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jbodin18

new top end bike won't start...please help

9 posts in this topic

I just put the first new piston, rings, timing chain in my 04 yz450f. Put a new OEM cylinder on. The piston and rings are JE 13.5:1. Did nothing to valves or cams, still original. I freshened since it had never been done and I'm starting to get back in mx again, wanted the gains of HC at high altitude. So I kicked my leg off trying to start. TDC is accurate. When I kick it and when I manually rotate the flywheel, the compression seems to be lacking. rotating the flywheel clockwise offers barely any resistance (if that helps). Any thoughts on where to start? Yes, it is my first top end rebuild, but I have done past valve adjustments no problem and am good with a wrench. Thanks.

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Check the valve clearances. If it happens to you, too, it will be the fourth tilted valve shim this week.

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My intake cam was one tooth off. i seem to have gotten more compression back now when manually rotating the flywheel it is much more difficult to spin, but bike still won't start. How many kicks does it usually take? doesn't even act like it wants to start. guess i get to tear down again...

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i'm a damn idiot. I dropped a shim down a hole I didn't even know was there. It is the hole right next to the middle valve on the intake side. I pulled the carb hoping I could get to it from there. I'm going to go get a smaller magnet that might fit down the hole, but do you know where that leads to. I can't see the shim. If I can't get to it with a magnet, will I have to remove the head?

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I dont know where that hole leads to, but if it does in fact lead to the engine internals...i would say absolutely YES, remove the head and get it out of there. Sucks that it will require that much work, but i would rather do that than suffer the incoming consequences.

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Check the valve clearances. If it happens to you, too, it will be the fourth tilted valve shim this week.

I agree!!! That is what happened to me!!! lol The tilted shim cracks open the valve letting compression out!!!

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Drain the oil first and see if the shim falls out that hole is just a oil return passage if its the same one im thinking of coat hanger and luck you may not have to split the case or just leave it and hope it finds its way to the magnet (sorry not yelling )

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So luckily I was able to buy a magnet small enough to fish out the shim. Valves are on the close side of being too small a clearance. Bike started, I checked the fuel screw and i thought i returned it to 2.5 turns, but it was only at 1.5 turns. Idle screw was too far out, screwed it in. I'm good to go, ready to tear up the track. Thanks guys.

Edited by jbodin18

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