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  1. #1
    CB750 Member ryguy85's Avatar
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    CB750 Warm Idle Problem

    I have a 1977 CB750 that I just finished converting into a cafe bike. I took it to a shop to have the carb work done. For the first few weeks I had zero issues. Now when the bike is fully warmed up, the idle is around 3000 rpm. When it's cold, choke on, the idle is around 1400. It's only when the bike is fully warmed up does the idle stay that high. When warm, the choke being on or off does not change the idle rpm.

    Thoughts on what could be doing this? I was told elsewhere it could be a vacuum leak in the hoses between the carbs and the block.

    I would take it back to the shop that did the work, but there are legal issues I have with them right now.

  2. #2
    Moderator dirtdigger's Avatar
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    Lot of things to check. Check for the vacuum leak at the boots with some spray carb cleaner. Recheck the carb sync. Something sounds wrong with the choke setup if it doesn't have any effect...it should kill the motor or just about kill it if you put it on all the way. Also you need to check the ignition advance to see if it is working correct...if it is stuck full advance or very sticky it can cause idle rpm issue. Also...on these old bikes you do have to play with the idle speed screw as they warm up, they are not a set it and for get it deal, as it warms up you will have to turn it to back down the idle speed and turn it up when cold. These carbs have a fast idle cam that bumps the idle up with the choke on so you need to see if it is working or if it is stuck or broke.

  3. #3
    CB750 Member ryguy85's Avatar
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    If I spray carb cleaner around the boots, what would be the tell tale signs that there is a vacuum leak? Will I be able to see the mist being pulled into the boot? Like I said it was working fine for weeks then it started giving me issues. There was little to no change in temperature outside in those weeks

  4. #4
    Moderator dirtdigger's Avatar
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    If you have a leak the engine speed will change with the carb clean.

  5. #5
    CB750 Guru amc49's Avatar
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    X2 to all that. I personally never chase idle speed, I set it for hot engine then hardly ever touch it, useless to chase a cold engine with it as the requirement changes in a few seconds. I just keep engine running by hand until hot enough it stays running on its own.

    Make sure you have a wee bit of slack in the throttle cables or they can pull slides up a bit to do that. Choke not working if you can't kill a hot engine with it.

    80% of engines that do the high idle when hot thing are airleak somewhere, sync not right (one carb open a bit) or leaks at boots, manifolds.

    Being it ran fine for a while could be something silly stupid like a jet coming unscrewed to fall in bottom of carb...........

  6. #6
    CB750 Member ryguy85's Avatar
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    Finally got warm enough for me to start the bike and ride around yesterday. Sprayed some starter fluid around the boots and sure enough the idle would keep dipping when I sprayed 2nd one. Looks like I'll be ordering some new boots. Thanks for the help

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