I have a 400 sport 2 stroke. it starts good and idles good but if you hit the throttle it spits and spuders and will backfire. The reeds look ok they had a little dirt under them and I cleaned it out and fliped the reeds over. Bike runs the same. I ran a compression test on it and got around 85 psi I removed the reeds and got 90 psi. I removed the carb and cleaned it out. I am lost on what to check for next this is my first 2 stroke. Thanks

 

sounds like there is a leak somwhere...im surprised it will run with 85 compression.

try checking everywhere where there might be a leak...and it also sounds like that you're due for a top end job anyway.

 

You need at least 110lbs comp to run .sounds like time for a topend.

 

not sure but u might have a broken skirt on the piston my 400 was kinda like that would not make no power

 

Bottom line is you need a top end rebuild. New piston, rings, gaskets and borejob. Will cost around 250-300 for everything. You may not even need the bore if the walls still look good.

 

When I bought my sport 400 it had about 4 years on it and I did not have to bore out any. Just ran a hone on it and it was good to go. Just make sure there are no grooves and no scratches.

 

is the comp. tester o k?
85 psi is far away to be ridden,if you got the reeds remove you might
look at the piston further front and look at the cylinder wall and
pistons ring for wear out to be there.remove the pipe will show you
even more what the shape inside is...