Detecting Worn Carburetor Parts
By Ron Van Zee
After you've gone through the process of cleaning the interior of a carb and replacing worn fasteners, it’s time to check the condition of the carb’s throttle and choke shafts and their bushings. These components have taken the most wear in the past due to the constant surging of air and fuel as well as operator adjustments.
Start your examination by feeling for wear on the throttle and choke shafts. You can detect wear by running a fingernail across the shaft to detect ridges where the shaft contacts its bushing. Pronounced ridges are a good indication that the shaft will need to be replaced.
Check for shaft wiggle
To determine if the bushings need replacement, insert their respective shaft (that is not worn) into the bushings. When examining the choke shaft bushing, be sure to attach the butterfly valve. Now wiggle the shaft from side-to-side in both directions. If you detect more than a minute amount of wiggle, the bushings may need to be pressed out and replaced.
Most old bushings can be easily removed with a bushing punch.
A local bearing supplier should be able to supply replacement bushings if you can provide the correct dimensions or bring them an old bushing for matching.
When inserting new bushings, make sure not to apply so much pressure that it breaks the carburetor’s casting. Carburetor doctor Cork Groth prefers to press bushings just past the surface inside the throat. This extra length is compensated for by filing a tiny amount off the top and bottom of the butterfly valve. If air can pass between the case and the shaft at this point, carburetor performance will be erratic.
After pressing in the new bushings, reinsert the shaft. If the shaft does not slide in freely, you may have created a burr at the end of the bushing where the punch made contact. If this is the case, remove the burr with a reamer.
With the shaft reinserted into the carb’s new bushings, attach the butterfly valve. Double check that the orientation of the valve is correct. After attaching, operate the valve to make sure it rotates smooth as butter.
Look for light
“Finally, hold the carb up to a bright light to determine if the butterfly valve makes even contact all the way around the venturi passageway,” Groth adds. “Remember, leakage of air at this point affects not only the airflow, but also the air-to-fuel mixture. If it’s a bad fit, you’ll need to replace the valve.”