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we have a 03 xr70 that seems to be lacking alot of power. I knew that it had been rode alot in the past so i put new piston rings on it thinking that this could have been some of the reason. Well this really did not help much. Also the muff mod and airbox mod with a twinair filter have been done.. Could use some advise on what to check next. The reason I say it lacks alot of power is we just got a new crf50 and it seems alot more powerfull than the 70.. any help would be appreciated.

Mike

Why would you feel the need to rebuild an XR? They're indestructible. That could be your problem, did you break in the new piston correctly? If not, that could result in a less desirable seal between the new rings and cylinder wall, and thus cause a loss in power.

no new piston just rings, and yes I did break in properly.. It gained a little power from this but it still seems to really be lacking. On takeoff it is really slow. not much power compared to the 50. Maybe thats just the way they are but I have read of some saying about how much power their 70s have/had and it just seems like it should have alot more

Does it rev out? If not, then you could have either a clogged main jet or water in the airbox. It could also be a leak on your head pipe, though if you haven't messed with that, it probably isn't the problem. Try checking areas that prevent running. You might have to replace the spark, which is easy. Also your timing could be off after disassembling the motor, but don't assume that is the problem immediately and take apart the engine. Try a "trickle down" test, and inspect things that would result in power loss.

So how's she doin? A little better?