Yz250 sputter on top

J

jms158bmx

PR Member
Recently put a top end in my 2006 yz250. Ran great before not so much now. Has a pretty good bog on top when the powerband hits. Runs great on stand revs up fine. I did clean up the power valve with carb cleaner when cylinder was off. Didn't remove anything just hosed it out. Anyone have any idea what it could be? I've done the following with no change
New reeds
Cleaned n rebuilt carb with new jets n seals
New plug
Checked all wire connections
Yes piston is in forwards
No there is not a towel in air box.
Fuel is brand new and its had the same 110 octane @40:1 since new
 
If it revs fine on top but not under power Id say the jetting is off. The only other thing would be weak spark. If its a bog I would try to richen the needle or main depending on where in the throttle you are when it does it.
 
I had a similar situation after a rebuild on a YZ250 and talked to Tim Shepard down at the Lancaster Yam shop about it. Of all things, he suggested changing the spark plug cap as he had seen some Yamaha caps go bad. Turns out for my situation, the new spark plug cap fixed it! Good luck!
 
A fresh rebuild can change the jetting if it was jetted to run with low compression/ blow by. Just google jetting a 2 stroke and follow the steps. Thumper talk .com has a Yamaha 2 stroke forum, at the top there is a sticky on how to jet a 2 stroke.
If you put in different reeds that could throw off your jetting too.
I've heard of the bad spark plug cap issue too. Never had it happen to me though.
 
Ill try spark plug cap. Its not the jetting. Been to both ends of spectrum n no change. Plus every thing i did to fix it was after it started bogging. And i went one by one not all at once. Killing me to ride it i got suspention done in june n havent had time to try it. Now this happens
 
Just to be sure...Dbl check your reeds. I had a new set break on me and it drove me crazy trying to figure it out because I knew the reeds were ok... just saying it's worth a look.
 
jms158bmx said:
Recently put a top end in my 2006 yz250. Ran great before not so much now. Has a pretty good bog on top when the powerband hits. Runs great on stand revs up fine. I did clean up the power valve with carb cleaner when cylinder was off. Didn't remove anything just hosed it out. Anyone have any idea what it could be? I've done the following with no change
New reeds
Cleaned n rebuilt carb with new jets n seals
New plug
Checked all wire connections
Yes piston is in forwards
No there is not a towel in air box.
Fuel is brand new and its had the same 110 octane @40:1 since new
Click to expand...

Sounds like (maybe) TPS is unplugged or the PowerJet Circuit is unplugged. Powerjet will have a big impact on top-end. If it is unplugged, the bike will be way too rich on top.
 
Came out on the 00 model year through 2017. The Powerjet was an auxiliary main jet connected to a solenoid. The idea was for the solenoid to close off the aux main jet above about 8000 RPM to lean out the mix for high RPM (leaner = better at high RPM). When the Powerjet circuit is not working, the aux main is open at all times, which translates into too rich of a mixture at high RPM.

Why do I remember this kind of crap?
 
hershey said:
Dude, I got rid of that bike in 08, I cant remember what I had for dinner last night!
Click to expand...

While cleaning my '06 yesterday, I took a picture to jog your memory.
2016-09-23 23.55.58.jpg

This has proven to be a very durable motor. The only problem I've experienced was with residue partially blocking the pilot jet. That made for some pinging off idle.

Any resolution yet for the '06 bog? These motors should run well.
 
I would think to check the main jet and or needle as well. One of the more puzzling issues I ran into years ago was a ring on the piston that wasn't concentric. Using calipers to measure, part of the ring was wider in some areas than others. Never would have seen it with the naked eye. Sure enough that was it. Certainly not a common issue, but if you have ruled out some of the more obvious issues may be the next one to look at.
 
You might try taking off the over on the powervalve linkage and make sure it is opening correctly as you rev it. You could have dislodged some crap that is now stopping the PV from moving correctly by just spaying cleaner in there. Matbe even disconnect PV linkage and move it by hand with front PV cover off making sure everything is moving ok. You can do this by just removing the exhaust pipe and 2 PV covers.
 
Clair Story said:
You might try taking off the over on the powervalve linkage and make sure it is opening correctly as you rev it. You could have dislodged some crap that is now stopping the PV from moving correctly by just spaying cleaner in there. Matbe even disconnect PV linkage and move it by hand with front PV cover off making sure everything is moving ok. You can do this by just removing the exhaust pipe and 2 PV covers.
Click to expand...
Or take it for a spin. :)
 
Got to tear into it tonight. Not good. The main valve in the center was stuck. The sides where opening good that's why it seemed like it was working fine. Now the odd part is the main shaft that runs through the cylinder was broke in half, where the fastener goes in it. Now do I buy the individual parts or do I just go big bore! Seems like it's gona be close to the same price, and few extra ponies could help this fatty. Decisions...
 
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