my 305 is only getting five inches of vacuum at idle presently. It has a performer RPM, holley 750 VS 3310 carb. I have the total timing at 34 degrees and initial is between 12 and 14. The cam is a crane energizer single pattern .454/.454 with an advertised duration of 272/272 and at .050 it is 216/216. There is a 2400 stall if that matters at all. If I play with the idle mixture screws starting with them at one and a half turns out and start turning them in, they have no effect on the vacuum until I almost close them, then the motor will start to stumble and then die out. I sprayed all around for vacuum leaks and found none.

By turning the idle speed screw up, from 800 in gear right now to around 1500 RPM in gear the vacuum will increase to 16-17 inches. What do I need to do to get this motor back on track?

 

With the screws at 1.5 out,hows the idle(smooth,rough)might wanna back them out? Did you mess with the heads,maybe valve lash?

What are you working on?Not the camaro. thought you were going with a 350.

 

valve lash is fine, no ticking and compression check is 155 all cylinders. Turning them out more has no effect on the idle either from what i can tell. not sure if the throttle plates aren't open enough in combination with the timing being too advanced making a lower idle tough. if i retard the timing to 10' BTDC i can open up the throttle plates a little. not sure which direction i need to go with this motor to get a better vacuum signal. whether turning the screws in or out another full turn, the vacuum guage never reaches the 15 degrees like it holds steadily at 1500 RPM.

 

Turning the mixture screws in is leaning it out(stumbles and dies) back them out 2-3 turns and get your throttle almost closed.If it starts to die back the mix screws out.

Where you taking your readings from,intake or carb?Not sure if it matters though.

You sure about the valve lash.Low vacuum-late valve timing/intake leak.

Not much help I know, sorry Bro

Don

 

I would go with a smaller CFM carb like 650cfm that 750 is quite large for a 305. I really don't think your going to get the vacuum results you want with that loping cam at idle speed If you need the vacuum for your pwr. brakes and keep the 750 you'll need a vacuum reserve can and an idle speed of 1,000 rpm at least.
Your cam doesn't develop much vacuum at idle anyway. you might want to get a vac. reserve can even with the 650 to improve power brake perf. I had a 280 dur. and 460 lift in a 360ci
and was lucky to get 12in. at 800rpm. in gear.

 

Don't think the cam is the problem,I had a 440/454 lift 210/216@.050(adv. dur. 270/276)in my 305 with a computer q-jet ,ran fine.

What are you working on Steve?

Whats been done to the engine?

 

Yeah, I've had a few of those itty bitty primary Q jet's. Not much problem getting them to idle and draw vac. with the small primary venturi's. What cfm was it? I'll bet you had to get a chip burned for thet CC carb too huh? Isn't the 3310 a square hole? (all venturi's the same size)? I never liked the Q jet very much because the part throttle performance just wasn't there on the street although it did deliver good milage crusing on the highway it seemed that it only delivered performance when your foot was on the floor! and sucked gas around town.

 

3310 is a 4160 style carb.

Don, going out 2-3 turns seems excessive and doesn't help the idle vacuum. I need to get this thing on a 4 gas analyzer to set the screws i'm guessing. What i've done now is set them so that the RPMs drop slightly then richen 1/4 turn. this was in one of chickenman's responses to someone else.

Not sure if running more or less timing at idle would help the vacuum quality? any takers on that one? otherwise i'll get a vacuum canister and be done with it.

 

carb size has no affect on idle.
When exactly did the problem first pop up?
Is this right after the cam install?
If so, are you absolutely sure the cam itself isn't retarded? That's a pretty mild grind, so it shouldn't be an issue.

 

[ I never said it did. All I said was the 750 is quite large for a 305.
Sorry for misleading you on the 2nd post! I was just stating that the Q-jet had good idle and vac. cararistic's on everything I've had one on.

 

when i installed it i lined the dots up dot to dot. I don't think it's retarted, when spinning the engine over the rockers both drop down at TDC compression when the balancer comes around to it. I didn't degree the cam, but this wasn't my first cam swap either.

Not sure if timing advance should be changed? would that affect idle vacuum....perhaps more timing then 14 degrees right now? Otherwise should i be taking the readings from somewhere other than the full manifold port on the carb? perhaps the intake etc? I'm really lost here and what to do. any suggestions would be greatly appreciated.

 

How does the engine run? If the engine runs halfway decent...then try a different vacuum guage. Sometimes the simplest things......

At 5" of vacuum that engine should barely run at all...hence I suspect a faulty guage.

 

i will try a different vacuum guage and report back to you.

 

....and make sure that you are reading " Manifold " vacuum and not " Ported " Vacuum"

 

Agreed.
That number sounds too much like a ported reading.

 

I run about 7" myself, but I'm also at 6300 feet above sea level, 350CIN, Victor Jr intake, Holley 4150, and a Comp XE284!
My brakes are rock hard at idle.

That gauge can't be right!

Ron

 

the full manifold port is the bottom of my two choices on a holley carb right?

that's where i'm taking the readings from. or should i use the top nipple on the carb which is what I have my vacuum advance hooked up to.

Another thought: should I have the vacuum advance disconnected when checking vacuum? All my tests have been done with it hooked up to the top port on my 3310.

 

Do you have a non-carb vacuum port? If not,use the brake booster or pcv.

 

Holley changed that bottom port around on different revisions of the 3310. Early models like my 3310-1 had full manifold vacuum on that bottom port. Later models ( up to 3310-6 I believe now )changed this to ported vacuum.

Easiest thing to do is get a tee connector and a short piece of vacuum hose and " Tee" your vacuum guage into the side port ( top passenger side ) that the distributor vaccum is connected to.

I believe that you will find that the side port is full manifold vacuum and that the bottom port is ported vacuum on your particular Revision number.

 

Exactly 'Snake Oiler'
I would just hook it up directly off the manifold so that there is no dought. I know the Performer RPM has one behind the carb.

Ron

 

working another 14 hour shift at the lab today-got huge FDA samples due out this week, so I will be able to pull the car out friday(weather permitting) and check. I have a second vacuum guage now as well. Will check off the brake booster port in back as well.

Thanks for the help as always.

 

i tapped into the port behind the carb today. used a new snap-on vacuum guages as well. tried different timing and idle screw settings to achieve 850 RPM in gear. lowered the idle to 600 RPM to set timing, then raised the idle speed to 850-900 in gear. came up with 16 initial, which is 36 total at 3000RPM and 11 inches of vacuum at idle. So i went from 5 inches to 11 inches now. Not sure where else to gain more vacuum, idle mixture screws had no effect either.

Checked for vacuum leaks again just to be certain and found none--all around carb and intake. Surprisingly the vacuum still jumps up to 15inches at 1500 RPM. i'm pretty lost now on what to do. Power brakes work better now at least and can keep them even while idling.

 

I said, I was lucky to get 12 lbs, You've got 11! It's called tweaking! I'll bet the throttle response is better too!

 

Hey Steve,glad to see things are progressing,how much pre-load did you use on the valve lash?

 

Where are you running the distributor vacuum from? Ported or manifold vaccuum.

I would advise running it from manifold vacuum. Buy a Crane adjsutable vaccumm advance unit. Adjust the diaphram spring ( with allen key ) so that you are getting full pull in by 7" to 8" of vaccuum. Check with a vacuum pump....alternatively, just loosen the setting off all the way. Set up advacnce limiter cam on Crane unit to give 15 degrees. Now connect to manifold vaccuum. This should get you a couple of more inches of vaccum at idle.

BTW.. IMHO,that is a fairly large cam for a 305 with a tight convertor. Checking the specs it looks like it is ground with a 110 degree LCA. This creates a lot more valve overlap and this will absolutely kill vacuum on a 305. A LCA of 112 is really the minimum that should be used on a street 305 of mild specification. Is this a flat tappet cam or roller? A flat tappet cam would make the situation even worse.


A 110 LCA requires more compression ( 9.5 to 1 minimum ) and more cubes ( 350 CI or more ) to really be affective.

 

i am running manifold vacuum off the carb. i will pick up a crane adjustable vacuum advance unit this week and try out those suggestions.

I did not think at the time that this was too large of a cam, considering many run the XE262 which has more duration(at .050) and more lift on the exhaust side as well, still being used on a 110 lobe separation. This is a flat tappet cam not a roller. Also, crane recommended that I use the 2400 stall convertor with this cam in my 305, should I have gone larger?


Snake Oiler---If i remember correctly i'm running 1/2 a turn or 3/4 a turn of pre-load, whichever crane recommended, I put this motor together last winter so details are a bit fuzzy. Compression test came out to 150-155 for each cylinder as well--so valves aren't hanging open on me.

 

Looking through the GM high performance manual, you will find that all of their cams recommend for a 305 have LCA's of 112 degrees or greater. These cams are made for GM by Crane.

When I had a 305 in my Camaro I ran the GM #12353917 Cam . Now I had fully ported #416 heads, 9.5 to 1 CR, Hedman headers, Performer 2101 manifold, and a Holley 780VS all through a Flowmaster 3" single exhaust. So a fairly stout 305.... Would run to 6,000 to 6,500 ( with power drop above about 5,700 ) but had a good 14" to 15" of vacuum with a rock solid idle. Excellant throttle response with good fuel economy. Ran a 700R4 at the time. Would light the tires in 1st any time I wanted.

I ran the GM #12353917 Cam Kit. .204\.214 @.050" .420\.442 lift. LCA of 112 degrees. I could have gone with the #12353918 " Marine " cam. 214\224 @.050" .442\.465 and 112 deg LCA.... but the smaller cam worked out better for autocross. For drag race I would have gone with the bigger cam. Note that they both use a 112 deg LCA...that is critical.

Every decent camshaft article that I have read ( and I have read many ) all recommend an LCA of 112 degrees or more with a small motor. They simply can't handle the greater valve overlap created by using the tighter LCA. Read some articles by Vizard, Harvey Crane or Lingenfelter. Lingenfelter specifically deals with our cars and motors in his book. None of his cams for a 305 have an LCA less than 112 by his recommendation.

Now lots of guys may say that they run a bigger cam ....with an LCA of 110 degrees....but I've seen lots of cars that ran like a soggy wet blanket on the bottom end with little manifold vaccuum.

From my experiences you would have been better off with a split profile cam with a slightly larger LCA

What are the rest of your engine specs, especially CR and head work? What do you use the car for?

BTW...a good indication of a well matched camshaft to engine combination is cranking compression. Article in Chevy High performance ran this about a year ago. Key factors are:

1: Compression Ratio

2: Valve overlap. Duration and LCA

With a well matched cam\engine combination, cranking compression should be in the 170 to 180lbs range. Anything lower is and you need to either cahnge to a smaller cam or increase the compression ratio. Generally the first option is easier.

Note: This is a general guide for a typical street driven high performance car. Race cars with huge CR's may see higher pressures but generally you should never see more tha 200psi.

At 150 to 155 psi cranking pressure you are a bit on the low side. Now your combo may work...but it could be better. IMHO of course. A 110 LCA is too tight in my opinion for your combo.

 

chickenman....heads have been ported and polished at home using several of your own and sitting bull's posts. CR should still be at 9.5:1 as it came from the factory, although the combustion chamber may have been opened up a bit(Few cc's) when i unshrouded the valves as per sitting bull's link. I am just trying my hand at this motor until I build up a vortec/XE274 combo and am trying to learn as much as possible as I go on my "practice" motor. Carbs around here aren't used much and the guys that know what they're talking about (or say they do) are all interested in making money off me for tuning rather than pointing me in the right direction. Seems like a taboo subject to me. I always appreciate your knowledge and the kind way you lay it out. I'll post back up in a few days after the vacuum advance shows up along with some nice weather hopefully.

 

11" manifold vacuum at idle seems a little low for that cam. One of the other posters is right about the lobe separation choice for a 305.

Other than that, the cam could be out of position in the motor.
When you installed the cam did you replace the timing chain and gear set (both top and bottom gears or just the top gear or??? The only way to tell for sure
would be to degree the cam to verify cam card valve timing specs.
What grade of oil are u useing? a thicker oil will create less lifter bleeddown at idle and more effective seat duration. if you're using a 50weight oil change to 30weight.
I'd also check the brake booster for leakage.

 

i installed a new timing set from cloyes, believe it was the true roller set, installed dot to dot. I am using 10w30 oil in this motor, and have used 5w30 as well. The brake booster was disconnected for these vacuum readings, the only thing hooked up was the PCV valve. The brake booster port on the intake was used for the most recent vacuum readings.

 

F-Bird88 raised a good point. Cam may be installed wrong.

Doesn't the Cloyes crank sprocket have multiple keyways to advance or retard the timing? You should have installed this cam " straight" up. IE: 0 degrees advance\retard on the keyway. Cam sprocket has corresponding punch marks on the face that must match up with the ones that the crank keyway is set at.

Double check the cam installation. I'm thinking that you may have installed the cam slightly " retarded" which could account for the low vacuum reading.

On the other hand, this could all be a " wild goose chase" as the problem could simply be that 110 degree LCA. Doesn't hurt to double check though.

 

All this talk about LCA is all well and good, but yanking a timing cover to double check that the cam isn't retarded seems like a lot of effort, especially considering you have to mess with the front oil seal, and considering he said this wasn't his first cam swap. That'd be my last resort.

That's not a 'huge' cam for a 305, and with a 2400 stall converter, it should idle reasonably well. But you'll probably want a vacuum can if you have power brakes.

Have you checked the power valve? If you're only getting 5" of vacuum, and you have a (stock) 6.5" power valve, you're getting power valve enrichment at idle and the idle screws would be ineffective, as you've experienced. Swap in something like a 2.5" power valve. At 1500 RPM, the motor is pulling more vacuum, power valve closes, probably 'idles' much better, except for that the fact that it's 1500 RPM!

To set the idle on a Holley, make sure the throttle blades are closing properly. Start like you did, with the screws 1.5 turns out on each side. Get the motor running with a vacuum gauge hooked to 'manifold' vaccum. The front port, at the bottom, is fine. Set the initial timing with the vacuum advance disconnected. You said you had 10-12 degrees. That's probably a good start.

Adjust the idle mixture screws to achieve the highest vacuum reading. Remember to adjust both sides relatively even! If turning one screw out a full turn makes a positive improvement, turn it back in 1/2 turn and turn the other side out a 1/2 turn to match. Re-adjust the idle speed if necessary. (ie-- motor picks up vacuum and RPM (maybe 1000), adjust the idle speed screw for lower idle speed (800). Adjust idle mixture screws again as necessary. It should only take 2-3 iterations to get it dialed in.

Lastly, hook up the vacuum advance and adjust idle speed if needed. Your car should idle reasonably well at this point. Assuming everything else is good.

-Dave

 

cam was installed using the 0 degrees straight up setting. even had one of the thirdgen mods over at the time of timing chain installation....so two people have seen the straight up setting as per their directions in the timing chain kit. Hopefully your suggestion of the adjustable vacuum can will help me out as well.

ChevelleFan -- I now have 11 inches of vacuum at idle, so i don't think changing out the power valve to a 2.5 is necessary, especially due to the higher vacuum reading i just got. I have played with the idle mixture screws and idle speed as best i can which along with the timing helped me to double my idle vacuum. thank you for the tips.