Let us make certain at this time that almost everyone understands that this
is what is called an "old mechanics trick". This trick was used years ago (mostly during the 60's) by certain dishonest or unscrupulous mechanics to convince the customer that the carburetor needed to be rebuilt, and the idea
has gotten around that this is really how to do the test for a worn shaft. This is not true, since this is not an acceptable way to check the shaft clearances. It may convince a naive customer, but it won't convince the pros
who know what they are doing.  

Yes, it is true that if the shaft is badly worn you can get an RPM change this way. What is not understood is that if you do the same test on a new carb, you will get a similar RPM change. So what did this test tell you? Not much really. The actual RPM change matters little, and it will vary depending on the under-hood temperature when you do this. You see, the carb doesn't really care where it gets its air from at idle; the air can go down the throat, leak around the shaft, go through an adjustable air bleed, etc., It just doesn't matter. So the shaft leaks air... So? They all do.  Please understand, what really counts is the clearance that is measured at the shaft. 

This is where everyone falls down: nobody is measuring for the correct clearance. Yes, this means using the correct tool and correct procedure for this. If you don't have the tool, or don't know the procedure, then you can't do it right.  You cannot have zero leakage at the throttle shaft. If you did, the shaft would seize up as soon as the engine was hot. Most cars have a steel shaft and an aluminum base. These two metals have a different rate of expansion when they are hot. Aluminum expands much more than the steel and the aluminum expands in all directions, including the shaft hole which expands inward towards the shaft. The steel also expands, but not nearly as much as the aluminum. 

What the result is that when the carb gets hot, almost all of the clearance at the shaft is taken up by expansion. If clearance is not  designed in" at room temperature, then the shaft will lock up on a hot day when the temperature under the hood hits 300 degrees. 

It can get very exciting driving a car with the shaft locked.  How much clearance is designed in is dependent upon the product manufacturer. But in most cases, the clearance will be between .001" to .008" on a new carb. We have seen up to .025" on new Volkswagen carbs, and up to .030" on some new Rochesters, but tighter clearances works better.

Technical Data

Throttle Shaft Clearance

Carburetor Decoding Examples

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