Vacuum Leak - Can't Find It
 

I have been dealing with an annoying inconsistent idle and have finally narrowed it down to a vacuum leak. The guy that rebuilt the Qjet pointed it out to me. With both idle adjustment screws turned all the way closed, I drove out to his shop. He put his hand over the primaries nearly covering them up and the RPMs came up. He said a vacuum leak will do that. Recommended I spray carb cleaner around carb base and where intake bolts to head. I had already done all that but did again. Nothing. Removed and plugged every line from carb. It will idle all day at 700 RPM but when driving and lift foot it will be at 1200-1300 and eventually get back to 700. I guess there is a possibility that the intake gasket could be leaking on the bottom edge and sucking air from the lifter galley.

Suggestions?

Do you have a hand held vacuum pump. If so draw a vacuum on the vacuum advance can and see if it holds

I think a better way to find a small leak in a low vacuum location is to try the propane torch (UNLIT) method.

Using a handheld propane torch, like the kind used for sweating (brazing) copper pipe joints - UNLIT - open the valve but don't light the torch, and hold the tip close to every potential vacuum leak location, from the intake manifold to head interface, the carb to intake flange, every hose and carb/vacuum fitting, etc.

Obviously, doing this with engine running at idle requires an eye on safety, so be careful (also, don't smoke). If there is a vacuum leak, the engine will speed up when the propane gets sucked into the combustion chambers.

Propane, as a gas, works better than a volatile liquid, and doesn't leave residue or a flammable liquid on your engine. Any propane not sucked up by the engine gets dispersed by the fan.

This is the easiest, cleanest, and most efficient way I have ever found to chase pesky vacuum leaks.

Good luck.

I wouldn't be surprised if it is the Q jet. Great advice from easy rhino on checking for vacuum leak.

I would think that carb cleaner sprayed directly on possible vacuum leaks would accomplish the same thing but at this point I'll try anything.

I've plugged every line but it so I will just plug it also.

Lower ignition point than propane...carb cleaner has too many chemicals compared to propane...:rock:

You're absolutely correct - for larger leaks. For the tiniest leaks my experience is that propane shows smaller leaks with less effort, and without spraying a volatile liquid on the carb and top of the motor.

YMMV

I reread your original post, and hope it's not from the internal areas of the intake gaskets. I am not sure of an easy way to isolate that leak.

I'm already mentally prepared to yank the intake. If the propane doesn't produce any results, it comes off this week.

Good luck with it, then. Sorry for your troubles.

Any chance that the heads were milled causing the intake to mismatch the intake?

Or the engine was decked

Unless I have been wrong for over 30 years...What I thought I knew about a QuadraJet was that IF you turned in your both of your idle mixture screws until they stopped...thus closing them off...and the engine still ran...you have a carburetor problem.

DUB

I had this problem recently as well. It ended up being my vacuum port. Stripped threads.

I didn't even think about that. You're right.

Remove the distributor cap and clean the centrifugal advance weights to make sure they operate freely, this will eliminate that as a possible cause. Just some WD-40 for lube.

When the engine is warm at 700rpm idle do the emulsion screws make a difference in idle quality. Turning the screws in to seat cuts the emulsion off to the curb idle ports only. It's possible the throttle shaft is wore and needs attention but the only way to know is to remove the carburetor and have a pro look at the throttle shaft in the base.

The Qjet was just rebuilt by Bill, a local guy who has been rebuilding carbs forever. I trust his rebuild and he would have checked the throttle shaft wear. The HEI is new. The plug wires and plugs are new and I did a compression check.

Up until recently I would have agreed with you but I tend to believe my carb rebuild guy Bill. The vacuum leak is causing the engine to run rich because it is screwing up the way the fuel is metered by the Qjet (or at least that was the way it was explained to me).

Same setup since 1987 other than rebuilds and cam changes. Have not driven the car much the last 3 yrs and I can't completely rule out the possibility that this vacuum leak hasn't been going on since the last time the intake was pulled and reset. I've also heard of HEI modules doing erratic things hence replaced it.

Is your intake set up for a Q-jet? If not do you have the correct gaskets between the carb and intake?

Many Q-jets have a hot idle compensator; that induces a controlled vacuum leak when the engine is hot to create faster idle until things cool down and then closes off for a lower idle RPM. Then the cycle starts over.

Yours could be missing, installed backwards (yes you can do that) or malfunctioning. Very easy to check on a Q-jet and Bill the carb guy could eliminate it quickly as an issue...