400ex Smoking

OK I'M ABOUT TO GIVE UP MY LUCK HAS WENT TO CRAP. I JUST PUT IN THE STOCK BORE 12.5 PISTON AND GT THUNDER CAM AND DONE A PROPER BREAK-IN BY DUNCAN RACINGS SITE AND NOW WHEN I CRANK IT UP IF I BLIP THE THROTTLE IT SMOKES LIKE A TWO STROKE BUT ONCE IT WARMS UP ALL THE WAY IT QUITS. ANY SUGGESTIONS?

Don't freak yet, A friend with a 416 kit says his smokes when cold then stops when it warms up. See what others have to say.
John

[kat400ex]

Maybe it could be bad valve guide seals.......ask terry marcum some possibilties MARCUMSATVS@HOTMAIL.COM does it run bad once it warms up??

[MR. HONDA EX'S]

are u running racing gas? My buddy put some 108 in my 400 (wrong gas can). When I started it up cold, it smoked like a 2 stroke also. But once it warmed up it ran good and didn't smoke. And I have no motor mods.:D

[Guy400]

You say that it started when you replaced the stock piston with the new one?

In our high-preformance auto machine shop we've seen this time and time again. Your stock piston and cylinder don't wear in a perfect circle. Most times they wear slightly out-of-round especially on the side of the cylinder where the piston is returning to TDC (top dead center). There's a point where on the upswing the connecting rod is at it's greatest angle thus putting the most sidewall load on the cylinder. This is where the oblong wear comes in. After running the stock piston the rings wear to match the cylinder wall. Now when you put a new piston and rings in they are assumably perfectly round. Well now your new round piston and rings don't match your ever-so-slightly oval cylinder. When your motor is cold and you start it you get a little bit of blow-by where your rings don't exactly match your cylinder. After the motor warms up a little the rings expand and the gap is filled. This is probably why it smokes a little on start up and after it warms it goes away. More than likely that the new rings will seat after a little while and the smoking on start up will just about go away. If not you can bore the cylinder and get an oversized piston or you can resleeve your cylinder and re-ring your piston (at the same time). Also, if you get the chance mic (micrometer) your stock piston and mic your new 12.5 compression one. I'd be interested in seeing how close they are. I know in cars the Keith Black hypereutectic pistons are slightly bigger because of their alloy mixture they can run tighter clearances. Typical forged pistons on the other hand are slightly smaller because they need additional clearance because they expand so much when they get hot.

I'd just about bet dollars-to-donuts this is what's causing your cold-smoke/hot-no smoke issue.

OK HERE IS WHAT I HAVE FOUND OUT LIKE GUY660R SAID FORGED PISTONS RUN LOOSER CLEARANCES THAN CAST SO WHEN IT WARMS UP IT NATURALLY FITS BETTER SO TO SAY. I'M ALSO NOT THE FIRST PERSON FOR SAYING THAT THEIR 400EX SMOKES AFTER MOTOR WORK UNTILL IT WARMS UP. I REALY DON'T THINK ITS THE OVAL PROBLEM THOUGH BECAUSE ITS A 2002 MODEL WITH ABOUT 20 HOURS ON IT BEFORE MOTOR WORK. I DO THANK EVERYONE FOR REPLING SO QUICKLY AND HELPING ME OUT. I GUESS I WILL JUST HAVE TO WAIT AND SEE.:devil