yz250 rebuild fail

[DirtyBlasterd]

i rebuilt the top end on my 89 yz250, actually an 88 engine, fresh bore, new piston, rings, cleaned power valve/ chamber, ect. it was late when i finished but i wanted to fire it up. after a few kicks and a minor kickback, i gave it some gas and it fired right up. sounded great, i ran it a few minutes parked, it wouldnt quite idle on its own. since it was late i didnt want to piss off my neighbors so i figured id set the idle in the morning. the next morning i kicked a couple times and got the most violent kickback ever. messed up my ankle but good. gave the leg a day to recover and tried again, after a couple kicks it fired right up with a little throttle. while i was messing with the idle setting it died and when i went to start it back up it happened. i kicked it and there was a loud noise as the kick lever came too far forward and dented/cracked my clutch cover. i pulled the cover and see that when it dented it also broke one of the fingers on the clutch basket. under further investigation the kick start stopper stripped on the kick start shaft allowing the kicker to advance. i guess im getting some pretty powerful kickbacks. i have all the parts to put it back together but i dont want it to happen again with fresh parts. is there something i have overlooked, or something i may have done wrong?

 

[Blaaster]

Is it possible that the timing could have slipped?

 

[Braaaptor]

Ignition timing!! Check it well before reassemby. Chances are, you have a sheared key (caused by a loose flywheel nut, perhaps) or the stator plate has been indexed incorrectly.

 

[DirtyBlasterd]

i was thinking that might be it, but it was fine before the rebuild, i will try to check it out tomorrow, i dont have a flywheel puller though

 

[Braaaptor]

i was thinking that might be it, but it was fine before the rebuild, i will try to check it out tomorrow, i dont have a flywheel puller though

You'll have one soon enough! I:I

Pit Posse Flywheel Rotor Puller Yamaha YZ125 YZ250 | eBay

Flywheel keys can appear to shear for no apparent reason, if the flwheel happened to come loose without your knowledge. This may have been the reason the engine blew a top end in the first place....if the timing has been off for some time, the engine can still run but will often overheat during riding, which can, in turn, melt a piston and damage the cylinder.

 

[DirtyBlasterd]

thanks for the link for the puller, not sure if that will work on mine it says 95-2010?
i pulled the flywheel cover, and i can see around the flywheel that the stator alignment mark is perfectly on the case mark, also the flywheel doesnt appear loose and i can turn the crank without it having any play.

the old top end never actually blew, it was just really worn out

 

[Braaaptor]

Ah, OK. The puller should fit all years, but you might want to ask the seller before buying one. Just to be sure.

I'd still remove the flywheel to be sure the timing is set right. If that bike has an adjustable stator plate, it is also possible that the stator plate may have come loose and rotated slightly, causing the timing setting to slip.

 

[DirtyBlasterd]

so reading the clymer manual for it i see that the alignment marks on the case and stator are for preliminary timing setting. there is another mark on the flywheel that should also align with the stator mark when the piston is in a certain position. apparently the preliminary position worked fine for the previous stock pistons, but not the wiseco. i guess the dome must be different enough to throw off the timing. now i need the flywheel puller, flywheel holder, and a dial indicator that can thread into the spark plug hole to set the timing. i guess i have to learn some how, too bad i trashed my leg and the bike in the process

 

[bimmer325es]

good luck getting that put back together man, take your time, get the right tools and ask lots of questions if unsure on anything till you are sure, you'll get it!