400 ex engine running really rough

[400EXracer74]

my friends 400 ex is running really wierd. it will start and all and idle a little bit but when its on its almost like its backfiring every time the spark plug goes off. we took the carb apart and cleaned it completely. so now im thinking maybe a valve spring is old or sticking or maybe the timing chain is off. it can be ridden but the throttle response is bad it revs unevenly sometimes a lot of power will all of a sudden kick in sometimes its just really slow. any ideas?

[400exrida06]

hey, i have no clue whats wrong, mines doin kinda the same thing, if you find anythin or get it goin could you reply back on here so i can try it to thanx.

[400EXracer74]

bump

[duke416ex]

Did you check the plug?

[400EXracer74]

yea its a new plug

sounds like the chain jumped time.. i had the same issue with mine, and that exact thing happened.. you could go full throttle, and it felt like it was going from half to full throttle, picked up a crf450 timing chain, problem solved.. its a easy thing to check to see if your outta time or not..

Pull your plastics like you are going to do a valve adjustment.. once you get that done, pull the motor mount out of the top, its a 14 mm bolt/nut.. once you remove that then take the 8 mm bolts and the 1 12 mm bolt out.. pull the rocker box off, now, take the 2 plugs out of the LEFT SIDE case, theres one in the center, and one about 11 oclock... Rotate the motor counter clockwise until you see the sideways T , line that up with the little notch in the window.. If all the cam lobes are up, then rotate it ONE more turn till you get back to the T, if the cam lobes are not ALL FACING DOWN, then you jumpped time, and its time for eaither a tensioner or a timing chain..

Btw, this job is a ton easier without the spark plug installed.

Hope that helps you

Jesse

[400EXracer74]

what would cause a chain to jump timing?

[j753301]

faulty tensioner or really stretched timing chain..

[400EXracer74]

could the stock chain thats in there just be re adjusted and have it work good again?