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04YZ250f rebuild on top end

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Well my son has blown up his 04 yz250f. Not quite sure if timing or valve adjustment was at fault. Both exhaust valves bent, piston resting in pieces(some in crankcase),IMG_20140308_114433565_HDR.jpgIMG_20140308_115107550_HDR.jpg valve bucket on one exhaust valve broke, cam lobe damage, head damage, cylinder gouged too deep to bore. So I now have a complete matching top end out of a wonderful running 01 Yz250f. Need rings. stock bore, mic and light hone done. Bike is currently upside down suspended in shop. Going to try and get pieces out of bottom end. Advice? Torque spec? Not a pro shop here but have knowledge and the tools but you Yz owners sure could help with advice....

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I've done many. Suggest you do a tear down since there are pieces in the cases. Barring that, remove the left and right side cases. Look for steel stuck to the flywheel magnets and pieces in the bottom. KEEP everything you find and try to match up the parts so you know if you got everything. Remove the oil pump and check for scoring on gearrotor, replace scored pieces. Blow out ALL oil passages and lines in case, tranny, and head. Wash out the oil tank and lines connecting and the screen in the line at the front down tube... that's what its there for, catching garbage.

It's much easier to remove the engine and split the cases and clean out because you can clean and check all case bearings that way. Other wise, you risk having garbage in there you don't know about, and you still get a trashed engine after cleaning it out half way.

Just my $.02, but I'd do a teardown and hope for the best if it didn't spin much after it broke. Remember, these are stressed race engines, and need maintenance.

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That's the neutral switch contact. It goes into the left side of the shift drum. There's a hole in the center and one outside of the center. The one to the outside holds the spring, then the contact pin goes in. The pin engages neutral switch on the inside of the case.

Not sure why that would have fallen out since it should be held firmly in place unless you've removed the left side cover.

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That's the neutral switch contact. It goes into the left side of the shift drum. There's a hole in the center and one outside of the center. The one to the outside holds the spring, then the contact pin goes in. The pin engages neutral switch on the inside of the case.

Not sure why that would have fallen out since it should be held firmly in place unless you've removed the left side cover.

I found it bud I took the sensor out not realizing the pin a spring would fall out. Thanks

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Just to reaffirm. Everything ronbuell is telling you is spot on with what I would do as well. If you don't completely disassemble the engine and clean everything you may well end up right back here.

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Still trying to get to the bottom of this implosion. While waiting for parts investigated carb to get a feel for settings, here they are

operating elevation-1,800 to 2,200 ft

humidity avg. 40 to 90%

needle - OBEL P at clip position 4

MJ-178S

PJ-42

LJ-100

PILOT- 1 3/4 OUT

93 OCTANE

STD PLUG, burn color perfect to slightly dark

Timing chain new<--was told by previous owner

Valve adjustment just done<--was told by previous owner

Son said it was running great then just bombed after 3-4 laps around a 1/10 mile field

Both exhaust valves bent, RF exhaust bucket shattered, head damage in RF exhaust bucket area, and indentation from piston smack on exhaust side of head.(minor), all valve shims 1.80 from hot cams,

Head matches that of my replacement from an 01...(decompression not hooked up)<---strange must have had an issue prior?

approx 1"x1" square of (oil guide fin under crankshaft counterbalance for lack of better description) part of actual crankcase broke off from shattered piston pieces. no other case damage. Replace?

Cylinder gouged too deep to bore<--only my assumption- resleevable?

Engine breakdown complete....

Complete matching used top end from great running 01 going back end

Any input is appreciated, Manual downloaded and in hand. Waiting on parts and gaskets

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Timing chain new<--was told by previous owner

>Trust but verify. Remove chain, drop on table, then pick up by 1 link. It will kink up if worn out, hang straight if good to go. For $20, I'd replace it anyway since it got stressed.

Valve adjustment just done<--was told by previous owner

>Trust but verify. Very good indicator was your post finding 1.80 shims.

Son said it was running great then just bombed after 3-4 laps around a 1/10 mile field

>Ooops

Both exhaust valves bent, RF exhaust bucket shattered, head damage in RF exhaust bucket area, and indentation from piston smack on exhaust side of head.(minor), all valve shims 1.80 from hot cams,

Head matches that of my replacement from an 01...(decompression not hooked up)<---strange must have had an issue prior?

>I removed the cable from my son's '02 after I installed the Hot Cam Stage 1 with auto decomp on it. You don't need it after that.

approx 1"x1" square of (oil guide fin under crankshaft counterbalance for lack of better description) part of actual crankcase broke off from shattered piston pieces. no other case damage. Replace?

>Someone else will need to comment on this one. Case damage could be sign of stress cracks elsewhere.... check it VERY closely. And a pic would be helpful.

Cylinder gouged too deep to bore<--only my assumption- resleevable?

>Send it to a plater for replating. They will tell you if serviceable or not.

Engine breakdown complete....

Complete matching used top end from great running 01 going back end

>+1 Good find. Go over it with a fine tooth comb.

Any input is appreciated, Manual downloaded and in hand. Waiting on parts and gaskets

Good luck with it. Sorry you're having problems.

BTW, take a pic of the small cam gear on the crankshaft, its possible its worn out and caused it to jump time.

Indicator is the teeth are sharp and pointed. Good crank cam gear teeth will look like teeth on the cam sprocket.

Another I forgot, Check all case bearing for smooth operation, no ruffness, noise or lock up. Maybe you got luck there.

Edited by ronbuell

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Just an FYI this could have been caused by the timing chain tensioner. I have a thread on here somewhere covering it and the modification I do to the tensioner to prevent it. My son had the tensioner kick back when he let off the throttle in the air and the chain jumped a few teeth and put the valves into the piston. Its rare but it does happen.

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I want to give a big shout out to Ronbuell, Charlie, Morphrider and all who helped make it possible for me to get this bike into the break in stage. It is running flawlessly. 1.5 hrs... going to check the plug. My son says it spits and sputters some and acks like t is running rich. I have a post in here of all the carb jets and setting if anyone cares to give me some insight. In all very nice rebuild. Kudo's to all and a big thank you!!!

 

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