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Need help with 08 400ex frustrating engine problems

5159 Views 41 Replies 4 Participants Last post by  EliteEX
Ive got a problem with my 400ex and need some professional advise. Sorry about the long post but there is a lot of small details that may give more hints on what the problems may be.

I bought it with a head gasket leak so i tore it down to replace it. When i had it apart i noticed some abnormal wear on the piston and slight scoring on the cylinder(figured the previous owner ran it low in oil). So i honed the cylinder and replace piston, pin, and rings with new oem. While i had it apart i also cleaned out the screens in the oil cooler and in the bottom of the engine. I then replace oil filter and filled with honda oem oil(It did seem to take slightly more oil than normal but i didnt think much of it since i had the oil cooler and side case off to clean out the screens).

Started it up and it ran great. Did about 8 heat cycles and some mild riding on it and then checked the compression which was around 120. Then i rode it fairly hard for a while till i noticed some smoke coming from the pipe and a very slight abnormal knock/ping while under load (the smoke continued seep out of the pipe for a while after i shut it off. It was a used pipe i just bought and put on, maybe it had some oil residue inside and was just burning out).

After it cooled off i removed the valve cover with intentions of getting a look at the cylinder but then decided to do another compression test before tearing down. So put the valve cover back on and tightened a few of the 6m bolts, did the compression test and at around 50 psi the engine seized. Took the cover off the side case and tried to spin the engine backwards, and it broke loose fairly easily. I tried to continue spinning it counter clockwise, and after a bit it got tight again. I pulled the valve cover off and the found the timing chain was tight all the way around (with tentioner in). Then i pulled the tenioner out and whenever the engine would get tight the timing chain work get tense the entire way around. (Tentioner seems to be working correctly)

Ended up pulling the head and cylinder off and found some fresh scores in the cylinder(much worse than the previous scores). The top of the piston also had a thin layer of black residue in the center.

At this point im stuck and need some professional input on whats wrong. If your feeling extra helpful and dont want the confusion of commenting below please send me a pm ...thx


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Just a reminder while you are going through the oiling system... include the passage up through the jug and check out the head as well. The service manual shows dowels and locations... sometimes they can fall out. Passages can get plugged by RTV as well if the previous owner ever used it.
Ok so heres the current situation. Im planning on doing the 416 kit so that should take care of any cylinder out of round issues. I think the slight knocking under load was cause by a very tiny bit of play in the connecting rod. So the next question....What kind of connecting rod/crankshaft should i do and anything i should know about replacing them....?

Thanks guys
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