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'05 yz250f won't sart, only an ocassional back fire.

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My brother and I rebuilt his yz250f. it ran good, but it leaked coolant from the weep hole. We also noticed steam from the exhaust, when it first started. We decided it must be the head gasket. We tore it back down and, sure enough, there was a speck of dirt under the head gasket, between the cylinder and the water jacket. We cleaned it up and put in a new gasket. Now, it won't hit a lick. every 8 or 10 kicks, we get a backfire. that's it. we've had the top end off a few times and timed the cams the same way every time, just like the manual says. The bike always ran. we've check the cam timing 2 or 3 times. It's spot on. I literally rode the bike around the yard right before I tore it down. My question is this. Does the crank need to be at TDC on a certain stroke to time the cams? I know it wouldn't usually matter, but since this bike has a counter balancer, which is driven by the crank, I thought it might be an issue. Any input would be greatly appreciated.

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I always put it on tdc and then align the cams. Just make sure all the slack of the chain is on the tensioner side so the timing dont go out.

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What Russell said ++

As long as the piston is on TDC, the timing marks on the flywheel will be in the right place, you're good to go with the rest of the cam install.

Positions to check:

make sure the carb manifold intake boot is tight, the one on the head side.

then check the air filter boot and clamp.

 

only thing you didn't mention was a carb check.

Sometimes when mine will just pop like that, it not getting fuel

turn on the petcock, then loosen the drain screw and make sure that fuel is getting into the bowl.

turn it off and tighten the drain, then turn the drain tube up beside the bowl.  Open the petcock and the drain while holding the drain pipe up beside the bowl and see where the level settles.  It shouid be level with the bowl carb body joint.  If not, float level is off.

QUESTION:  when you had the head off, did you check the accelerator pump to see if it squirted?  Just asking.

Mine starts with 2-3 twists of the wrist, no throttle, and choke for cold start... I know you know the drill.

Good luck.

Fuel, air, spark, timing: if its all right, it has to run.

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