Kev82vette

I do and with a Edelbrock 600 w/manual choke. Timing at 12* initial, 32* total and all in by 3300rpms. My idle is at 950 rpm and I only have 9-11 hg of vac. This seems to low. Anyone else have this low?

Durango_Boy

Idle in gear or idle in park or neutral? A 1K idle in P or N is not bad if not a tad high. In gear, you want around 600.

Kev82vette

in park. The vaccum is what concerns me the most. I think 9-11hg seems low. Could the cam cause this? 222/222 .450/.460.

Durango_Boy

How thoroughly have to checked for leaks? Even a bad check valve could case a problem.

Aside from that, 10hg is enough, at minimum, for headlights and brakes. I wouldn't have thought that cam would have done that which is why I'm leaning towards a leak.

wesmigletz

You could probably bump your initial timing up 4* and bring your total timing to 36*. That might help the idle a bit. Are you running ported or manifold vacuum? Are you sure the timing tab you're using marks true TDC on the balancer? If the balancer is used, is it possible it has slipped?

Kev82vette

I sprayed carb cleaner all over the intake and carb. Around all the vac lines and pvc valve. I thought there was a leak down by the trans module but I don't think it would cause it because I plugged the line on the back of the carb that runs to that and it ran exactly the same.

Durango_Boy

Try disconnecting all the vacuum lines except the distributor and the transmission. Check those two for leaks, and make sure the carb is cleanly sealed to the manifold and check again.

Kev82vette

I think I will bump my timing up a little. I am running off of manifold vac.

ESU

I had the same motor and was plagued by low vacuum also. It was in the 11 range right out of the crate. Had the manifold off twice, tried two different carbs , never got it over 12.
ESU

Durango_Boy

You should have just run a spare line into the cabin, and sucked on it like a straw for the extra vacuum.

77 Vette Girl

I have the 290hp GMPP. I experienced low vacuum (12.5 - 13). Checked everything for leaks, installed an A/F monitor, re-jetted carb. (Edelbrock 1400), changed to lighter advance springs in dist. I was begining to think it was a big cam causing the problem. But I stayed with it and prevailed......Results:

1. Rejet carb to yellow step up spring
.092 Primary jets
.095 secondary jets
68 x 52 Rods
2. Timing 12 deg initial, 36 total all in by 3000rpm
3. (This is very important) Run the vacuum advance on ported vacuum.
I have always run manifold vacuum on SBC but for some reason this engine and carb combination likes ported vacuum.

Now I have smooth idle, 20" vacuum and a very much reduced unburnt fuel smell in the exhaust.
I have purchased 12 sets of rods and 9 sets of jets to get to this point. As I suspected the factory jetting for the Edelbrock carbs are WAY rich. I may go even leaner, I have 4 new sets of rods I haven't tried yet.

Kev82vette

77 Vette Girl , did you have a stumble off idle? If I floor it from a dead stop it falls on its face for a brief moment and then pulls hard. I will try switching to ported vac.

Kev82vette

Must be the cam or something that makes our motors have low vac.

ESU

Actually the edlebrock carbs are generally jetted lean since they are California emissions compliant but glad to see you got your problem handled. I tried the edelbrock 1400 series as well a the Q jet but neither helped even though Lars worked over the carb.
ESU

Kev82vette

I wonder if switching to ported vac will help? I am going to have to check my timing tonight. I have it to manifold vac right now, So to see how much vac the dist can is giving me do I need to rev up the motor? Or will it be the same at idle?

okinawa86vette

I don't think it's the cam causing your low vac. It would be lower than stock, but not that low.

I'm running a 234/244 .520/.542 cam in Colorado at over 6000ft (about 1" loss for every thousand feet is the rule of thumb) and I'm seeing about 9" of vac.

What jets/rods are you running? I've had edelbrock carbs that were creatively (off edelbrock's charts) tuned and sometimes they don't respond well, needing a high idle to keep running, giving driveablility issues and poor vac readings.

I agree with durango boy that you should plug up all your vac lines to know for sure you don't have a vac leak somewhere else in the system.

Chris

Kev82vette

I will try that. But I am having a hard time thinking its a vaccum leak when other people with the same engine and carb have experienced the same thing. It has to be something else???

77 Vette Girl

Sorry for the delay, my internet was down for a couple of hours.
My guess is the stumble is related to carb jetting, but it could be a combination of factors. I suggest 93 octane fuel and make sure your breather is not restricted. I am not running an EGR (Edelbrock 2101 manifold) and I took out the heat riser valve on the driver side of the air box. Crossover is open. Try experimenting with the ported vacuum connection while watching your vacuum gauge.

Theiskell

I may be missing something, but why couldn’t you just plug all vacuum lines except one and check how much vacuum there. Would that not at least tell you whether you have a leak down stream. If you have good vacuum then you know you have a leak if not you know it’s in the motor.

77 vette

I had this problem breaking in my 383. What I found is that the edelbrock carb needs a fuel reg, I baught a cheap $20.00 chrome adjustable reg from NAPA (or any other national parts store) and set it to 6 PSI. I found that my adjustments where "way off", after re-jetting and adjusting the fuel to air mixture I gained 9-10 inches of vacuum and a much better air fuel mixture. My eye's stoped burning from the exhaust fumes as well. I hope this helps. Pease.