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Thread: flat spot

  1. #1
    hansleopold's Avatar
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    flat spot

    Did a test ride on my CB 450 K1(converted to CL now) yesterday, and found out that there is a flat spot around 1500-3000rpm. The engine hesitates, and you don`t get the desireable torque when riding away.
    I`ve been messing around a lot trying to solve this problem, but nothing helps. The ignition is perfect, the mains are #130, pilot #38 and the float is as stated. Have read in the Clymer Manual that flat spots are due to fuel starvation at a specific rpm. Does it help to change to larger mains? Blocked air jets?

    Anyone who has a clue on this subject..............?????



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    Hans

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  3. #2
    Sensei 66Sprint's Avatar
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    Re: flat spot

    Quote Originally Posted by hansleopold
    Did a test ride on my CB 450 K1(converted to CL now) yesterday, and found out that there is a flat spot around 1500-3000rpm. The engine hesitates, and you don`t get the desireable torque when riding away.
    I`ve been messing around a lot trying to solve this problem, but nothing helps. The ignition is perfect (... Read with a strobe at both 1100 idle and at full advance it shows EXACT index alignments?)..., the mains are #130, pilot #38 and the float is as stated. Have read in the Clymer Manual that flat spots are due to fuel starvation at a specific rpm. Does it help to change to larger mains (... CL pipes are somewhat more restrictive than CB pipes, jetting changes are common....) ? Blocked air jets? (...IF you actually properly cleaned the carbs, this couldn't be an appropriate question..)

    Anyone who has a clue on this subject...... Are the carbs correctly synchronised?........?????



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    Hans

  4. #3
    hansleopold's Avatar
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    Re: flat spot

    Well, I have checked the ignition, and it is perfectly set. Nothing wrong there.

    Yesterday I changed the mains from the stipulated #130 to #140 instead. ----and there was a significant change. To the better with quicker response and reving from the lows. But not perfect....

    I try to be a perfectionist, and has a goal to restore my Honda not only to the classic looks it once had, but also to the running condition of the motor it had when it left the factory 42 years ago.....

    I have to try to find the optimum setting of the pilot screw and the throttle valve. The throttle valve seems to be difficult to set. Even a tiny twist on the adjusting screw makes a whole difference.

    I`m still puzzled about the air jets. If they are partly glogged or not. I`ve made a carb cleaning in diesel, gas and ultra sound several times, but the small air jet passages are impossible to check.




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    Hans

  5. #4
    Super Moderator Znabb's Avatar
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    Re: flat spot

    Hans, did you check the ignition with a strobe light at both idle and at 3-4000 rpm as Steve suggests above? If the advancer unit (tändförsällningen) is not working correctly it would show in the rpm range you have hesitation in. When you rev the engine to 3-4000 rpm and use the strobe light the ignition indication should "move" to in between the two lines to the left of LF on the rotor.

    Also, what kind of air filters are you using?

  6. #5
    Sensei 66Sprint's Avatar
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    Re: flat spot

    Quote Originally Posted by hansleopold
    I`m still puzzled about the air jets. If they are partly glogged or not. I`ve made a carb cleaning in diesel, gas and ultra sound several times, but the small air jet passages are impossible to check. May I suggest a spray-can of CARB CLEANER followed by compressed air to clean and check the smaller passages?.....




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    Hans

  7. #6
    Super Moderator nigelrharris03's Avatar
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    Re: flat spot

    Have you tried using stranded copper electrical lighting wire? Seperate out the strands snd you can gently push them through small orifices.
    Nige
    CB450K0 Bomber 1967
    CB650Z 1978

  8. #7
    hansleopold's Avatar
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    Re: flat spot

    Thanks for replies and suggestions. Yes I have a strobe light tester and the advancer works okay. I`ll try the spray can-, and the electric wire job.
    One thing that bothers me is, if one or both carbs are defect. I mean I had huge problems with another Honda (750). The engine never run properly. I changed everything you could think of. I had the carbs cleaned at least ten times. And synchronized. Still not perfect.

    At last I swaped the old carbs with another set that had been lying on the shelf. And that made a huge difference!! The bike suddenly runs like new!! Yes!!!!

    I´ll be back.......

    _________
    Hans


    Ronny, I have the original filters. Are they okay to use, or do you have any suggestion of a better substitute?

  9. #8
    Super Moderator Znabb's Avatar
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    Re: flat spot

    Quote Originally Posted by hansleopold
    Ronny, I have the original filters. Are they okay to use, or do you have any suggestion of a better substitute?
    Hans, my used original filters was too dirty so they restricted air flow making the engine hesitate on higher revs. I'm now running pod filters until I get a chance to dress up the original filters with new paper as new filters obviously are made of unobtanium. I know many of the guys here on the forum are running UNI sock filters with excellent result.

  10. #9
    hansleopold's Avatar
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    Re: flat spot

    The left cylinder did not run at all below 2500-3000rpm, it was not just hesitation. It didn`t get enough of fuel at the low rpm range. The spark was okay, fat and blue.
    I`ve cleaned the carb several times, but nothing cured the problem.

    At last I got my hands on an old carb that had been lying in a garage for years. It was the right one, the 14H, but it was in misareble condition, plenty of dirt inside and out. I rebuilt it after a long period of cleaning in different solutions.

    It was a thrilling moment when I pressed the start button. Would it work, or not?

    And it did! The motor runs very good now from the low rpm! Just a few moments of fine tuning and the response is great.

    It is hard to tell exactly where the problem was, but my humble guess is that the tiny fuel line from the pilot/slow jet was restricted, and the fuel couldn`t reach to the outlet point.

    I`m looking forward to ride my bike now to feel the mighty torque when twisting the throttle grip...


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  11. #10
    Super Moderator nigelrharris03's Avatar
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    Re: flat spot

    Sounds liker a happy man to me. Glad it seems to be sorted.
    Nige
    CB450K0 Bomber 1967
    CB650Z 1978

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