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Discussion Starter #1
In need of a re-ring job(burning oil bad), and am going up to a 86mm piston from wiseco.

Just wondering whether the 400ex runs better on 10:1 or 11:1 compression ratio? I think the stock is 9:1? correct me if Im wrong.

I have 93 octane gasoline where I live. Would 11:1 run ok on this?

Also with an 86mm piston would the standard topend gasket kit from say rocky mountian atv still work? or do i need a special head gasket.

While im in there I will be doing valve seals. any other sugesstions for a 2002 motor?
 

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anything under 12:1 is fine to run on 93 just make sure you dont get to hot so you might have to go up like one or two on your main jet

as long as its not the big bore kit the regular gasket should work 440 is where you have the problems
 

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I suggest a Namura topend kit you can get everything to do the rebuild in one box at very reasonable cost. I do suggest not using the tensioner gasket in the kit just leave your on the tensioner. 11:1 or 10:1 is your choice but keep in mind the higher the compression ratio the harder your starter works. I just rebuilt a 400ex with an 86mm 11:1 kit and it works fine. I also run 110 VP race fuel in all of my quads. straight in mine mixed in the rest​
 

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Discussion Starter #7
Thanks for the info, I'm gonna go with 10:1 just to keep it closer to stock. I don't need any more power just wanting to stop the oil burning.

For an engine this old, should I look into replacing the cam chain? Are there any checks to test the old chain?

Where is the namura kit sold?

Thanks
 

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Discussion Starter #9
just wanted to update, Got the cylinder pulled off the bike.

It has no cross hatching left from where the rings rode, and one small gouge bout 1/2" long and deep as a hair.

The valves were covered in black soot and the spark plug was too.

I'm guessing since it had no cross hatching left that why oil was coming through and getting burnt.
 

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well the cross hatching will wear off over time its for the rings to break in properly but if there's a gouge then that's why shes smoking,was the smoke blue or white?
 

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The scratch will definitely cause some smoking for sure, I also agree replace the timing chain.
 

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Discussion Starter #12
It was smoking what seemed to be white-ish. While rididng last weekend ~40 F it filled an entire field with smoke and ate about half a quart in maybe 4 hours riding trails.

It seemed to smoke more once heated up and while getting on it. At cold it would smoke only when hitting the gas then letting off.

I have to say I'm impressed with how well the dissasembly went. Only took maybe 2 hours. Hopefully it goes back together as well.
 

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when I rebuilt mine it comes up as 12.5:1 comp so should I run race gas or 93 and should I get any special oil to keep from over heating
 
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