I have sport 400 and I have done a top end rebuild. Also bored cylinder out and put in larger piston and rings. Compression is at 90 lbs when book calls for 115 lbs. I did the trick with putting oil into the cylinder and then checking compression - if the compression goes up with oil then it has bad seal around rings. I did not have that, the compression stayed the same.

What happens though is the engine runs great at idle but kind of boggs down when accelerating. When it heats up it will not even move.

I have also rebuilt both clutches and carb. Any ideas out there I have done about everything you can do but the thing will not run????

 

Sounds like you may have bored too large?? Did you check the bottom of the piston to the bottom of the cylinder skirt before you installed it? Most 2-strokes and 4-strokes(with the exception of watercraft) are set up TIGHT! Clearance on a new bore should be between .0015-.002 clearance after boring and finish honing if done properly! When 2-strokes reach about .005 clearance between piston and cylinder they are essentially worn out! It may run at idle but as it heats up it looses even more seal on the rings to the cylinder.Another thing is did you torque the head bolts properly? Plus compression on a new top end should be somewhere between 130-145 pounds or more. 115 pounds is probably the MINIMUM in the book.The only other thing I can think of is a broken ring if every thing else is correct OPT

 

I sent the cylinder out and the company did the bore job and supplied the piston and rings. They said it was in specs. I believe I torqued the head bolts right but the head is the only thing I have not done anything with. I did the steel ruler across the top of the cylinder and the head but did not see anything as far as warping or scarring. Does anyone know how to check the head for warping??

 

We used a solid slab of marble at work and if a .001 feeler gauge could be inserted anywhere around the head then we sent it out to be milled down OR used a light grit garnet paper on the slab and could do a few our selves. Plus another thing did you bleed the head to remove any trapped air? This can warp heads and cylinders and overheat quickly if you don't. If the steel ruler showed a tight fit at different points,then the head and cylinder are probably ok? Just my 2 cents on this,a lot of shops may allow too much clearance on bore jobs. I learned this very quickly on our first send out job and then we got our own boring machine which I used for almost 23 years and its still going strong last I heard!!
PS: crank seal leakage can affect running as it must have bottom end compression along with the top end compression,but you must have more than 90 psi on the top to begin with! OPT