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  1. #1
    CB750 Member Logolibre's Avatar
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    Question CB750 carbs problem. High idle (unstable rpm). Need help

    Hi all!

    I have a Honda CB750 F2 Boldor (1982)
    Factory carburetors, exhaust 4-2, velocity stack and jet kit. And there are problems (yes, I know how capricious these CV carburetors, especially in the absence of a factory air filter).

    The first time after engine assembly, carburetor synchronization, etc. the motorcycle worked perfectly. But later I changed the piston rings, and after that something happened.

    The main problem is with rpm. At idle very high rpm - about 2-2.5. Warming up the engine does not give anything, they either remain the same or still increasing. But at the same time, the motorcycle rides more or less normally.
    If you try to reduce the idle speed with the idle screw, to 1.2-1.3 rpm - the motorcycle begins to stall when driving at low speeds.
    I took apart the carburetors, cleaned, synchronized, checked the air leakage, new spark, the fuel screws were unscrewed by about 2 2/8 approximately.

    Can someone tell me which way to go in order to fix this problem?

  2. #2
    CB750 Guru amc49's Avatar
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    Likely an air (vacuum) leak when the carbs were removed and installed after the ring job. Cracked intake manifolds to the head, they do it all the time. If you have the added vacuum demand valve that came on US models in '82, a vacuum leak at the #2 carb vacuum supply line can do it too. You cannot use the idle speed screws to control that, the extra air is going around them and then the too small air past butterflies chokes engine to die at letting off throttle.

  3. #3
    CB750 Member Logolibre's Avatar
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    Yes, I have a model for the US, with a vacuum valve. Since I removed Spawn of Satan, I shut off this valve. Today Iíll shoot carburetors, Iíll look at the block, but there are no visible cracks

  4. #4
    CB750 Guru amc49's Avatar
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    You won't see the cracks, even more so looking at cylinder block, they are in the rubber manifolds higher up and do not show but they are there all right. They only show when expanded like putting carbs in and maybe not even then.

  5. #5
    CB750 Member Logolibre's Avatar
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    He took off again, including rubber manifolds. Indeed, there are cracks along the edges, but very small. Could this be the reason? Why then didnít react to the ďquick startĒ spray in any way? I ordered a new kit, but delivery will take about a month. Can i try using high temperature sealant?

  6. #6
    CB750 Guru amc49's Avatar
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    New manifolds is the fix there, anything else is a waste of time. If you haven't dealt with the aircuts then they can easily leak too and do the same thing.

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