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bought an ultrasonic cleaning device for the B&S carbs. He has shown me the result. I want one too!
What I have done over the years is to use solvent, thinner / naphta. You can use diesel or kerosene as well. Submerge the carb completly in it. Use a brush, tooth brush, nail brush, or whatever I have at hand.
This usually do the trick. Blow them clean with compressed air. Then use dairy (or is it fairy...) fluid handdishwashing cleaner (called "Yes!" in Swedish); or if that is not doing the trick I spray on the special owen cleaner by the same company ("Yes! Power) (somewhat acid though - will discolour the aluminum).
You will however need to have taken apart the carbs nearly compleatly to do this cleaning job. You can leave the butterfly valve in the carbs, but be aware that the o-ring sealing may be damaged by the solvent.
You may need to use the small clock screw-drivers that you can find on the market, to scrape off oxidations, resin, dirt, oil, asphalt, paper gasketts, etc.
Be sure to use new gaskets when rebuilding the carbs!
The o-rings are a pain in the ... to get in place... will proably cost you a few to learn how to do it... (=I avoid messing with them).
When you close the butterfly valve you may notice that is does not shut-off the tube compleatly... can be adjusted. You need new screws and a new shaft - just in case... It is brass and the screws are lock-tited...
The choke has a casting residue inside. Grind that away! The casting residue can "jam" the choke as it pivots. Never had a choke getting stuck for me since I began fixing this BMW "sloppyness". Regrease choke. Soak the gasket in grease - seal better and you will not need to scrape the paper gasket off the carb at the next renovation. Use cupper paste of preferably Omega 99 (anti-seazure) on the screws, and choke axel.
Replace the diaphragms - they are usually worn-out. They do -not- like solvent!!! So if you do not need to change them - be very careful with them. Soap or hand-dishwashing fluid is just about it that they accept.
I sometimes get my parts from:
Bing Home Page
And... do buy their tuning manual - it is really good and very helpfull. Bought mine in 1994, so I guess there are several up-dates since then making it even better -