Checking for Vacuum Leaks

Checking for Vacuum Leaks

Leakage of air into the intake manifold can be an exasperating problem. This phenomenon occurs because the pressure inside the intake manifold is lower than atmospheric pressure. If there are any holes in the manifold or at any of the connection points, then air can be sucked into the manifold, causing the fuel-to-air mixture to become too lean.

Air being sucked into the intake manifold can cause -

The car acts like it’s not getting enough fuel; i.e., running too lean.

The car may run well at highway speeds, but the engine dies at idle and will only idle at higher than normal rpm (e.g., >1200 rpm).

Our experience: At 1200 rpm the engine would start running rough, and below that it would die altogether. We had to set the idle at 1200 rpm or above just to keep the car running.

The engine hesitates and dies when you take your foot off the throttle, or hesitates when pulling out of corners at low rpm.

Difficulty in properly tuning the carburetor (actually tuning will be impossible).

Since the carburetor cannot be tuned correctly, the timing cannot be properly set.

You may get engine “looping” (alternating between high and low rpm).

The engine may backfire.

Backfiring in Beetles usually means running lean. On the overrun, it fails to burn properly so the exhaust system fills with unburned fuel mix, and then a successful spark and the hot exhaust gases sets the stuff in the muffler off with a bang.

Air inleakage can occur at the following (certainly not all inclusive -

At either or both ends of the carburetor throttle shaft (as the base of the carburetor);

Note: A carburetor can wear a lot during a quarter century of use, and one of the major wear points is the throttle shaft bore hole(s) through the bottom of the carburetor. An out-of-round throttle shaft bore is often the source of air inleakage.
A worn throttle shaft bore could present some of the symptoms listed above.

At the carburetor/intake manifold flange;

At the outer ends of the manifold where it attaches to the cylinder head;


At various points along the intake manifold (e.g., pinoles rubber connections in the dual-port manifold);

From simple oversight, like failure to prevent air from being sucked into the carburetor and/or the intake manifold by not plugging the vacuum line(s).

Vacuum leakage shows up first in the idle. Inability to set the idle speed at the specified rpm indicates improper fuel/air mixture (too lean) or a fault in the ignition system — usually the former.

Note: It doesn’t take much of a leak to affect the performance of the 34 series of carburetors. They seem to be much more touchy than the smaller carburetors.

Testing for Vacuum Leaks

There are a couple of good tests for air vacuum leakage -

The first is the “blip” test. Rev the engine up to about 1500 rpm, then pull the throttle lever back and immediately release it. If the engine promptly dies, or if it hesitates whenever the throttle is pulled back quickly, it is likely that you have air leaking into the intake manifold.

You can pin down the location of the air vacuum using the “starter spray” test.
You can use ether-based starter spray (probably the best) WD40 or even LPGas from an unlit propane torch.

The idea is to use something that is very volatile and very flammable that can be easily sucked into the intake manifold at the leak point. With the engine idling (whatever it takes), alternately spray both ends of the carburetor throttle shaft, the carburetor/intake manifold flange, the outer ends of the manifold, and various points along the intake manifold.

Listen for any increase in engine speed as the extra “fuel” is sucked into the system.
(Alternator, use a dwell-tachometer to see the momentary increase in engine speed.)

If you get ANY increase in engine speed during this process, you have an air leak, and you ll never be able to set the carburetor correctly until you get it corrected.

It pays to verify any problem before replacing/repairing the part. To verify that there is air vacuum around the throttle shaft, clean the area around the shaft with MEK or other prohibited, cancer-causing solvent :-) such as acetone or toluene.

When the area is clean and dry, smear on THIN film of RTV compound and allow it to cure. Plan your moves ahead of time and wear disposable gloves–RTV is messy when smearing.

The object here is to form a temporary flexible external gasket. The throttle shaft should still move and the RTV membrane should move with it rather than tearing. If this temporarily solves the problem it is good evidence you need to have new throttle shaft bushings installed (re-bush) or replace the carburetor.

The Fix for Vacuum leaks

If you have the carburetor off the car, look carefully for any excess sideways/up down movement in the throttle shaft i.e., is it a real sloppy fit in the carburetor? You should not be able to detect any lateral movement; if you do, it is likely that air is leaking into the system at this point, and all of your attempts to correctly set the fuel/air mixture will be in vain.

If the throttle shaft is worn, it is likely that the carburetor is worn in other areas as well. An after market replacement carburetor is the best fix for this problem.

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Author: jcchirco@gmail.com on September 21, 2009
Category: Fuel System, How To's
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