YZ250 with no power

[icantslowdown]

I'm normally great with these things, but...

The end of last season I grenaded the top end on my 95 YZ250, and replaced the piston, rings, cylinder and cap with new, as well as having the bottom of the crankcases welded since a piece of the piston skirt wedged under the crank and cracked the cases. I went through and replaced all the bearings and seals while I was in there.

The motor holds 8psi for well over 10 minutes with no leakage, top end compression is 190psi.

For some reason, it was gurgling really bad on the top end and just doesn't feel like it has the power it used to. I dropped the main from a 175 to a 172, 170 and finally a 168 and it's not gurgling on the top end anymore, but still no power everywhere. I dropped the needle from the 4th (stock) to the 5th clip, and midrange leaned out a bit. I pulled the exhaust and made sure the power valves were opening and closing properly, everything's good. Pulled the carb, cleaned it thoroughly, made sure the fuel line wasn't kinked, etc. Tried a hotter plug (B7ES instead of the stock B8ES), no really noticeable difference.

One of my neighbors who's a car mechanic suggested that part of the piston skirt might have shot out the exhaust port and lodged in the baffles in the silencer (turbine core spark arrestor) - is this likely? I still have the stock silencer somewhere, so I'm going to swap them out to see once I dig it up, but I thought perhaps some of you gurus here had some other ideas.

I rode it for about 1.5 hours the other day and it ran fine, just no power hit like normal. Doesn't even wheelie on pavement when running through the gears. I change my tranny oil after every ride, and it was black, where the tranny oil from my girlfriend's YZ125 was still tan after the same riding time.

I'm running 117 octane, Amsoil synthetic oil at 32:1

Any help is greatly appreciated!

Thanks
Jim

[Michael Parker]

check your reeds and your exchaust flow

[Satch0922]

clean the powervalve assembly.

[icantslowdown]

I cleaned the powervalve when I rebuilt the top end - it's immaculate. I checked the operation as well and they open and close fine.

Checked the reeds, what looks to be minor abrasion at the ends, but no cracking or bad chips. Forgot to mention, running a RAD valve, but it was on there last year while it was running fine. I'll try a fresh set of reeds and see if that gives me any love.

[Aus_Rider_22]

Definetly pull your silencer apart and check for blockages!

I had a similiar problem where my YZ would have no power. Just fell flat soon as you opened it up.

The perforated core had dislodged from the canister and caused the packing to cover the holes and severly impede the flow of gas out of the silencer.

[icantslowdown]

Well still no dice

I pulled the RAD valve out and replaced with the stock reed cage and a fresh set of reeds - no love.

I pulled the silencer apart, cleaned the baffle and checked the packing, and reinstalled it - no love.

Pulled the cap off, don't notice anything out of the ordinary other than some premix in the cylinder, which I'm assuming is from the compression test I just did. Compression is still at 190, same as when I rebuilt the engine 1.5 hours ago.

I can wiggle the piston ever so slightly in the cylinder (microns maybe) with my finger, is this normal? It's a brand new 'B' sized cylinder, and a brand new 'B' sized piston/rings/circlips/wristpin.

The only other thing I could think of is something internally dragging the engine down, but the piston moves easily enough by hand with the kickstart lever, and it idles and runs through the gears fine, just no power hit.

Talk about frustrating...

Thanks
Jim

[icantslowdown]

More info:

Pulled the flywheel, made sure the ignition timing was spot on - it was a teeny tiny hair off.

Adjusted the float level - again, just a teeny tiny hair off.

Pulled the pilot jet out, made sure the holes weren't plugged.

Switched back to the RAD valve, and the 175 main, took the bike for a spin - definitely more power than before, but not like it used to be.

Does it take some time after the rebuild for it to regain it's power? I've done top-ends before and seems to me that they were all back to normal almost immediately.

[Michael Parker]

the only other things that i can think of is that it has a crank seal leak so do a leak test and see if it holds preasure. check your electric system and make shure your getting full volts to the plug cap.

[icantslowdown]

motor holds 8psi for well over 10 minutes.

any idea how many volts are supposed to be at the plug cap?

[dancrf100]

just a thought, did you run the new top end in? Ive heard a lot of storys about people going straight out on a 2 stroke with a new top end and it fries the rings and you have to rebuild it again, Ive always ran mine in slowly and its been fine. Just a thought

[icantslowdown]

Yup - normal breakin. Rings are still good too since the compression is still at 190.

I'm going to put another hour or so of riding on it tomorrow to see if it livens up.