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Flat spot cbr 400 @8000- 10000rpm

Discussion in 'Technical and Troubleshooting Torque' started by Dylan05, Jan 17, 2012.

  1. Took the 400 to PI for its first run since a rebuild and noticed a massive flat spot @ 8000-10000rpm in every gear. The engine pulls hard till 8g and boggs down then hits 10g and pulls hard again. The jetting I think the mechanic said had been put up to 1.15, too big? and valve clearances were just done. Could it be the rubber boots/diaphragm are just old and may possiblely let air in?

    Just wondering if anyone has an idea what the problem could be or can put me onto someone that could help.

  2. Not that its bike related but my BMW E28 had that problem, hit 4500, bogged down, hit 5500 then pulled hard to redline at 6800. Timing was out slightly, set timing again and it pulled right to the redline again. I have no idea how the cbr is set up and how the ignition works but its an idea.

    Other then that, mixture, ignition, spark intensity, fueling, air leaks, could be anything will just have to cross them off one at a time...

    Untill someone else who knows what they are on about comes anyway :D
  3. I doubt air leak, otherwise it would happen everywhere...

    Sounds like the main jet might be too big, is it only at WOT?

    Get it on a dyno for an A/F readout would be the best bet, otherwise start playing with jet sizes, but make lots of notes!

    This is helpful:


    You want high RPM, obviously.
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  4. Any notable exhaust system or airbox mods?
  5. A dyno tune will sort it out. Exhaust and airbox mods can put huge flatspots in. I got my cbr600 tuned the other day and it goes like a madman now.
  6. Thanks for the tips guys, the link was an awesome page its a favourite now. The airbox is standard the exhaust is a yoshi RS3, anyone know if any dyno's out Werribee way?
  7. head over to cbr250.com and post this up in the cbr400 forum... theres a few blokes on there that have raced them etc. and should be able to help ya pretty easy.

    id put my money on carbs needing a clean or a dirty air filter but carbs arent a strongpoint of mine
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