Lost my low-end mojo - 2008 YZ250

[jdaugherty33]

My 2008 YZ250 seems to have lost its low end. We were climbing hills a couple weeks ago and about the 6 run up the same hill it seemed like the low-end just disappeared. It had been running great all day. From that point on, it bogs big time until I get the revs up. Once I'm on the pipe, it runs great.

I've rejetted to stock and the plug looks good. I have VForce reeds and they are in good shape - no cracks. I've also checked the power valve from the outside - I can move the linkage smoothly and see the valve moving in the exhaust port. I also checked the valve in the front of the head and all the parts are there and it looks like it's working fine - (I had heard of bolts falling out). I read someone's PV was sticking causing the same symptoms so they ran race gas with Yamalube and it solved it. Mine doesn't feel like it's sticking when operating it manually but maybe I wouldn't be able to tell? It seems like the problem gets worse throughout the day.

The top end's about 60 hours old and the compression is reading at 170 psi. The bike was running great up until that day a couple weeks ago. It might be worth mentioning that I've probably run the bike too lean for the last 20 hours or so - I was running 168/48 on the jets which is closer to my summer jetting. I zoned out on the jets and ran them several days around 30-45 degrees. I'm not sure if this could've caused any issues that wouldn't manifest in low compression.

Any ideas on other things to check before I start tearing into the motor and getting myself in even more trouble?

[dbart250]

Sounds like top end to me. Theyll run easier once you get them going, until they burn up completely. Red flags : running lean, 60 hours.

[adam728]

Could very well be top end time. Hard to start as well?

What does your bike do on the compression gauge when fresh? 170 psi is REALLY low on my gauge. A good YZ250 on mine tests at least 210 psi.

[jdaugherty33]

The top end makes sense but it's strange that it would come on so suddenly. I would've expected a more gradual decline in power.

I didn't test the compression when it was fresh so I don't have a good comparison. It starts great cold - one or two kicks with the choke on. But it's more difficult to start warm - it's now taking 3-4 kicks whereas before the problem it would always start on the first kick warm. Is this more of an indication of top end issues?

[FGR01]

168 main in 30-45 degree temps and Tenn altitude? I'd say you're way lean and the bike is starving for fuel on a wore out top end. And in my experieince the V Force is prone to air leaks as well which would make it even worse. What does your plug look like?

[jdaugherty33]

The plug looked more rich than anything which is why I wasn't too worried about changing the jets - well, that and just plain laziness. I'll probably just change the top end if I have to pull the head off to inspect the PV closer. I'll be surprised though if the top end is the primary cause since the problem seemed to appear so quickly while riding.

[highflyernick]

It has to be your top end, 170 is a little low, we've tested a fresh piston in the high 230's, all stock. Plus with the extra kicks etc, pretty good sign. More than likely just you rings but might as well do it all. If you are looking for even more bottom end punch, I did the EG 295. Talk about a whole different animal. I'm not sure what can stop this bike now, it's gotta have more power than a 450, and so smooth with the "Mo Betta" Good luck!

[jdaugherty33]

Thanks for the replies.

I pulled the cylinder and it is extremely worn, especially in the center. A couple gearhead friends looked at it and said it is most likely the problem. I'm surprised the problem came on so quickly but enough people I trust agree that it's the problem that I'll take their word for it. I've ordered a new cylinder and top end kit. But I'm going to research the EG kit tonight and may cancel my other order if it looks like a good option.