400ex smoking after rebuild

[dirtdog400ex]

Ive got a 400ex with a 440 big bore kit and its smoking like hell. It was smoking when i bought it and the guy said he just had it rebuilt but it still smoked when he got it back. So I bought it took it to my guy and he rebuilt it as well. Shaved the head, honed the cylinder put the new rings in it. Got it home wasnt smoking road it for 1 day. The next time I pulled it out to ride it was smoking like hell again. Im stumped. Please help if you think you know what it is. Do you think a new cylinder would fix it. Or is there something else thats causing it.

[maxamillion]

I would make sure the valve guide seals are good and do a leak down test with the head off to make the the valves are sealing up against the seat.

[BSTURDIVANT]

Typical 440 witha bad sleeve installation! buy a new cylinder and piston.

[hondaguy20]

maybe the rings werent staggered when the cyliner was put on the piston... possiblility

[03'400exrookie]

What kind of oil are you using?

[Big - D Racing]

I too am guessing your valve seals are bad, or you need a new valve job

[Kickstarts-suck]

Were the valves and seals replaced?

[dirtdog400ex]

valve seals were replaced and the machine shop didnt say anything about valve looking bad

[cbr929rr]

Originally posted by maxamillion
I would make sure the valve guide seals are good and do a leak down test with the head off to make the the valves are sealing up against the seat.

How do you perform a leak down test with the head off?:rolleyes:

[Sjorge450R]

Originally posted by cbr929rr
How do you perform a leak down test with the head off?:rolleyes:

yea i was thinking the same thing.

Chances are its the valve seats. My 300 has been like that forever but i never got around to fixing them. Thing still runs like a rapped ape. Never really bothered me, because it would only smoke when the motor was cold or not fully hot.

[yfzrider45]

My suggestion is to take out the 440. My friend had a 400 and it was extremely reliable and ran perfect until he put the 440cc kit in it....

[Big - D Racing]

Originally posted by yfzrider45
My suggestion is to take out the 440. My friend had a 400 and it was extremely reliable and ran perfect until he put the 440cc kit in it....

I have a 440 that beats most 450's and is reliable as can be. No problems what so ever and starts first start everytime.

They are not reliable if they are not built right. Just like any other engine regardless of displacement.

[400exStar]

I can tell you that I had the very same problem and mine was a valve guide. I had my guy reapair the guide and now it is fixed. I have other issues. Def sounds like a valve prob.

[xc400ex]

Make sure the headgaskets not blown although most of the time they wont smoke like your describing from a blown headgasket. I have this sayin about the 400ex's.....if there not smoking then u better put some oil in it...

[dirtdog400ex]

its not the head gasket im sure its either the something with the cylinder or the valves

[nick249420]

i just had my 400ex rebuilt for the same problem
my valves and seals were bad the valves had carbon all over the top of them and they were way out of adjustment
when i ran it it looked like a 2 stroke with the wrong gas/oil mixture

it was dumping smoke just idleing

[dirtdog400ex]

Yea nick mines doing the same thing. But what I dont understand is it was perfect for 2 rides after rebuild then started smoking again worse than it did b4 I took it in the first time.

[416exfreak]

My 440 is extremely reliable. I havent had a single problem with it.

The only complaint I have is how noisy the valves are, but its because I put in a stage 4 Webcam (billet) with hardfaced rockers.:ermm:

[dirtdog400ex]

Mine has a stage 2 cam and its noisy also but I new that when I got it.