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  1. #1
    CB750 New Member mrbigelow's Avatar
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    Aug 2020
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    Question New bike -- high a** idle issues

    Hello all! First post here, thanks for having me!

    Just picked up a 1981 CB750F yesterday. The bike is mint as heck. Completely re-built top to bottom. Motor is completely rebuilt by a Honda tech, as well as the transmission. Only about 400 (ish) miles on the new build. It's got a full aftermarket exhaust (Delkevic 4-1) and has been jetted. Here are a few quick photos for funs:

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    Anywho...

    I've having 2 issues.

    ISSUE 1: The bike starts right up great, and idles fine. But when I start riding, when I disengage the clutch (coming to a stop, or shifting), the RPMs shoot way up and hold steady, quite high. There's no tach, so I'm not sure exactly where they're at, but it's way too high. But as long as I keep the clutch in, the RPMs stay up there. Then I start engaging the clutch, and they start dropping, as you'd expect. I come to a complete stop, in neutral, and still super high. I have to shut off the bike, let it sit for a while, then when I come back to it, it seems to be ok. I've adjusted the main idle screw (adjusting all 4 at once) to make sure the start-up idle is kosher, but then this issue only happens while driving. The previous owner states that he sat there and witnessed the honda tech sync the carbs during the rebuild, so they should be in sync. He also said that the tech did the timing, so that should be ok, too.

    ISSUE 2: The power band flattens out about halfway through the gears. Getting to maybe 5 or 6k, it levels out and doesn't continue to produce increasing power. It doesn't bog down at all or LOSE power, but it doesn't really GAIN power.

    We're wondering if this could be an issue with improper jetting? He put a DYNO JET set in, and here's what's in there from the kit:

    MAIN JETS: DJ130
    PRIMARY JETS: DJ074
    FUEL NEEDLES: DNO102

    One thing the previous owner suggested was to check the bottom adjustment screws on the carbs. I'm not familiar with exactly what this screw does, but he said from forum posts he's read, they should be about 2-2.5 turns out. Haven't checked those yet.

    Any ideas?????

    Thanks so much, all!!

    Cheers,

    Matt
    Last edited by mrbigelow; 08-09-2020 at 09:03 PM.

  2. #2
    CB750 Guru amc49's Avatar
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    Compression check to make sure you are not wasting your time, no rev up high means valves may be burned and how most of them die. Nobody ever sets the valves and they close up to burn. They come off the floor too tight to begin with.

    Hanging idle is typically bad intake rubbers in front of carbs. Sync can't touch that.

    The engine is NOT supposed to idle when stone cold and you make the hanging worse by trying to get it to do so. The main idle speed adjuster gets adjusted for engine hot then LEAVE IT ALONE. Set screws under carbs to 2.5 turns.

    Carbs are now messed up. The secondary main is too big and the needles don't work and why those kits are garbage. Don't care what anyone told you. If you got pods on it even worse, one reason the engine quits revving high but not all. Those should double hp. between 5500 and 8000 rpm and rev cleanly when right to easily over the 9500 redline.

    Dunno what you are calling 'mint', half the bike is gone.

  3. #3
    CB750 New Member mrbigelow's Avatar
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    Thanks for the feedback, amc!

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