to more initial, ponts to the fact that it did not have enough initial advance to begin with. That would also mask a lean spot due to slow or inadequate " Pump shot"

I've also seen a lot of cars with too weak of distributor weight springs. Typically what will happen is that someone will buy one of these " HEI Advance Weight " kits that come with a cam, weights and springs. In most cases these are absolute crappola....can give LESS total advance than stock cams and weights.

Put in too light of return springs...and at idle the advance curve can already be starting. I've seen as much as 10 degrees of mechanical advance curve used up idle. That leaves you with only another 8 to 10 degrees of mechanical advance left from the distributor. Will also make the idle very erratic...particularily with Auto cars. That's one good reason why the mechanical advance curve should never start before 1,000 rpm.

Have a look at the Thread " Tuning my 406 ". Goes into some more detail there.

 

The springs I put into my distributor had a curve that started at 800RPM. I bent the spring perches to increase tension, which should have moved the curve up to 1000-1100RPM. My idle is rock steady at 900RPM out of gear, and 700 in gear.

I get 14" of vacuum at idle in gear.

I have a .31 pump shot, with the most aggresive cam. Should be sufficient.

My total VA is adjusted via an allen key. The rate is adjusted by switching the out springs, the distrib came with 2 other springs. 30* at idle sounds like alot but I'll give what you suggested a try this weekend when I have time to play around with the car. My mechanical is limited to 16* as it is. So I'll run 16* initial which should put me around 32* WOT timing, that should be good. I'll set my VA for for around 13-14*. Hopefully I won't have problems.

Thanks.

 

A couple of questions, if you don't mind. What is the " most agressive cam " that you refer to? Color or number please?

VA rate is usually adjusted by the allen key inserted in the tube where the hose goes on. Total advance is limited either by an adjustable cam ( in the case of the Crane unit) or by the length of the slot that the actuating rod slides in.

Now I'm not familiar with all mfg's products, but this is usually how they work. What mfg of vacuum can is this?