CB450 startup after rebuild
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Thread: CB450 startup after rebuild

  1. #1
    Senior Member amannl's Avatar
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    CB450 startup after rebuild

    This has been a long rebuild; several years since a crank main disintegrated.


    Today I finally got the battery charged and gas in the tank and it started right up. I needed to adjust the throttle stops on the carbs more than I felt I should have but it holds a somewhat steady idle. But if I blip the throttle even a tiny bit it races to around 3000 revs and stays there.
    I sprayed wd40 and it’s not an air leak; I’ll remove the points and check the aau but i checked it when I installed it (months ago) and it was free on the spindle and fit nicely. The bike starts right up so it’s (probably) not initial timing.
    Any suggestions on what else to look at?
    Thanks,
    Adam

    https://www.dropbox.com/sh/guhhm8gh3...FUaPbBija?dl=0
    Last edited by amannl; 07-13-2019 at 03:05 PM. Reason: Link added

  2. #2
    Supporting Member 12ozPBR's Avatar
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    Did you spray the carb boots AND the carb throttle shafts? The felt seals on those shafts dry out and can cause a leak. That’s what cured my 450 with similar symptoms.
    Runners
    1971 Triumph Tiger
    1972 CL350
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    Current Project
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  3. #3
    Senior Member ancientdad's Avatar
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    ^^^and let me add to that, I certainly hope the engine had already been warmed up a bit at low revs before that video was taken. If that was the initial startup of a cold engine that had just been rebuilt and had not yet been run, you were about a minute and a half away from having any more oil on the cam lobes and followers than you put on them as pre-lube... hopefully no parts got any excessive wear in the process. The advance being free on the spindle and the camshaft is only part of it, the springs need to have no slack in them (stretched) so the weights return fully or the engine will try to stay running where the advance in the timing makes it run better (so to speak)
    Last edited by ancientdad; 07-13-2019 at 06:10 PM.
    J-T and 12ozPBR like this.
    Tom

    Ride along at the drag strip - https://www.youtube.com/watch?v=20jFPazXlvU



    running points... because I'm too old for mysteries that begin with pushing

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  5. #4
    Senior Member amannl's Avatar
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    I did ultrasonically clean the carbs so I’ll spray the spray the felts when I get a chance later today.
    and Tom, I did let the bike idle for several minutes around 1,000-1,200 before I blipped the throttle and I shut off the ignition immediately. I also applied assembly lube to the cam and followers quite liberally.
    i’ll take the advance off and check it and the springs if oiling the felts doesn’t work.
    Thanks again,
    Adam

  6. #5
    Senior Member Twowheelrich's Avatar
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    I'd also be looking at the throttle cables for routing/ kinks/proper tension or whether they drag in their housings, as well at the throttle tube. Then, see if the carb slides are working as they should.
    1973 CB450
    1968 CL 175
    1991 KLR 250
    2008 Lance Vintage 150!

  7. #6
    Senior Member amannl's Avatar
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    I had a chance to look at things this afternoon.
    The throttle cable is fine and snaps back closed.
    I removed the carbs and drizzled heavyweight oil into the the throttle shaft felts.
    I also swapped out an auto advance unit with a spare.

    Nothing has changed; the bike starts up right away with the electric starter, idles ok (I haven’t gotten to the point of fine tuning it) and takes a little throttle to about 2000-2500, but even a bit more than that has the engine race to 3,000 and only 3,000 and stay there, at which point I switch off the bike.

    I did spray wd-40 around the carb boots with no change at all. Would it make any difference if I use carb cleaner instead?

    Does anyone have any other ideas?
    Thanks,
    Adam

  8. #7
    Supporting Member 12ozPBR's Avatar
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    A stab in the dark here but how are your valve clearances set? I was just reading in a thread yesterday that too tight of clearances can cause a high idling scenario. With spec for clearances at .0012” you can see how easily it might be to have them too tight. Common wisdom on here has most people setting them at .002” for a DOHC450
    Runners
    1971 Triumph Tiger
    1972 CL350
    1974 XL350
    1974 CB450

    Current Project
    1966 CB450 Black Bomber

  9. #8
    Senior Member amannl's Avatar
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    I’ll double check the clearances, thanks for the thought.

    i was wondering if a poor condition gasket on the carb cap might be the cause of my issues, or would it have the opposite effect?

  10. #9
    Senior Member amannl's Avatar
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    I think the problem is solved, though I haven’t had to time to take in on the road.
    I replaced the o-rings on the air screws and the gaskets on the carb caps, and also decided to remove the carbs and double check my float height. I have an old aluminum Honda float gauge which makes it very easy to check.
    While the carbs were off I noticed that the butterfly on the right carb was not seating perfectly covering the holes in the floor so I loosened the screws and adjusted it so it fit better in the bore.
    The bike started right up and idled well and I could blip the throttle and the rpm’s would settle right down!
    Just off idle it will die if I don’t give it a bunch of gas.
    I assume this is a lean condition and I should turn in the air screw a bit?
    Thanks again to all who offered help

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