79' CM400t RPM Jump after warming.
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  1. #1
    Junior Member greven909's Avatar
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    79' CM400t RPM Jump after warming.

    Hey all, my 79' cm400t has a new issue this spring. It starts up and idles nicely(1200rpm), after 4-5 minutes it will begin to make some growl/rumble sounds like its going to backfire, then rpm jump to 4k and sit there.

    I've checked for vacuum leaks and haven't found anything. I will be doing a leakdown test this afternoon and see if I learn anything. I am running pods with 95 mains and 125 secondary. Any thoughts?

  2. #2
    Member ckemps's Avatar
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    im having the same problem only for my right cylinder. although, once it starts, the right side won't rev immediately and then starts to backfire.
    Did you figure out what your problem was?

  3. #3
    Senior Member Chippe's Avatar
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    I have the same engine and carbs (VB21/31) With UNI pods I couldn't get stable RPM without stacks. https://www.hondatwins.net/forums/63...tml#post189348
    I made mine out of a pvc pipe.

    How many turns out on the idle mixture screw are you?
    Last edited by Chippe; 07-27-2019 at 04:14 AM.
    Honda CM400 1979

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