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Discussion Starter · #1 ·
We just put back together my sons 400ex, with a new 87mm piston and freshly bored cylinder, the problem is it won't start. The cam is Lobe down the (IT) is lined up with the (I) in the timing hole and the cam sprocket is lined up with the (--) and the plain of the head. I have spark, I have fuel, and I have air movement through the carb and exhaust. It turns over fine but it does not even attempt to start. I am told that the 400ex (2002) rotates 760 degrees on the crank for every 360 degrees of cam travel, but that the spark plug fires on both the compression and exhaust stroke is it possible to be out 180 degrees seeing that you have spark at both strokes? I am at a complete loss for what to do, yes another Rookie!

Thanks you your help!
 

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It sounds like its 180 out, line up the T mark on the flywheel with the mark in the hole you see the timing mark in and then be sure your piston is TDC and your cam gear marks line up with the index mark for the cam gear.
 

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What year??? Welcome to the forums .

is it a weak or faint spark or pretty strong?
 

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Discussion Starter · #4 ·
it's a 2002 400ex. The spark seems strong we got it to jump about an 1 1/2" last night and yes it is a fresh plug. What I don't understand is if I have the T lined up with the notch in the timing window (TDC) and the -- are lined up with the head how can I be 180 degrees out?
 

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Is the piston on top dead center compression stroke?
 

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I would double check the valve clearance for starters it's something like .004 on the intake and .005 on the exhaust for the 400ex or vise versa? but if you have fuel and have spark then I would say incorrect valve adjustment or you have poor compression due to improper ring spacing. but I have been know to be incorrect from time to time. lol since you do not have reverse that rules out you being in Reverse. Do you get any kind of detonation? have you ran a compression test?
 

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Discussion Starter · #7 ·
No detonation, Rings where installed correctly. Have not run a compression test but I will. Now that you have me thinking about the valves, I suppose that with a new gasket between the head and the valves could have changed the valve clearances. Seems like the old gasket was paper and the new one is metal, we did not change the cam or mess with the rockers so we assumed we did not need to adjust the valves. What should the compression be on the 400ex? Also am I correct that I can't be out 180 degrees because I have the "T" properly lined up as well as the lobes down?
 

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Yeah, if you have the line beside the T lined up with the mark on the case, then the piston has to be on TDC. There's also an F on the wheel, so don't get it confused with the T. The cam lobes should be down and the lines on the cam gear even with the top of the head, like you said. The cam will turn 360 degrees for every 720(not 760) degrees of the crank. Compression should be 100-128 PSI, according to my manual.
 

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Discussion Starter · #9 ·
Thanks for all the help, we just solved the problem, it was in fact the valve clearances, we theorized that the new gasket between the head and the cam cover was thicker by enough to throw the clearances out enough for it not to start.
Helmut thanks for the math refresher yes 360 X 2 = 720 I hate it when I do that :).
 

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lad you got it going next time just use Hondabond...lol that leaves Helmut and his calculator out of the equasion...LMAO
 

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Yeah, I always use Hondabond on a valve cover. Hondabond is good stuff. I really wouldn't trying to give a math lesson, I just thought someone had told you wrong. LOL.
 
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