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  1. #1
    CB750 Member nighthawking's Avatar
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    Sep 2013
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    High idle run on backfiring.

    I have a good set of the 1979 DOHC Limited carbs that have been cleaned, not ultrasonic, in good shape. Over jetted for 4 to 2 exhaust, everything else stock. Sea level. Jets, bowl gaskets, plugs, mixture screw O-rings, float needles replaced. Have not split them for the air cut off diaphragm replacement. Seem to have sort of a high idle when coming off throttle. Comes back down to proper RPM sitting still. Does anybody know what gives, and how to possibly fix? Thanks.

  2. #2
    CB750 Member pjmule's Avatar
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    Dec 2012
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    Had the same problem on my 80. Ended up splitting the carbs and blocking off the small o-ring in the air cut off valves. Then really took my time and bench synced them, and it runs perfect now. You might try syncing them if you don't want to split them , but it isn't that bad of a job blocking them off and it's free.

  3. #3
    CB750 Member nighthawking's Avatar
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    Figures. I just don't like dealing with butterfly screws. Tx.

  4. #4
    CB750 Member bodeane's Avatar
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    Hi, I am new here I came across this post because I am having the exact problem...
    I recently just cleaned/Synced the carbs and the bike stated off by lunging around 5K then started working great after about 15 minutes of riding.
    THEN. started doing the backfire thing on acceleration and loosing power all the way to the point of not even idling.
    I have a few suspect causes maybe if I list what I am thinking someone can point me in the right direction?

    1. Air cut off valves. I didnt know about these and then after soaking my carb body in carb cleaner I finally found the port on #1 I took out the rubber and it was not bad but looked like it might have got a bit warped by cleaner. I also did not realize there are 4 of them and the other ones I never bothered to open.
    Possibly something to do with these being damaged?

    2. Airbox bottom hose broke off.
    Tthere is a drain hose on the bottom of the airbox that broke off in the installation and that hole is just exposed now not blocked off. possible cause?

    The weird thing is when the bike is working it works amazing then will just totally crap out.

    Bike stats:
    1981 CB900f
    kerker 4-1 exhaust with baffle
    main jet 105, secondary 68
    OEM airbox

    any suggestions welcome.

  5. #5
    CB750 Member bodeane's Avatar
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    Here is a video of the bike dying out when throttle applied.
    http://youtu.be/Tkom4_Q-Ktw

  6. #6
    Moderator dirtdigger's Avatar
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    Sounds like the thing has a dead hole, have you done the basics....check compression, check valve clearances, check timing, check for spark at all 4 plugs. If you got carb clean on the air cut diaphrams and they are misshaped then they are junk. These style of carbs were junk from the factory and didn't run perfect, you will be fighting those carbs till you put new aftermarket carbs on it or get rid of the bike. Its harsh but its the truth. These are not good carbs for beginners to clean or tune, many shops wont even touch them anymore.

  7. #7
    CB750 Member bodeane's Avatar
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    Thanks dirtdigger, What would you recommend getting for this bike then?
    What is the issue with these carbs?
    would the Air cut off valve being misshapen cause these issues I described to happen?

  8. #8
    CB750 Member bodeane's Avatar
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    If I wanted to find a set of used carbs that would fit this bike what would I look for?

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