Jump to content
Sign in to follow this  

Newly aquired yz goes kaput please help.

Recommended Posts

Hey all, first post here for me, so first off, HOWDY! Heres my dilemma. I picked up this 03 yz250 last friday. Went riding on sunday. Got about 2.5 hours in to our ride, was going on a slight up hill at 1/4 throttle, revs about middle, then brrrrrrrrrrrooohhh POP(with a bit of a "tinggg" sound mixed in. Dunzo.

We thought fouled plug, but tried a new plug and no deal. Has spark, its getting fuel, i thought top end, so i took it apart yesterday. There are some vertical scoring marks from near the edges of the skirts, but the rings look good and most of the upper part of the cylinder looks good. Could this top end still be the problem?

Big end freeplay is ok, wristpin clips are there, no discernable lower end leaks i can see. Reed valve assy. is good. (havent gotten too far into lower end). I dont have a ton of experience with 2 strokes, so i guess i need opinions on if this is why the bike wont run. Here are a couple photos. Marks in cylinder can barely be felt with your thumbnail. However, the piston's marks seem notably worse. Also, i guess about 20 mins before the no start/no run condition, i thought i had heard some slapping in the top end, but bike was running great, and i didnt think much more about it.

Mix was 32:1 with motul 800 2t. Running 50/50 mix of VP ms109 race fuel and premium 91 pump gas. Plug was black but not fouled. I attributed that to it sitting around with the previous owner who didnt ride it very much over the last 1.5 years. Was really puking oil for the first 20 mins, my assumption was the pipe and silencer were loaded up with spooge. It was blowing minimal smoke after that was all cleared out of the system. Any input will be greatly appreciated.

1.jpg2.jpg3.jpg4.jpg

Share this post


Link to post
Share on other sites

FYI ive ordered a piston kit, but want to be sure before i spend money on resleeving or replating the jug...

Share this post


Link to post
Share on other sites

If it needs it, I would opt for replating. However, take it to a shop and have them run a rigid hone through it (lightly) that will tell you more.

Share this post


Link to post
Share on other sites

clean first your cylinder mate so you can really see what is up. So far I see it look OK and if you can not feel anything with the finger nails you are good to go.

Share this post


Link to post
Share on other sites

Scoring can definitely be felt with a finger nail. Its clean I just took the pics first sry...compression test this afternoon. (I put it back together last night)

Edited by meinmoto

Share this post


Link to post
Share on other sites

Compression is at 195. Cant get the damn thing to run. Its all back together now. Jets are clean, plug sparks against the head when I check it, fuel is getting into the cylinder. I can smell fuel out of the spark plug hole when I kick the bike over with the plug removed. Im stumped.

Edited by meinmoto

Share this post


Link to post
Share on other sites

Sounds like fuel and compression is ok based on your above post. I know you verified spark but IMO the electrics could be bad and not providing a consistent spark to run - I'd be testing your electrics.

Share this post


Link to post
Share on other sites

Humm?? Sounds like a challenge. Take a look and see if you sheered off the flywheel key. You will still have spark but God knows when you will have it. I ran into this prob. building a motor and had the wrong flywheel. Oh, it will mess with your head for sure. Sounds like you have covered about everything short of checking the brain box.

Let us know what happens for future reference.

Share this post


Link to post
Share on other sites

Thanks guys, 'tis the direction im headed. I got the flywheel weight off tonight which came off way too easy... ill pull the flywheel tomorrow and check if the key is sheared. Ill also be trying to get my hands on a known good cdi box to see if that could also be the issue. Ill keep you updated. Let me know if y'all think of anything else i could be missing.

Share this post


Link to post
Share on other sites

Looking at the piston crown, you have zero "piston wash" . This suggests an extremely lean running engine. There are 2 scenarios, either your carb is jetted too lean, or you have some debris partially clogging the jet(s) . I strongly suggest completely disassembling the carb remove all the jets and clean with carb cleaner and compressed air. Verify the jetting someone prior might have swapped jets. I recommend stock . Not sure about your specific year YZ250, but the more current '05+ YZ250's have issues with too lean stock jetting, and too aggressively advanced oem ignition timing. Good luck !

  • Like 1

Share this post


Link to post
Share on other sites

Carb was cleaned for the second time as of 11pm last night. Going to reassemble and try again this afternoon. So you all know, I have an education in motorcycle mechanics, so to save your time and mine lets leave the fundamental things out of it as they are already done. Please dont take offense, but ive gone through everything I can think of first before even asking here on the forum. I appreciate the response tremendously but I originally posted because I had no ideas left. Ive seen that theres no piston wash, but i dont think this is the cause for my no run condition. Back on topic, like i said in my OP, the bike is an 03. Ill double check the jetting when I get home. Thanks!

Edited by meinmoto

Share this post


Link to post
Share on other sites

Carb was cleaned for the second time as of 11pm last night. Going to reassemble and try again this afternoon. So you all know, I have an education in motorcycle mechanics, so to save your time and mine lets leave the fundamental things out of it as they are already done. Please dont take offense, but ive gone through everything I can think of first before even asking here on the forum. I appreciate the response tremendously but I originally posted because I had no ideas left. Ive seen that theres no piston wash, but i dont think this is the cause for my no run condition. Back on topic, like i said in my OP, the bike is an 03. Ill double check the jetting when I get home. Thanks!

OK, so in post #1 you said "I dont have a ton of experience with 2 strokes," and now in post #11 your telling us "So you all know, I have an education in motorcycle mechanics" . So i'm confused . Did you not learn 2 stroke engines ?

Edited by Polar_Bus

Share this post


Link to post
Share on other sites

Talk about getting pissy ! Polar I think you need to take another crap.

  • Like 1

Share this post


Link to post
Share on other sites

I can understand your reason for any confusion. Please allow me to clarify. I know and understand two stroke fundamentals. I can do many things a two stroke engine requires. By the time i graduated and got into the field (2005), two strokes were already being phased out. So i didnt work on very many. All i said was i didnt have a TON of experience with them. I am not an expert at anything, nor do i claim to be. But i am educated and have many years in real world motorcycle experience, both formal and informal... may be not as much as some, or you, or most, but i know my way around bikes. My statement before was mostly meant for any 2 stroke gurus that would be able to tell me that in my particular situation, my piston wear was enough to raise a flag in someones head. That is all. My apologies for what could have been mistaken as conflicting statements. Again, your suggesions are still appreciatied Polar. i know im new here, but not to forums so take it easy and dont be so quick to jump down my throat.

Edited by meinmoto
  • Like 1

Share this post


Link to post
Share on other sites

If you are getting spark and fuel, and you have compression I would think it has to be electrical.

Share this post


Link to post
Share on other sites

I can understand your reason for any confusion. Please allow me to clarify. I know and understand two stroke fundamentals. I can do many things a two stroke engine requires. By the time i graduated and got into the field (2005), two strokes were already being phased out. So i didnt work on very many. All i said was i didnt have a TON of experience with them. I am not an expert at anything, nor do i claim to be. But i am educated and have many years in real world motorcycle experience, both formal and informal... may be not as much as some, or you, or most, but i know my way around bikes. My statement before was mostly meant for any 2 stroke gurus that would be able to tell me that in my particular situation, my piston wear was enough to raise a flag in someones head. That is all. My apologies for what could have been mistaken as conflicting statements. Again, your suggesions are still appreciatied Polar. i know im new here, but not to forums so take it easy and dont be so quick to jump down my throat.

For the record, my intent was NOT "jumping down your throat". Your initial comment "I don't have a ton of 2 stroke experience" is completely subject to ones' opinion, and can be perceived in either direction of expertise. You did not detail exactly what your experience level currently is. Honestly your reply #11 can be equally perceived as "jumping down my throat" I was just trying to help. My post reply # 10 clearly is below your experience level, so as to not waste any more of your time and mine, I will respectfully leave this thread alone...

Edited by Polar_Bus
  • Like 1

Share this post


Link to post
Share on other sites

How was the flywheel key?

Did you check the condition of the reeds?

195 psi is plenty to start the bike, but is getting low for a yz250. From all the old skuff marks (likely improper warm up) I would say its time for a new top end.

Have you checked for exhaust blockage? Not common, but s silencer cor e can craxk and fall down, blocking the exhaust.

Am I the only one seeing piston wash? It looks like most the crown is lightly carbon covered with slight wash at the transfer ports. Also, a cimpletely clean crown means rich jetting, the cold fuel keeping the piston from developing deposits. A carboned piston with no clean spots is running hot and lean. What jetting is in the bike anyway?

I'm still thinking flywheel spun, but if not that, then electrical.

  • Like 1

Share this post


Link to post
Share on other sites

For the record, my intent was NOT "jumping down your throat". Your initial comment "I don't have a ton of 2 stroke experience" is completely subject to ones' opinion, and can be perceived in either direction of expertise. You did not detail exactly what your experience level currently is. Honestly your reply #11 can be equally perceived as "jumping down my throat" I was just trying to help. My post reply # 10 clearly is below your experience level, so as to not waste any more of your time and mine, I will respectfully leave this thread alone...

Ok Polar, we're all good. Like i said i meant no disrespect in your direction either. Thanks.

  • Like 1

Share this post


Link to post
Share on other sites

Polar thinks you are running lean and Adam728 rich. Lets see......... Polar just destroyed a brand new top end, so who are going to listen to?

  • Like 1

Share this post


Link to post
Share on other sites

How was the flywheel key?

Did you check the condition of the reeds?

195 psi is plenty to start the bike, but is getting low for a yz250. From all the old skuff marks (likely improper warm up) I would say its time for a new top end.

Have you checked for exhaust blockage? Not common, but s silencer cor e can craxk and fall down, blocking the exhaust.

Am I the only one seeing piston wash? It looks like most the crown is lightly carbon covered with slight wash at the transfer ports. Also, a cimpletely clean crown means rich jetting, the cold fuel keeping the piston from developing deposits. A carboned piston with no clean spots is running hot and lean. What jetting is in the bike anyway?

I'm still thinking flywheel spun, but if not that, then electrical.

I will be pulling the FW this afternoon. I'll post info on that as well as current jetting on all that as soon as i know. As for the blocked pipe, thats a good idea. I have had the pipe off (obivously) but havent actually poked around inside. I'll report on that as well. New top end is on its way to me and should be here and installed friday evening. It needs it for sure, i just dont think that in its current state, it would cause a no run condition. I was thinking maybe whoever put the FWW on didnt put the key back, or did something incorrectly. The FWW appears to have been rubbing the stator cover a bit. It had the double gasket set like the Stealthy instructions suggest. Stay tuned race fans!

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

Sign in to follow this  

×