Hey Everybody, Here is the issue , I have a 2008 bottom end , ( crank,cases,gears,etc ) New piston , rings , Cylinder ,timing chain, (all OE stock) The head is off the original motor , thats all that was salvaged after the new crank I put in it last year decided to come apart just below the wristpin , there didnt appear to be any head damage. everything is timed up correctly , has great compression. It is a bear to start and when it does it runs like crap, Backfires on the exhaust side, and the exhaust gets Glowing red hot . Any Help? P.S. The hot start was stuck but I fixed that with no better results. Thanks , Mark . I forgot to mention the head is off an 05............

 

I would check the cam chain and tensioner for proper placement and adjustment.

Welcome to KF

 

Hey Dragone , Hows that ? Is there not only one way to put this back together? Mark

 

You mentioned that it is a kxf, that the engine was tore down and put back together. My first thought was that the timing is off, ya missed a tooth on the cam timing for example. Or that even the valves are not adjusted properly. Just tossin out ideas since I have not personnally worked on one yet.

 

I have rechecked timing over and over because thats exactly what it acts like , but thats good . If the valves are off by a couple thousands could that do it? They dont exactly have a good method for checking this. All hints are appreciated , Thanks

 

What Cams are in the bike? Stock or aftermarket? Have had problems with the stock cam gears spinning so when you time them the lobes are actually off and bike runs horrible if at all.

 

i have a 2006 kx 250f i put a new cdi box in it a new ignition and coil it was running great i been riding it everyday racing it and it has been great no problems i rode it to my buddys house shut it off and now it wont start again i have no idea what it could be does anyone on here have anyideas? i have never had any other problems and it was getting hard to start before this happend

 

is it getting spark?
is it getting fuel?

 

it is not getting spark but when i first baught it i had to put a new cdi box in it and a new coil and ignition so i cant see those goin bad again in a month

 

I think you have a fuel delivery issue. It starts, your timing can't be off too much, no noticeable new holes in the piston from valves punching holes in it....

Red hot exhaust isn't a timing symptom. Neither is backfiring on the exhaust side. Both MAY be lean jetting issues / carburetor corrosion. You haven't explained what you used for a carb on this rebuild. You mention the head was the only thing from the original engine you reused. This statement may or may not include the carburetor. Did the carb from the '08 donor make it's way onto the '05, and if so, how was it jetted? If the original-to-the-bike carb was used, how long did it sit around before it went back into service? Maybe a jet cleaning to verify the state of things? No air leaks at your manifolds? Exhaust seated and properly torqued?

Where is the air/fuel mixture screw set at? Maybe you could run that in to stop [gently, mind you] and come out 2 full turns and experiment.