CM400T cant rev past 3000 rpm
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  1. #1
    Junior Member Goodspeed's Avatar
    Join Date
    Aug 2019

    Talking CM400T cant rev past 3000 rpm

    Hello guys, first post here!
    I started working on my CM400T a couple of months ago, and took the entire bike apart, cleaned the carbs and so on.
    I am by no means a motorcycle expert, and even though i am car mechanic, i have had no education on carburetors prior to this. So im using this project as a learning experience.

    After assembling the bike, i tried to start it up, it idles just fine, but i cannot seem to rev past 3000 rpm, it just bogs down to 2500 and repeats this cycle.
    I have taken a look at the accelerator pump diaphragms and it all looks fine to me (could be wrong) but without any prior experience it is hard for me to tell what could be the cause of this behavior.
    I have changed the jets aswell, but the problem was there before, so im guessing i can rule that out.

    I am not sure how the carburetor works, but i assume the different jets start overlapping around the 3000 rpm area? but if u have any ideas please let me know

    I have a video showing the problem, maybe that could ring a bell with some of you.

  2. #2
    Member mikemo's Avatar
    Join Date
    Aug 2019
    Tampa Bay, Florida
    Can you check to see if your ignition timing advances as you rev past idle?
    Is the air filter clean and free of mice and other squatters?

    That doesn't sound like an accelerator pump problem.
    Good luck!

  3. #3
    Super Moderator longdistancerider's Avatar
    Join Date
    Oct 2011
    Vancouver Washington (text 408-239-9580)
    Start here to verify that the ignition system is good. Just need a VOM, volt ohm meter
    Jim O'Brien
    1979 CM400T "road bike" modified for travelling
    1978 CB400T1 restored
    1972 CL350 nuts and bolts restore in progress and
    Road Trip or "where's Jim now?"
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  5. #4
    Senior Member drydreamer's Avatar
    Join Date
    Aug 2015
    If you are a car mech then you will know of wasted spark, cdi etc.etc.etc., just apply what you already know.

    you know of cmsnl which will be of help to you.

    The engine does not sound sweet at all, but that may be due to poor audio and location, here's hoping for you.

    Did you change the engine oil and examine it, much "glitter" and black lumps?

    Did you adjust the valve working clearance, cam chain tension and balance chain tension?

    How many miles has the engine done and how long was it in storage before you got it.

    you should be aware that valves can stick ( open usually ) when engine are static for some time, yes this happens to bikes as well.

    You have a strobe, so can check for ign advance, but more importantly, loss of ht, in your case. You know of the red neon plug caps which are handy, but would need to make an adaptor for each plug.

    as you are aware, be methodical, first principles and outer onion layers first.

    I assume you are running the engine on fresh fuel, how is fuel flow from tank and tank internals etc..

    "dont know how a carb works" then you must be young. find out how an aircraft flies........

    these ign systems have some hair pulling faults which sometimes takes us down the road of a fuel problem, but actually turn out to be loss of ht.........rule that out first after a basic tune up and carbs clean. In your case the "surging" does seem to be happening at a lower revs than "usual" for the particular loss of ht fault in mind. This usually happens at about 4 to 5 k revs.......

    get a manual and wiring diagram......

    try disconnecting the cdi unit black/white wire from the main loom and see what happens, earth this wire to kill ign, engine will run without the aid of a battery as its a self contained cdi system, but keep it wired in for the moment ( and always really ) and be aware that if the engine is running the charging system is working............research research research.......but most of all take your time and enjoy.
    Last edited by drydreamer; 08-22-2019 at 08:45 AM.

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