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2005 YZ250 Piston wear. Normal or not?

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Hey all,

First post to TT, I bought my bike about 5 months ago and it had a new Weisco top end and it ran fantastic. After about 15 hours or so it lost compression and power rapidly one day at the track. After tear down, I discovered the top ring broke at the exhaust port. I ended up purchasing a new cylinder and top end because I didn't want to wait for it to be replated. I just recently rebuild my 2005 YZ250 and it has about 4 hours on a new "B" OEM cylinder and new "B" OEM piston kit. I took the exhaust off last night to look at the piston and cylinder a little just to check things out and right below the top ring is the only wear visible. It looks almost chrome. Is this a common spot for wear. I'm concerned because of my prior engine issues. Could there be a problem with the power valve?

The picture supplied is looking up at the piston from the exhaust port. Any advice or experience? Or am I over thinking things?

image.jpg

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I run 32:1 maxima super M. 1/3 c12 to 2/3 premium pump fuel mix. Jetting seems to be spot on and plugs always look tan and dry.

If it were running with not enough lube what would cause that area to be damaged and not above or below that?

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It does look a little lean. You said your plugs are tan and dry which is a indication of a lean setting. You want more of a brownish slight tan color. Did you clean the carb or rule out any lean conditions such as air leaks with the new top end?

Although you are mixing your race gas but c-12 is not a good 2 stroke fuel, simply because it has a low gravity range that does not keep the oil suspended and has way to much oxygenation. I personally have best results with AV gas and also used U4.4 with great results but cant justify the price when AV performs just as well. My suggestion go a 50/50 AV/U4.4 and premium pump, clean your your carb and do a plug chop. If you get a slight darker color on the plug that might be your winning ticket. Hope this helps

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Ok thank you for the reply. I still keep in contact with the guy I bought the bike from and he said it wouldn't hurt to go one fatter on the main. So after cleaning the carb and checking for air leaks the main will be my next step

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It does look a little lean. You said your plugs are tan and dry which is a indication of a lean setting. You want more of a brownish slight tan color. Did you clean the carb or rule out any lean conditions such as air leaks with the new top end?

Although you are mixing your race gas but c-12 is not a good 2 stroke fuel, simply because it has a low gravity range that does not keep the oil suspended and has way to much oxygenation. I personally have best results with AV gas and also used U4.4 with great results but cant justify the price when AV performs just as well. My suggestion go a 50/50 AV/U4.4 and premium pump, clean your your carb and do a plug chop. If you get a slight darker color on the plug that might be your winning ticket. Hope this helps

I believe C-12 was designed for 2 stroke racing engines. Also it is non oxygenated. I don't know about the gravity level. You may have something there. But if a fuel was designed for and used frequently for 2 stroke engines I would assume it has characteristics to work in conjunction with 2 stroke premix. But after doing a little research on MXA I did discover that C-12 should be ran in highly modified 2 strokes which I do not have so I am gonna ditch that fuel for now too. It says it runs too cool for stock engines.

Edited by Bremer120
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Anybody else have any ideas? 2005 YZ250 brand new motor, DEP pipe, stock silencer, VForce3, jetting=185, needle clip is centered all is stock except main jet, pilot air screw is 2 turns out. ImageUploadedByThumper Talk1378311483.623331.jpg

Plug may look a little lean? BR8EIX

The bike starts and runs perfect and is very responsive but that piston looks bad to me for such low hours

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My KTM 200 polished a ring and lost compression from being too lean at the Idaho City 100. You may be able to get away with just a new ring on that but you may also need to hone the cylinder if it looks glazed. I would also advise at least dropping the clip or go one bigger main like you said earlier. Although 185 seems pretty fat as it is....Hmm. 

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The picture supplied is looking up at the piston from the exhaust port.

 

 

 

Well that picture alone does not give enough information for me to see if the piston has a problem or not. I like to see the head and the piston crown of both sides.

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Thank you for the reply muffintop. I got a size fatter main today and also talked at the guys at the local Yamaha dealership and I got a few more ideas I'm gonna try. First I'm gonna check for air leaks including crank seals. And next I'm gonna take it apart because after looking at it most of the day it appears the top ring may be squished in the ring groove. While its apart I'm going to inspect the power valve and make sure it isn't coming into contact with the piston

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I think I figured it out. I sprayed the base of the cylinder, reed cage, and all around the carb with carb spray in search of an air leak and didn't find anything then I pulled the head and discovered the power valve had a shiny spot where it came in contact with the piston. Sadly this is the power valve from the old motor so I can't say for sure if this mark was a pre existing condition but I will be disassembling and milling the power valve at the minimum. ImageUploadedByThumper Talk1378339648.558507.jpgImageUploadedByThumper Talk1378339684.096134.jpg

Edited by Bremer120

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I polish the @#$% out of everything when I put a motor back together.  Make sure the ring lands are clean and not smashed or have burrs.  I polish the dome of the piston too.  Harder for carbon to build up on a slick surface.  I smooth the edges on everything especially the PV.  Time, patience and a small file and some different sandpaper grits and you will do just fine.  I have heard that some have had issues with the PV hitting the piston, might be what happened to you???  Check clearance.

 

As for your plug, it might be a tad lean but not bad.  Might be on the edge.  I ran a banshee HARD for 4-5 years with plugs like that.  It has a lot to do with the oil too.  I am a firm believer in KLOTZ..  I have run it for more than 3 decades.  Ya, I am an old fart.  As for the fuel...c-12 is not oxygenated fuel.  Oil should stay in suspension unless there is a compatibility issue.  The gravity, or more correct specific gravity, is the measurement of the density of fuel and has nothing to do with oil suspension.  Lighter fuels like AV gas need a larger jets. How much is up to you.  

 

Keep an eye on the plugs.  It is always safer and cheaper to error on the side of rich.  Plugs are cheap.  Remember that the only lube the crank gets is the fuel and the oil suspended in it.

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Thank you very much mrjata! I take a ton of time putting motors together from V8s to jap bikes. I measured all parts and chamfered all edges with proper grit paper specified in the manual. Wash ALL parts with soap and water, lube all parts immediately after, then take my time building. I have OCD when it comes to motors that's why I took the pipe off just to take a peek. I check plugs every time I ride that is why I'm so confused. I do think the power valve is the culprit and I will be milling some material off of it before reassembly. Other than that I am stumped. Only other thing I can think of is possibly a bent crank? The piston I pulled out prior to this motor had to have the wrist pin beat out with a hammer! I have a feeling that the pin got to hot. But what caused this I have no clue. The same symptoms the old motor had are coming back. I'm just trying to figure out what is causing this. It's driving me nuts! Jetting is crisp and the bike runs amazing. Fuel is always fresh and mixture is precise. All parts when I pulled it apart were covered with a film of oil and oil dripped out of the exhaust port.

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I am surprised to see your plug that light colored and that much carbon on the piston (especially in only 4 hours) with such a rich main jet and clip setting.  Seems something is making it lean.  Possibly leaking main seals.

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I am surprised to see your plug that light colored and that much carbon on the piston (especially in only 4 hours) with such a rich main jet and clip setting. Seems something is making it lean. Possibly leaking main seals.

If I had leaky main seals do you think I would have a hanging idle or poor performance?

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I was thinking that was a reflection. Looks like piston is polished?

It appears polished because of some sort of wear.

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If I had leaky main seals do you think I would have a hanging idle or poor performance?

 

Possibly, unless your are basically covering up the symptoms with that unusually rich jetting.

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Possibly, unless your are basically covering up the symptoms with that unusually rich jetting.

Yeah that makes sense to me. I did take the ignition cover off and ran the bike and sprayed carb clean all around behind the flywheel as best I could and didn't find a difference in rpm. If the clutch side seal was leaking wouldn't it be smoking pretty bad? Possibly drawling in tranny fluid?

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