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Motorcycle Repair/high rev cb350

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Question
Hi Bill,  I rebuilt my 772a carbs on my 73 cb350. ( except the diaphragms)
I set the clearance on the butterfly's , when I start the engine it instantly goes to 5000 rpm  and will not go lower. If I give it throttle it'll rev higher and slowly come back to 4 to 5 thousand rpm. The idle stops are on there resting spot ( butterfly closed ). What is going on?
How is that much fuel getting in the cylinders ? Help

thanks
Don

Answer
David, did you check the diaphragms for any tears or pinholes? Any defect in the carb slide diaphragms is not adjustable. When the old diaphragms age, they can be slow to respond causing metering issues. With carbs assembled, check the diaphragm action by lifting up the slide with your fingertip. There should be a little resistance to movement and then when you let it down, there should be a little air sound with the slide dropping back down. If the slides snap down quickly, the carb top isn't sealing properly or the diaphragm is damaged.

When checking initial idle speeds, just back the speed screws out until they are clear of the carb body stops, then turn them down to where they just touch, then add in 1/8 turn. Watch the carb arms while you swing the handlebars from one side to the other so you are sure that a cable routing issue isn't causing the problem.

Check the carb inlet manifolds for any signs of cracking which causes an air leak. Any originals probably need to be replaced by now. Be sure that the carb stubs are fully seated into the manifolds. Float level is 26mm and idle mixture screws set at 1 1/8 turns out from gently seated.

If you are confident that all the carb issues are solved, then check the ignition timing...

Remove the point plate and grab the end of the spark advancer cam and twist it back and forth. It should spring out against spring tension, then snap back on its own. If there is slop in the weight springs, tighten up the ends and try again. Lubricate the point cam where it turns on the shaft after you clean both parts to prevent sticking. If the spark advancer is hanging up, the engine speeds will skyrocket.

Set the point gaps at about .014" first, then move the point plate around until the points open a the F mark (right cylinder). If the left side is a little off, then you may have to adjust the point gap slightly to get it to match up to the other side (LF).

Hopefully, you have cleaned and rebuilt the petcock already. Also check the gas cap for damage to the cap seal. If the cap isn't venting properly, the carbs run low on fuel, which leans out the engine at idle and that raises the idle speeds briefly before it runs out of gas altogether.

Bill Silver  

Motorcycle Repair

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Bill Silver

Expertise

Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.

Experience

I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

Organizations
VJMC (Vintage Japanese Motorcycle Club) of North America

Publications
VJMC newsletter, as editor for two years and as contributing editor currently.

Education/Credentials
3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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