Too high RPM?
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  1. #1
    Senior Member Chippe's Avatar
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    Too high RPM?

    I have the original speedometer och tachometer on my CM400T. It idles good at around 1200 rpm but I suspect the rpm goes up too high when I ride. Does it?

    1st gear, 30 kmh (18.6 mph) around 5500 rpm
    2nd gear, 30 kmh (18.6 mph) around 3500 rpm
    3rd gear, 60 kmh (37 mph) around 5500 rpm
    Honda CM400 1979

  2. #2
    Senior Member Alan F.'s Avatar
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    Those engine speeds sound right for those road speeds, these are high reving engines, that's where the horsepower is.
    Chippe likes this.
    CB250 Nighthawk projects 92,93,92.
    CM400C Sold 7-21-18 It's been fun, buh-bye.
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  3. #3
    Sensei 66Sprint's Avatar
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    Run first up to 25MPH, second to 38, third to about 56, and fourth to 67... fifth gear is for 70 and above.....
    IF you aren't getting to at least 7500RPM in the upper 3 gears, you are wasting the engine's capabilities......
    Your engine is happiest between 7200 and 8500 RPM........
    Chippe likes this.
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  5. #4
    Senior Member Chippe's Avatar
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    Ok, I understand I need to ride at high rpm but I found this info which made me wonder.

    LDR's bike with 17/35, tach is reading lower than original but I think the difference is big compared to my bike.
    30kph shows @3600 rpm in first (Chippes bike - about 5500 rpm)
    60kph shows @4000 in 3rd gear (Chippes bike - about 5500 rpm)


    Another guy with none original 1:4 tachometer
    in 3rd gear going 60kph I was getting around 4500rpm
    in 1st gear going 30kph I was getting around 4000rpm

    That guy claims the recommended (on many sites) 1:7 ratio is too high. A swede I'm talking to on the internet says the same.

    https://www.hondatwins.net/forums/65...io-cm400t.html
    Last edited by Chippe; 04-15-2019 at 12:08 PM.
    Honda CM400 1979

  6. #5
    Super Moderator longdistancerider's Avatar
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    The way to get the correct engine to tach ratio will be to remove the left engine cover and disconnect the tach cable at the gauge end. Affix a pointer on the end of the cable and have someone hole it to count revolutions. Now manually turn the crankshaft a number of times, like 10. Might be easier with the plugs removed.
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  7. #6
    Senior Member Chippe's Avatar
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    Let's see if I understand you. I turn the flywheel 360 degress about 10 times and count the revolutions on the end of the cable?
    Can I adjust the original tachometer which I have or is it just to find out the correct ratio?


    I wonder if it's possible to have too high rpm at a certain speed. I think I've read about it on the forum but I can't find it.



    I would appreciate if somebody could give me rpm at a certain speed and gear. It doesn't matter if it's in MPH or KMH.
    Last edited by Chippe; 04-15-2019 at 02:44 PM.
    Honda CM400 1979

  8. #7
    Sensei 66Sprint's Avatar
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    Divide or reduce....Result will be the actual ratio.....10 crank spins equaling 70 cable spins would be the 1:7 ratio your meter should have
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
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  9. #8
    Senior Member Chippe's Avatar
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    Thanks I understood I should devide it. That's great when you have to buy a new tachometer which I'm going to when everything is ok. I still think my rpm is too high based on the 4th comment. I found out that it could be a slippery clutch.
    Last edited by Chippe; 04-17-2019 at 12:53 AM.
    Honda CM400 1979

  10. #9
    Senior Member Chippe's Avatar
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    Sprockets are not the same on these bikes if this info below is correct and I have 36 teeth on the rear sprocket instead of 35 but it doesn't do much difference.

    Too high RPM?-sprockets.jpg


    My RPM is a bit high based on the info on this awesome site: https://www.gearingcommander.com/
    Below you can see stock sprockets, my bike 16/36 and LDR's bike with 17/37. According to the info we're both off but somebody might have put in the wrong data.

    Based on a CM400T 79-81

    Too high RPM?-gc.jpg
    Last edited by Chippe; 04-24-2019 at 08:40 AM.
    Honda CM400 1979

  11. #10
    Senior Member Chippe's Avatar
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    Quote Originally Posted by longdistancerider View Post
    The way to get the correct engine to tach ratio will be to remove the left engine cover and disconnect the tach cable at the gauge end. Affix a pointer on the end of the cable and have someone hole it to count revolutions. Now manually turn the crankshaft a number of times, like 10. Might be easier with the plugs removed.
    I did what you said and made a video so now we all know. I think it was very accurate so only did 4 full rotations on the flywheel and Honda use as far as I know 1:4 or 1:7.
    I wonder why all the sites on internet say 1:7, maybe it's not the same on all models?
    1:7 was too much on a CM400T 1980 and on a CB400N 1982. That's all I know.

    Last edited by Chippe; 04-24-2019 at 01:41 PM.
    Honda CM400 1979

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