Rebushing SU Carburetor Bodies

Rebushing carburetor bodies and fitting new throttle shafts is a major part of a proper carb rebuild that is often overlooked. Vacuum leaks due to worn bodies and shafts cause rough idle, poor performance and poor fuel economy.

Checking the shaft and body clearance is a simple matter of wiggling the shafts up and down. Any appreciable play indicates wear on the bushings and shafts. Some wear is to be excepted, but performance is seriously affected when clearances are over .005″. Verification is done with the engine running at idle speed. Spray the joints of the shafts with carburetor cleaner. If the engine speed changes, the shafts leak.

Quite often, the problem can be fixed by simply replacing the throttle shafts. Mark the butterfly plates with a felt tip pen to record which way they face and which way up they are installed, then remove the two securing screws. (You’ll have to pinch together the split ends which are widened to prevent loosening; needle nose pliers or pushing with a screwdriver will do this.) Pull the plates out, then slide the shafts sideways till an unworn section of shaft is in each side of the body. Now wiggle them to gauge whether any play is still present. If none, a new pair of shafts will solve the problem. Oversize shafts (if not already fitted) can be purchased to take up a small amount of play. Oversize shafts are .010″ larger in diameter. If the bodies are worn as well as the shafts, rebushing will be necessary.

To rebush bodies, the old bushes (or the bodies, if your carbs don’t have bushes) must be drilled out, just short of entering the venturi. Since each side of the carb must be drilled separately, accurate alignment of the two holes is difficult to maintain. We have found that an easy way to do this is to chuck the proper sized drill in a lathe, and run the carb body into the drill with tailstock center. New bushes are then pressed into the bodies, and reamed to fit the new spindles with the minimum clearance which allows rotation of the spindle. This is best accomplished with a 5/16″ adjustable reamer with a pilot long enough to ride in the opposing bush. Sometimes it is necessary to lap the spindle in the bushes.
This is done by chucking the spindle in a hand drill and running it dry in the reamed bushes. Do not use any abrasive compound, the dark oxide that is formed will be enough. Add a drop of oil, and continue until the shaft rotates freely in the bushes. Re-fit the butterfly plates and drill the shaft for the adjusting lever, then drill the taper pin holes with the throttle completely closed. The drilling is best done from the underside on a drill press with the parts clamped in a jig. The hole must be drilled exactly on center or the shaft will be spoiled.

After the reinstallation of the throttle plates and return springs, the rest of the carburetor rebuild is routine. Keep in mind that the throttle plates must be centralized before the screws are tightened.

If all of this seems rather complex, it is. Rebushing carburetors takes patience, specialized tools and experience.