YZ Woes - Please Help :(

[Blisterphish]

I have a 96 YZ250 that just quit after running kind of rough for about a month or two. Lots of sputtering and rough idle.

*Seems to have compression-will blow thumb off plug hole when kicked. Don't know if that is a valid test.

*I'm getting pretty good spark but to be sure I swapped ALL electricals with known good parts including
CDI
Coil
Pug Wire & Plug
Stator Assembly
Entire Wiring Harness
Kill switch THERE IS NOTHING ELSE :yell:

*On about the 50th kick the thing will fire off and run for about 5 seconds then it dies. Hollow sound like it is choked.

*The carb looks good and clean and the floats are at spec.

Any ideas?

****Caution, the following does not really pertain to this issue - it's just a question.***********

Here is and oddity. Keep in mind the bike ran with this setup all summer, it's just a question and probably not part of the problem.

The main jet is a 175 (stock)
The pilot jet is a 45 but stock is a 42. does this get you a richer idle?
The cutaway on the slide is 8mm but stock is 6mm which should lean the mixture back out??????
The jet needle was also set to the top position which should lean it even more.

Does this setup serve a purpose or do I just not understand it?

 

[mifun3]

Save yourself lots of trouble and buy a compression tester (you can get them pretty cheap $15-30) If compression checks off low rebuild the top end. Often a burnt ring will bring the compression just low enough to create hard starting or poor running.

My 0.02 cents

L8R

Mike

 

[skipn8r]

A CR80 with a broken piston will blow your thumb off the spark plug hole. If you're sure you have spark and fuel, either get a compression tester or pull the head/cylinder to check things out.

 

[rmguy125]

Is the plug fouled after it starts and dies?

 

[Blisterphish]

Thanks for the responses guys.
I was reading in the book that there could also be a loss in compression in the crank case. If a seal has gone it is possible to pull case oil into the combustion chamber and foul plugs. My plugs do seem to have a lot of oil on them after the 5 second run time. Question is----------is there a way to do a leak down test without purchasig what looks like a very expensive test item?

 

[rmguy125]

no you have to buy the leak down tester the last i knew, but you may want to try to put the carb back to stock settings if your plug is fouling

 

[Blisterphish]

Yes the plug is fouled after it dies. I don't have the stock pilot as I bought the bike very used but I did set the jet needle back to #3 which is stock. no luck-I think I'm going to set it back to top-lean like the guy before me had it. I assume he did it for a reason.

The book calls for top end compression of 100-125 PSI. This seems like a large variance to me. How far below that should be a concern? What level would indicate a need for a top end job. Either a bad ring, gasket, etc...

 

[rmguy125]

i would say 90 and below but i dont really know the best thing you could do is go to the nearest bike shop and ask them

 

[Blisterphish]

one more quick question:
can you buy the compression tester at an auto zone or is it something that you have to get at a bike shop? seems to me like it would transfer but..... The closesest bike shop is 45 minutes at least. pain.

 

[rmguy125]

well you could as long as you had the hose end to fit the spark plug hole and they sold it

 

[Uchytil]

Did I miss how many hours your top end has? There are only so many things that will make you engine act like that. If it has high hours it's probably ready for a rebuild. You better get a manual if you plan to do it! There are some good threads in this forum about TE rebuilds also. If that thing has never been apart since 96, and you plan to keep it around for awhile longer, you may want to invest in a bottom end. I guess the real decision should be based on hours and use. That's how I roll. One last thing, how do the reeds look?

 

[Blisterphish]

Back from the Dead

I finally gave up and took the bike to the Yami shop. Their tech called me back within two hours and had the bike running. :cool:

He changed the pilot jet from a 45 to a 38 and bumped the main up by a few (I forget exactly). He also moved the jet needle from #3 to #2. The thing kicks easier and runs better than ever.

I never would have guessed that a machine that was running (maybe not so hot but still running) would just stop without anything going bad. The guy I purchased the bike from lives at the same altitude so I don't understand why the jetting would be so off. I know jetting is important but I didn't think a setup would just stop working one day????

The shop charge was only $80 vs. the month of aggravation and unneeded parts purchases I went through. I do believe in diy but in this case it didn't work out. I'll have more knowledge next time. :laugh:

 

[j32]

i cant believe a 250 would only have 125psi, i think any thing below that needs new rings,i cant remember exactly but 80's and 125's have compressino around 150psi,and 250-500 are around 200-250psi
bad reeds will also result in loss of compression