Engine warmed up and idling normally. Suddenly races to approximately 2000 rpm. I’m thinking maybe dirty pilot jet so I clean the carb .
It does it again.
My question is- are there any electrical types issues that would simulate a fuel/carburetor problem?
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Vintage Honda Owners, Restorers, Riders and Admirers
1969 CA77 Fuel or Electrical
1969 CA77 Fuel or Electrical
S90 / CB550 / CA 77 Dream 305 / Yamaha RT2 360
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Michael Stoic
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Re: 1969 CA77 Fuel or Electrical
Could be related to your throttle cable/s. I'd try to adjust for more cable slack and repeat the test. It's been a while, but I recall trying to diagnose a similar issue on my CP77, where the throttle cables were the cause of unexplained sporadic high idle.
Re: 1969 CA77 Fuel or Electrical
If the springs which control the centrifugal advance unit are broken or very weak that could cause the timing to go full advance even when the engine is idling, causing the jump in rpm. Perhaps others with more experience with 305s could opine on this.
My first guess would be the carb, though. There must be a small amount of free play in the throttle cable. And when you open the throttle and release it you should hear the slide drop down and close. Cable lube used on bicycles, or even diesel fuel, is good for lubing cables. An air leak at the intake manifold can also cause a rise in rpm. A plugged pilot jet cannot.Confirm also that the slide moves up/down with complete freedom. If not, use wet-dry paper on the carb, not on the slide.
Did you check that the carb mounting surface is flat? Put some wet/dry paper on a piece of glass, a surface plate, or other flat surface. Add a little oil or WD to the paper, and use that to surface the flange.You can tell immediately whether it's flat or not. When installing the carb put a small amount of grease on the O-ring and don't over tighten the carb! That's what warps them.
However.....Many years ago (in Japan) I had a CB350 Honda that I recycled that did something similar. It would start to rev uncontrollably when the engine warmed up. It drove me to distraction and I never did figure out what the problem was. It had CV carbs on it and I checked everything I could think of to no avail. It ran very well but would rev like crazy when stopped at traffic lights.
My first guess would be the carb, though. There must be a small amount of free play in the throttle cable. And when you open the throttle and release it you should hear the slide drop down and close. Cable lube used on bicycles, or even diesel fuel, is good for lubing cables. An air leak at the intake manifold can also cause a rise in rpm. A plugged pilot jet cannot.Confirm also that the slide moves up/down with complete freedom. If not, use wet-dry paper on the carb, not on the slide.
Did you check that the carb mounting surface is flat? Put some wet/dry paper on a piece of glass, a surface plate, or other flat surface. Add a little oil or WD to the paper, and use that to surface the flange.You can tell immediately whether it's flat or not. When installing the carb put a small amount of grease on the O-ring and don't over tighten the carb! That's what warps them.
However.....Many years ago (in Japan) I had a CB350 Honda that I recycled that did something similar. It would start to rev uncontrollably when the engine warmed up. It drove me to distraction and I never did figure out what the problem was. It had CV carbs on it and I checked everything I could think of to no avail. It ran very well but would rev like crazy when stopped at traffic lights.
Do what you've always done and you'll get what you've always had.