'83 CM250Custom Carb Issue



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Thread: '83 CM250Custom Carb Issue

  1. #1
    Lyndsay's Avatar
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    '83 CM250Custom Carb Issue

    I have issues with the bike not idling properly. I can adjust the stop, but it tends to jump from no idle to revving about 2 grand (just a guess, no tach) with a minor tweak of the screw. I was told it was probably something to do with the pilot jet, but cannot locate any information on where to access this part to remove/clean it. Does anyone have any information on how to get at this without dismantling the entire carb? I don't feel like doing that as I have other things to spend my time on like rebuilding the family cottage after the fire.

  2. #2

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    Re: '83 CM250Custom Carb Issue

    Sorry to hear about the fire.

    Are you sure you aren't dealing with a vacuum leak? Try wiggling the carb to see if the idle jumps around.

    Obviously disassembly is the most effective solution, but I have run through seafoam or FI cleaner to try to buy me some time before a teardown. Alternatively, turn jet screw in and count the number of turns, then turn all the way out and blast carb cleaner through the passage, then thread screw all the way in and back out the number of turns you counted.

    What screw are you adjusting? Fast idle? Idle Mixture? Mixture?
    Last edited by GreyGhost; 06-12-2018 at 02:19 PM.

  3. #3
    Lyndsay's Avatar
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    Re: '83 CM250Custom Carb Issue

    I'll try wiggling it around. I have put Seafoam through it, with no help. It started last trip of the year last fall and came back this spring. I'm not sure which screw it is, I just know it as the idle limit screw.

  4. #4

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    Re: '83 CM250Custom Carb Issue

    If I recall, that bike has a single Keihin VM style carb. It's not a big job to overhaul the carb, spend 2 hours and $25 bucks -- it will last for 10 years if you run premium fuel and winterize properly.
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  5. #5

    Re: '83 CM250Custom Carb Issue

    Good news is, you only have one carb to service. The low speed jet screw adjustment is probably sealed with a little tamperproof cap, is part of their plan to prevent you from altering the emissions.

    ... oh wait does the CM have 2 carbs?
    Last edited by Trials; 06-12-2018 at 07:06 PM.

  6. #6

    Re: '83 CM250Custom Carb Issue

    Quote Originally Posted by Lyndsay View Post
    I have issues with the bike not idling properly. I can adjust the stop, but it tends to jump from no idle to revving about 2 grand (just a guess, no tach) with a minor tweak of the screw. I was told it was probably something to do with the pilot jet, but cannot locate any information on where to access this part to remove/clean it. Does anyone have any information on how to get at this without dismantling the entire carb? I don't feel like doing that as I have other things to spend my time on like rebuilding the family cottage after the fire.
    There isn't going to be an easy way around this. Seafoam, Karb Kleen, etc. won't usually save you. You're going to have to take out the carb, remove the bowl, remove all the jets and properly clean them. It is your pilot valve but you probably have a lot of gum in it so a proper cleaning will pay off. You have a Keihin 17A or 18A carburetor. I have the same bike, albeit modified. The carb is a piece of cake to work with. Very simple. Unfortunately you have to disassemble half the bike to get the carb off. Remove the seat, side covers, battery and battery case, air cleaner and casing, and then remove the carb. If it's your first time, count on about 3 hours, but the bike will run like new when you're done.

  7. #7
    Lyndsay's Avatar
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    Re: '83 CM250Custom Carb Issue

    Thanks all. Sorry I haven't replied lately; I've been busy. Thanks for all the info. I expected I would need to pull it. Good to know I need to pull all the other stuff first, otherwise I would have thought I was guessing wrong at how to get at it. Yes, the CM has a single carb; my CB has dual and I had them apart a couple of years ago. Fun getting them synched up. I'll keep you posted once I find time to do it. At present I'm just using it with the idle set high. Doesn't affect anything except for being loud when sitting at a stop light.

  8. #8
    boyoboy's Avatar
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    Re: '83 CM250Custom Carb Issue

    Quote Originally Posted by Roadghost View Post
    There isn't going to be an easy way around this. Seafoam, Karb Kleen, etc. won't usually save you. You're going to have to take out the carb, remove the bowl, remove all the jets and properly clean them. It is your pilot JET (probably) but you probably have a lot of gum in it so a proper cleaning will pay off. You have a Keihin 17A or 18A carburetor. I have the same bike, albeit modified. The carb is a piece of cake to work with. Very simple. Unfortunately you have to disassemble half the bike to get the carb off. Remove the seat, side covers, battery and battery case, air cleaner and casing, and then remove the carb. If it's your first time, count on about 3 hours, but the bike will run like new when you're done.
    and while you have the carb off and apart - replace the float needle and seat as well. Even taking the carb off and bringing it to a reputable shop for service can save you some dollars if your not confident taking the carb apart, cleaning it, checking/adjusting float level, screw settings etc.
    Last edited by boyoboy; 06-19-2018 at 11:58 AM.
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