Rebuild Honda CB350 Keihin CV Carburetors

| 10/8/2015 10:06:00 AM

Owners who shy away from rebuilding their own carburetors might be surprised to find it’s often a fairly straightforward proposal. The carburetors on most of our old bikes are quite simple, and experience says that success hinges on two key issues: cleanliness and thoroughness.

It takes more than a simple strip-down and cleaning with a can of carburetor spray to get good results. To do it right, you have to thoroughly soak and clean the carburetor body so that any trace of dirt is removed. That’s sometimes easier said than done, as modern carb cleaners are much milder than their forebearers and less effective at dissolving dirt and other deposits.

BikeMasterFortunately, ultrasonic cleaners keep getting more affordable, with 6-liter units (the smallest we’d suggest) running around $120. Briefly, an ultrasonic cleaner uses high-frequency sound to create pressure waves. The pressure waves “tear” the liquid, creating millions of microscopic cavitation bubbles, and as the bubbles collapse they release enormous energy, agitating the liquid. Thus agitated, the liquid scrubs whatever surface it contacts, removing dirt and scale. Ultrasonic cleaners are remarkably effective, and they don’t depend on toxic chemicals: The cleaning solution is typically just tap water with an ounce or so of dish detergent to act as a surfactant.

Our project 1970 Honda CB350’s Keihin constant velocity (CV) carbs were particularly nasty, so we took a two-step approach, first soaking the carbs overnight in a can of Gunk carburetor cleaner, followed by a trip through our ultrasonic cleaner (see photo above). This worked well; the Gunk dissolved the major dirt and the ultrasonic cleaner finished the job, leaving the bodies looking clean and ensuring the passageways are clean. If you don’t presoak, spray the carbs with cleaner to remove the worst dirt before they go in the ultrasonic bath.

Honda CB350 carburetor

1/4/2018 8:55:23 AM

On #7, using a "punch" to remove the emulsion tubes can damage the ends, if you are going to reuse them. I use wooden chop sticks or small dowels to work them free without damaging the ends. Bill Silver aka MrHonda