2005 RMZ 250 top end rebuild fail! Help

Hey guys, first time posting. I recently rebuilt my top end because the bike was spewing oil from crankcase breather tube, dispite propor oil level.

So i bought the cylinder works standars bore kit for the bike, installed the piston in the correct direction (arrow towards exhaust) and i believe the piston rings were installed correctlt as well (3 rings and 1 spacer) top ring letter was up, and two oil rings sandwhich the spacer, all ends staggered) so then the piston went into the cylinder well as far as i could tell. Moved up and down decently. Got it all back together and kicked it over. It has less compression than it used to (used to start first kick) and i can also hear the piston move up and down for a second or two after i kick it over. Its hard to explain but it just feels all wrong. Has spark, but wont even think about firing, kick even got rough for a second which leads me to think its the piston. Just a dry sounding movement. Im not much of a mechanic, im actually a wood worker but i thought id try. Plan to tear it apart soon and re-try. Any ideas in the mean time?

Thanks so much everyone

Also forgot to mention timing is in spec as per manual

Timing marks are in spec but are the cam lobes correct? If i remember the time can be set but if it wasnt truly tdc than it will give you issues. Sounds like the valves arent closed on the compression stroke.

Take this with a grain of salt...i own this bike and cant keep it running lol

I'm also having a very similar problem on my 2010 rmz. Just finished rebuilding the top end but it doesn't seem like I'm getting any spark. Won't fire at all.

I just took it back apart, found coolant on top of the piston???? Other than that i couldnt find anything i did wrong, checked timing again and its ok. Now when i kick it over there is far less compression than before and i can hear the air being pushed out into the airbox when kick it. I'm defeated.

Edited by Jason_31

Air being pushed back through the carb means your intake valves are either leaking on the compression stroke or your timing is off on the valves. That would explain the low compression.

Post pictures of your cams.

Coolant on top of piston could possibly just be some coolant left over in the jackets on the cylinder.

Thats an answer im looking for! Thank you, and as soon as i get back out to work on it i will post pictures immediately!

Well im an idiot....cam chain came off the sprocket down in the bottom end. That was the problem.... Told you guys i was a woodworker. Thanks for the help guys! But you cant fix stupid hahaha

Works now tho? I dont care how stupid i do something, im just happy it didnt break or cost me anything!

Yeah thats my thoughts exactly! I took it out today to break in the new piston. Ran okay but was acting up. I think i need to check valves and probably shim them, its harder to start now and would run okay then die suddenly. Would take a lot of kicks to fire again. Got on the bike pretty hard at one point for my last break in cycle and once i let off it died and was very hard to start so i just called it a day. The kickstart would feel decent at times and i swear would feel like it lost compression sometimes as well. Im hoping its just valves as they were noticably ticking. Maybe valves and a carb tune? Put a fresh filter on it and i doubt it's tuned for it and my new piston. What do you guys suspect?

Yep, checked valves and all of them are too tight, which should explain low compression

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