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Please Help, 06 RMZ250 backfire

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I have an 06 RMZ250. My bike ran perfect untill I put a new top end in it (piston, rings, and all neccessary gaskets). I also adjusted the valves to spec in my manual. Ever since then my bike is backfirring horribly and actually shooting foot long flames from the exhaust. I have checked jetting and valve clearances at least fifty times to correct this. I cannot seem to figure it out. Is it even remotely possible that the piston or rings are installed incorrectly? If anyone has any information for me, it would be of great help. I desperately want to go ridding.

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I would start by checking for an exhaust and/or intake leak. Either of those could cause it to backfire.

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I have checked valve timing and valve clearances at least fifty times. Everything is in spec.

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I appriciate the advise, anything helps. I have checked for leaks. I let the bike idle and have sprayed all around the intake and exhaust with brake clean and there was no rise or change in idle indicating to me that it is not sucking air. Any other ideas?

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Have you had the carby apart i once had a yz450 and put the slide in the wrong way around it pop'd and spluttered and took ages to find.:thumbsup:

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One more thing to check. Is the accelerator pump actually pumping fuel? I worked on a bike a few weeks ago with the same symptoms you describe and it turned out the A/P was gummed up and not squirting any fuel. Pull the subframe and twist the throttle while looking down the carb.

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im not sure if this could cause backfiring like that, but its fairly easy to put rings on upside down. so you might wanna check that out

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i had the same i adjusted the pilot jet at the bottom of the carb there is an adjuster screw, wind it all the way in and then out half a turn at a time try after each turn to see if it is ant better. it should be 2 and 1/4 turns

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I appriciate all the advise from everyone. I have double checked everything that everyone has mentioned. I am now back to the piston and rings. I have noticed a slight casting flaw in the piston. But I did perform a leak down test and had zero leakage. Does anyone know the exact orientation of the rings and how they sit on the piston? I belive the main ring should be offset by 45 degrees from the oil seal rings. Is this correct. And is there a top and bottom to the main ring? I cannot find any markings that indicate a top or bottom.

Thanks again everyone.

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the top ring gap faces towards the ex valves(center of front piston skirt) and the top of the ring has a 45deg cut on the inside. the oil expander ring faces the inlet valves (center of rear piston skirt) the top oill ring sits -15 deg to the left of the expander ring and the bottom ring +15 deg to the right

I would put up a pic but dont know haow yet.

hope this helps

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