Air Inleakage

(See also our article on Hesitation.)

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Topics covered in this article include -

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General Discussion

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.

In this regard, Rob has written -

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.

  • Lots of frustration!

The key symptom here is the idle. Failure to idle smoothly at the specified rpm indicates improper mixture or a fault in the ignition system.

A carburetor can accumulate a lot of shaft bore wear in a quarter of a century, and it is an out-of-round throttle shaft bore that is often the culprit. A worn throttle shaft bore could present some of the symptoms listed above.

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Testing for Air Inleakage

There are a couple of good tests for air inleakage -

  • 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, of if 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 inleakage using the "starter spray" test. You can use ether-based starter spray - probably the best - or WD40 or even LPGas from an unlit propane torch. The idea is to use something that is very volatile and very flammable that can easily be sucked into the intake manifold at the leak point. With the engine idling (whatever it takes), alternately spray the carburetor/intake manifold flange, the outer ends of the manifold, both ends of the carburetor throttle shaft, and various points along the intake manifold. Listen for any increase in engine speed as the extra "fuel" is sucked into the system.

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.

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.

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The Fix

A worn bushing where the shaft comes through the throttle body is not uncommon, and can be fixed by drilling out the hole and inserting a brass bushing. Re bushing a throttle shaft bore isn't all that difficult. Solex provides kits for this (if you can find them). We've heard it said that the throttle shaft is the same diameter as a valve stem, so you can use a piece of valve guide as the bushing, but we have first-hand no experience with this.

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.

But -- it pays to verify any problem before replacing/repairing the part. To verify that there is air inleakage 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 or replace the carburetor.

NOTE: Don't forget, a new carburetor is not likely to work perfectly straight out of the box (particularly if it's a Brosol Mexican Solex) from what others have said. Sounds like they need a careful clean and tighten up, and might need the jets altered to suit your car etc. Don't throw out the Solex until the new one is running right some parts might be useful.

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Disclaimer stuff: Rob and Dave have prepared this information from their own experiences. We have not assumed any specialised mechanical knowledge, but we DO assume that anyone using this information has at least some basic mechanical ability.

We hope you find this information useful, but we don't take any responsibility for anything which happens to you, other people, your VW or any other property or goods resulting from your use of this material.

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Have fun fixing your VW - just keep them fweeming, OK?

Last revised 4 May 2004.

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