Fix a Loose Carburetor Butterfly

Tech Article Carlo Catalanotto of CC Racing Engines


Been having problems with a sticking carburetor because of the butterfly coming loose? Read this tech article by renowned engine builder Carlo Catalanotto and be rid of those butterfly blues!

I have seen so many instances of carbs sticking and seen the problems from screws backing out and going through expensive engines once removed and not re-installed correctly that I figured I would help you guys that want to fix them yourself. This is a simple how to article on how to fix your carb. First I would like to talk about what is actually happening that causes this problem most of the time. We are twisting these little engines far beyond what these carbs were ever designed for and the harmonics and vibrations that they are subjected to is what rattles the screw that holds the butterfly tight loose. When that screw comes loose it allows the butterfly which is not perfectly round (It is actually egg shaped) to spin on the shaft. When it spins on the throttle shaft that it is mounted to it does not seat in the venturi correctly any longer. Since it will not seat correctly it will allow air around its outside diameter of the butterfly and in turn will not allow you to shut the engine off when you try to push your trigger forward. This problem is only multiplied when guys use a hard linkage to the carb instead of a bell crank system that is not attached directly. When you use a bell crank system and you have a problem with the carb it lets you know when you can not shut the engine off that there is a problem and you can fix it before anymore damage is done. When you use a hard style linkage you force the carb closed and the butterfly digs into the aluminum housing and eventually ruins the butterfly and the carb. So for those of you who have used this method to cure you carb problems I would re-think what you have done and hope that your blade has not dug into the carb housing and sent aluminum through your engine.




Here is the procedure that we use here at the shop at CC Racing Engines:


Find a clean dirt free spot on your workbench or wherever you work on your stuff. Completely disassemble the troublesome carb with the exception of the throttle shaft. This is also a good time to inspect your gaskets, diaphragm and check your screen for dirt particles. Next make sure that you have not lost the clip that holds the shaft in place on the side of the carb, this must be in place before you do anything else as this keeps your shaft In line.

Image Image

Remove the screw out of the shaft and remove the butterfly and inspect it for burs. If it has burs or is bent in anyway it would be best to replace it. The next thing we need to do is clean the shaft and the screw with carb cleaner, let it dry or blow it off with compressed air. When we re-install the butterfly first make sure that you have a little tension from the carb return spring on the shaft assembly. Then when installing the butterfly you will need to make sure that the small indented nipple on the middle of the butterfly butts up against the flat part of the throttle shaft as this is what aligns the butterfly on the shaft.

Image Image

When re-installing the screw use a very small amount of red Loctite just enough to coat the threads.   ***Note you do not need a lot of Loctite just enough to coat the threads.*** Make sure to line up the butterfly put the screw back in and very gently and lightly snug it. Now make sure the shaft turns easy and the butterfly closes all the way before you completely tighten the screw. 

Image Image

After you tighten the screw, Make sure that there is no loctite oozing out that could get in between the shaft and the carb housing as it will make the carb almost impossible to free up. Place the carb screw side down on top of a Philips head screwdriver to support the throttle shaft (I use the Zenoah tool that comes with the engine and mount it in a vice) and stake the end of the screw that sticks thru the throttle shaft. You can do this with a center punch, Philips head or a flathead screwdriver. 


Image Image


After you stake it, it may be hard to turn the throttle shaft and this is because the screw has pulled on the shaft a little and caused it to be slightly bent. That is no problem as there is an easy fix. Place the carb face side down on top of a rag or shop towel on a hard supported surface. Use a Phillip head screw driver and place the end of the screwdriver back in the screw as if you were going to tighten it (Again I use the Zenoah tool that comes with the engine) and give it a gentle but firm tap to the end of the screwdriver to straighten the shaft. Check the carb to see if it is easy to turn as you may need to do this procedure again to get it to be completely free. Put a couple of drops of oil on each side of the shaft and you are ready to re-assemble the carb.



If you do this procedure correctly you will not have to worry about your carb screw coming loose any longer as I have never had one give me any trouble after this has been done.

CC Racing Engines


Log in to comment

User Login


You are not logged in.

© 2017 All Rights Reserved.