Engine Vacuum Issues - 72 Automatic with 350 Base Motor

[Christopher R.]

I have an issue with vacuum on my 72 vette. We sorted out the brakes and electrical gremlins and the car finally fired up after three years. This is the good news. The bad news is despite being perfectly timed, the vacuum is very low and clearly we have some issues despite replaced all of the lines for the headlights and wipers - basically everything that is vacuum operated was replaced. We have one large vacuum leak that despite numerous attempts at resolving I can't fix. The carburetor to intake manifold leaks on the right side of the carb. The carb was professional restored and it works beautifully, the carb base is not warped nor is the intake manifold base. What can I do to stop the vacuum leak on the base of the carburetor? Once i sort that out i can go through the system, but the leak is so bad i cannot drive the car under its own power.

[Stuart F.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

Only time I ever had a similar problem on a Chevy V8 it was base gasket related. You didn't say anything about what you are using, but there are a lot of "incorrect" gaskets out there.

Stu Fox

[Mark E.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

Only time I ever had a similar problem on a Chevy V8 it was base gasket related. You didn't say anything about what you are using, but there are a lot of "incorrect" gaskets out there.

Stu Fox

That's good advice.

First, confirm you're using a carb and manifold that are compatible. Pre-1970 Q-Jets have a different heat-crossover design and won't seal properly with newer manifolds.

Is the gasket correct and of good quality? The quality of gaskets varies greatly. Contact Cliff's High-Perf Quadrajets, maybe the best Q-Jet expert out there, and buy one from him.

Double check the throttle body and intake surfaces for dirt, old gasket material and warping.

Could the throttle shafts be worn and leaking?

Whatever you do, don't try to fix it by over torquing the carb's mounting bolts. The casting will deform or even break; the soft metal just can't take much stress.

To positively eliminate the car's complex vacuum system as a problem, you can disconnect and plug the manifold vacuum port that supplies vacuum to the lights and AC and see if that makes a difference. Use a vacuum pump on the system to see if it holds.

Ditto for the power brakes.

[Christopher R.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

that's what it appears to be. I have a restored Qjet and I am using the base gasket that all the parts suppliers have in their catalogs - maybe it is just a heat insulator and not a gasket?????

[Christopher R.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

Thanks for the advice. According to the part numbers on the intake and the Carb, they are matched to what was delivered from the factory. The carburetor was restored and run tested on another motor before I installed it so I can rule that out. I will try the gaskets from a reputable source because maybe what I am referring to as a gasket is not a gasket after all. Next, I agree on the overtightening of the bolts but what can you do if the manifold area where he carb bolts to is warped? Scrap it and get another one or can it be machined?

[Mark E.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

For '72, most Quadrajets used a 1/4" thick open flange gasket (open, meaning one big hole in the center, not four separate holes for each barrel). I've seen some cars of this vintage (I believe those with NA9- CA emissions) with a thin metal heat insulator sandwiched between two gaskets (manifold -- gasket -- metal heat insulator -- gasket -- carb). In this case, one gasket is thicker than the other. Again, Cliff or another reputable Q-Jet shop can get you properly set up.

[Terry M.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

that's what it appears to be. I have a restored Qjet and I am using the base gasket that all the parts suppliers have in their catalogs - maybe it is just a heat insulator and not a gasket?????

[Patrick H.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

Photos of your carb, intake and gasket would be great.

Have you also tried running the car with all vacuum take off to the headlight/wiper system disconnected and plugged?

[Mark E.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

You should not have the aluminum plate that Mark is referring to as the NA9 system was 1970 only.

I didn't realize NA9 was a one-year only RPO. I learn something everyday.

According to the Black Book, 1966-67 and 1971 had K19 (A.I.R. pump), 1970 had NA9 (California Emissions), and 1972-95 had YF5 or NN5 (CA Emissions Requirements).

So Christopher's '72 could have YF5... was that equipment similar to NA9 (Q-Jet with sealed external fuel bowl vent, carb heat shield, and charcoal canister)?

It's curious that 1971 reverted to the older RPO K19, then 1972-later used YF5 or NN5. Is this accurate?

And was there really no CA smog option for 1968-69?

[Christopher R.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

Yes, I have the charcoal cannister but not heat sheild, just an insulator gasket. the vendor i used was WWW.Customrebuiltcarbs.com and they guaranteed the base as true, but I will recheck both the base and the intake opening as you suggest. Does anyone know the torque specs for the four bolts that hold the carb to the intake so i make sure i don't overtighten them.

[Mark E.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

The torque specs may be in the service manual; not sure.

I usually hand tighten them using a 1/4" drive spinner handle (looks like a screwdriver with a 1/4" drive male on the end). That prevents me from applying too much torque.

[Terry M.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

I didn't realize NA9 was a one-year only RPO. I learn something everyday.

According to the Black Book, 1966-67 and 1971 had K19 (A.I.R. pump), 1970 had NA9 (California Emissions), and 1972-95 had YF5 or NN5 (CA Emissions Requirements).

So Christopher's '72 could have YF5... was that equipment similar to NA9 (Q-Jet with sealed external fuel bowl vent, carb heat shield, and charcoal canister)?

It's curious that 1971 reverted to the older RPO K19, then 1972-later used YF5 or NN5. Is this accurate?

And was there really no CA smog option for 1968-69?

That K19 option is confusing as all get out. In some years it was an added option for California delivered vehicles. In other circumstances the AIR injection pump was an integral part of the engine option (LT-1s & LS-6s in particular) and no Ki9 option is shown for those engines, yet they all had an AIR pump from the factory.

NA9 was 1970 and included the aluminum heat shield already discussed, some insulating washers for the carburetor and a charcoal canister along with the necessary plumbing for all that.

There was a 1972 California emissions engine for the base motor. I know it included a label on the windshield that showed the emissions the engine produced, and I believe it included an AIR pump.

I am not aware of any special emissions details for 1971.
Christopher, the torque values for your carburetor are in the AIM and may also be in the Chevrolet Service Manual. If no one else looks in their AIM before I get home tonight, I will look for you.

[Patrick H.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

There was a 1972 California emissions engine for the base motor. I know it included a label on the windshield that showed the emissions the engine produced, and I believe it included an AIR pump.

.

If you recall I posted pictures of a CDH suffix motor elsewhere on this board, and it includes an AIR pump.

I have attached them again below.

[Mark E.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

I assume CDH is a '72 base engine with CA emissions? Christopher would be interested in knowing if it had a heat shield for the carb.

Terry- Did these cars really have a sticker on the windshield showing its emissions? I grew up in CA and never saw that on any car (not that I was really looking for them).

[Terry M.]

Re: Engine Vacuum Issues - 72 Automatic with 350 Base Motor

I assume CDH is a '72 base engine with CA emissions? Christopher would be interested in knowing if it had a heat shield for the carb.

Terry- Did these cars really have a sticker on the windshield showing its emissions? I grew up in CA and never saw that on any car (not that I was really looking for them).

No carb heat shield on CDH, and yes it was a paper sticker or label about two inches high and five or six inches wide. I once had a photo of one, but that was two computers ago.