09/22/2021
Lesson on Carburetor vacuum leaks..
How many times have you struggled with trying to get your car to run well and everything points to the carburetor as being a problem. Once you pinpoint that the carb is the problem often times you struggle with trying to rebuild it and then find that the problem is still there or worse. Buying a new or rebuilt carb(s) can be rather costly so this may not be an option. So you may ask yourself what is the problem and how do I fix it.. The problem often times is due to vacuum leaks at the butterfly shafts. So what can you do to fix the vacuum leaks? Let me shed some light on this for you.
I often get requests to fix carbs with vacuum leaks due to worn butterfly shafts or the shaft holes being worn excessively causing excessive vacuum leaks. These vacuum leaks can affect performance greatly. Often times a carb is disassembled and a rebuild kit is installed. After the carb is cleaned, rebuilt and reassembled vacuum leaks become evident or more evident. The reason the vacuum leaks become more evident is that the shafts or bores have worn and there is a varnish that builds up on the shaft or in the bore which takes up some of the wear slop. Cleaning the carb during the rebuild process removes this varnish making the shaft to bore fit looser and the vacuum leaks become more pronounced and obvious.
Given the multitude of carb designs there isn't one standard fix that applies to all. Each carb design may require special tooling and machinery, machining knowledge and skill, coupled with a comprehensive understanding of how each carb design works in order to find the optimal solution to the problems identified.
Often times multiple types of repairs are required for the same carb. In this example the carb was machined for a special seals on two of the butterfly shafts. And for the third butterfly shaft the housing was line bored and brass sleeves were machined to fit.
You may question why seals were used on two shafts and sleeves were used on the other shaft. In this case the shaft bores were very close to the carb mating surface and there wasn't enough material to allow for installing sleeves. I could have machined the bores a few thousandths larger and then machined new butterfly shafts to fit but due to the design of the butterfly shafts this would have been a very costly repair. Machining the carb for seals was the optimal and most cost effective solution here.
Another question you may have is why did I use brass for the sleeves. The number one reason for using brass is thermal expansion and secondly it wears similar to the aluminum or zinc die cast carb bodies. Brass expands at a very similar rate as aluminum or zinc die cast does which helps to prevent the butterfly shafts from seizing in the brass sleeve when the carb heats up. Also note that for many carbs the butterfly shafts are made out of brass or plated steel. For each type of butterfly shaft material different shaft to bore clearances are required to prevent shaft to bore seizing.
For cast iron carb bases a completely different set of machining steps and techniques need to be applied. Cast iron expands at a different rate than aluminum, zinc die cast, or brass. So sleeving a cast iron carb base is done differently. (As a side note, brass valve guides should never be used in cast iron heads for this reason. Cast iron expands quicker than brass and the valve guides will be compressed and eventually will become loose in the heads and or cause valve stem to guide seizures or both.)
Hopefully this gives you some idea about how carb vacuum leaks are often times a problem and a high level understanding of how some of the common types of carbs are fixed. Feel free to contact me if you need assistance with your carburetor issues. This post will also be available on my other page, see Brass and Classic Motorworks LLC.
