2005 YZ250 Top End Failures

I bought a 2005 YZ250 about 6 mos ago, the bike needed quite a bit of work. After one ride with what seemed like not full power, I found excess crankshaft play on the flywheel side and pulled it apart. I put in a new Hot Rods crank, and found to governor (for power valve broke) and fixed it. The cylinder measured big, just out of specs. I just went with a new ring. After about 6 rides the piston failed (exhaust side missing piece above top ring & ring broke). The cylinder was not damaged so I replaced piston & rings. About another 6 - 8 rides almost exact same piston failure. These two failures happened at a very sandy track. This time I sent the cylinder to US Chrome, had it repaired & re-coated. I did notice a slight sticky-ness with the power valve moving in the cylinder, but I thought the spring forces were strong enough to overcome it. 1st ride at a hard rough surfaced track. Bike ran well. I checked the plug afterwards - half of insulator was reddish brown, the other half a little lighter. I talked to a person at the track about this (who I trusted). He thought it sounded like pre-detonation and reccommened using some higher octan gas. Before my next ride I mixed my normal 93 octan gas 3 gallons with 1 gallon of 110 octan race gas. I thought this would give me some cushion in case it was pre-detonation. 2nd ride at my favorite sandy track. Bike ran strong until about 3 laps I thought I noticed a flattening of power so I came in. I pulled the plug, it was white with a few shinny dots on the electrode. I did not have a bigger main jet w/ me so I lowered the clip on the neddle two knotches and went back out. It seemed like it wanted to load up, but then by the end of the first lap just ran worse, I pulled in and l went home. The piston & cylinder are damaged. The piston has the start of two small holes in the crown just over the ring on the exhaut side. I think it is the same failure I had before, just I caught it sooner (but not soon enought). Also the power valve was stuck in the 7/8's closed position, this is new and might have to do w/ the re-plating.

Any thoughts ideas would be appreciated.

Sounds like you know the main is too lean.

What are your elevation, temps., jetting and timing numbers?

Way too lean. Air leak adding to your troubles?

sea level, 80 deg, stock jetting (178m, 50p, clip started out on 2nd groove from top, a.s 1 turn out), timming .007" BTDC

How do you test for air leak at flywheel side seal? I was thinking of after re-assymbly to blank of intake & exhaust and put about 2-3psi and see if it holds (leak down test) but this will take some fab work to rigg up.

Assuming lean is the problem, when I get it back together I want to make it rich and then work my way back down. Also assuming I find & fix any air leaks, how much bigger jets & carb settings do you think I should start at?

Couple things

- did you actually confirm the .007 with a dial gauge?

- many guys run 178 at 80F/SL because it's actually a little rich

- For a sand track in those conditions w/93 oct. you shouldn't need more than a 180, 182 would make many sputter.

- The crank seals are pretty easy to do

- It sounds more like air leak/timing than jetting since that's a common jetting spec

- Especially with the fuel you were running, it shouldn't have been detonating with that jetting if the timing was right

As far as crank seals go: the proper way is the complicated leakdown test. I think a better idea is: need new piston= replace crank seals. They're relatively cheap and easy to replace, and you can check the sealing surface while you're there and touch it up with 600 grit w or d paper. if you do leak test, flywheel or drive side, you can either put leak detect liquid on or even WD40 will sometimes show. But I'd just replace seals and be done with it. They have a tough life.

I always went at least two jets richer to start, if that doesn't do it, go another two steps up.

Yes I set timming with a micrometer gauge that threads in head (no dial but the same thing).

So, I will just replace the crank seal behind the flywheel and start w/ a 182.

178 definitely doesn't sound lean, but every bike is a little different. Exhaust with too much back pressure could also cause higher piston temps and melt downs. The FMF SST has a reputation for this on bikes that see lots of wide open. A silencer with a broken core that's misaligned, packing stuck over the end of the core tube, or a mashed up end cap could even contribute. I've even been told about bikes with lots of carbon in the stinger section o the pipe reducing flow and causing issues.

178 definitely doesn't sound lean, but every bike is a little different. Exhaust with too much back pressure could also cause higher piston temps and melt downs. The FMF SST has a reputation for this on bikes that see lots of wide open. A silencer with a broken core that's misaligned, packing stuck over the end of the core tube, or a mashed up end cap could even contribute. I've even been told about bikes with lots of carbon in the stinger section o the pipe reducing flow and causing issues.

Very true, and very overlooked: Ford went though a bunch of Escape engines due to a bad batch of catalytic converters recently. Took out the main and rod bearings.

One think on the timming - when I put in the Hot Rod crank in, I am pretty sure I replaced a Yamaha crank. I had to move the coil (stator) plate quite a ways, almost to the end of the slots to set the timming. Since I was confident in my timming tool I thought the flywheel key way in the after market crank shaft must have been located a little different then the stock one. Has anyone else noticed this with Hot Rod cranks?

I will definetly check out the silincer, like I said the bike had a lot things wrong when I bought it, and I have not been in the silencer yet (I know the power valve was not working when I got it so maybe that fouled up the exhaust. I have already removed carbon at the big end. It is a FMF silencer not sure if SST.

Has anyone else noticed this with Hot Rod cranks?

If you do a search through the timing thread, there are multiple cases.

Timing thread

Have a peek at the intake too, a crack there could do it. BTW, what is your squish?

The head is stock. I got an old head and plan to try reducing the squish at some point, but haven't yet.

tighten squish yet? Head cut? It could have previously been set up for race gas if you did not buy it new...

I'd run more race gas while sorting through this.

Hell - considering the investment these failures have cost you - consider running race gas ALL the time - and a good LEADED fuel (guessing you are).

Race gas may seem pricey but lets face it - a cylinder, three pistons, and maybe another cylinder is much more price in a year than race gas is

UPDATE - I have been thru the carb very carfully and am 75% sure the pilot jet was clogged, also the float level was low (betwen moderate & quite a bit)(I should have measured it befoer I adjusted it, but I guess it was 0.350" to 0.380") everything else in the carb looked OK. I pulled the seal behind the flywheel, it looked to be OK. I took the silencer apart, nothing broke but packing was 25% missing and almost solid, so I replaced the packing. No excess carbon or blockage in pipe. When the parts get here I plan to start w/ 182 main, 50 pilot, a.s 1 turn. That should be rich.

Before all this I thought I had some feel for jetting 2 strokes, but not so much now. I have been brushing up on it and after a break-in period, plan to spend some dedicated time on a dirt road jetting. I follow most advice up to the main jet, but I am hoping once I get there I will have a decent feel for the power and plan to just go up & down on main jets to find the best or maybe just a little rich since I love riding sand tracks.

hi there

when you work on jetting it is wise to just take one jet at the time, meaning change the pilot till you the engine run nice and crisp at 1000 to 2000rpm than go for the needle position and least would be the main. If you problem are in the upper range just change main.

First thing to do is the float level, if that is out of specs you don't have a starting point yet.

Good luck

Arnego2

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