YZ250-Extreme Pitting Problems

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1/9/2015 7:49 AM

Hoping you guys can give me some insight into this! 09 YZ I bought last Jan, Did a top end this summer after it started losing power/compression. Noticed some slight pitting, thought maybe it was detonating due to pump gas and advanced timing so I set the timing back to the stock ticker and threw everything back together.

Fast forward to 12 hrs on the bike later and I pull the top end to find this:

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I've also noticed a spike in rev's for 5-10 seconds when I start the bike cold which some seem to attribute to a leaky crank seal. I want to start with a solid baseline to eliminate factors, so as of now my plan is new top end, crank seals, stock pipe w/new o-rings, make sure carb has stock jets. Put everything back together and keep an eye on things every few hours.

Could a lean condition caused by a leaking crank seal cause this damage? Running a stock setup I should be fine on 93 octane without having to worry about detonation correct? Any other ideas on what I should check or keep an eye out for?

Thanks!

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1/9/2015 8:41 AM

Yes a leaking crank seal on the stator side could do that. A warped head can also cause it and I see sighns of that in the photos.
Remember that those bike have a reputation of running right on the ragged edge of detonation right from the factory. In most cases the squash is not correct, or running lean or poor quality fuel will be the cause.
I have seen a lot of these over the years and the best fix is to have the squash checked and corrected.

Paw Paw

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1/9/2015 8:48 AM

Paw Paw 271 wrote:

Yes a leaking crank seal on the stator side could do that. A warped head can also cause it and I see sighns of that in the photos.
Remember that those bike have a reputation of running right on the ragged edge of detonation right from the factory. In most cases the squash is not correct, or running lean or poor quality fuel will be the cause.
I have seen a lot of these over the years and the best fix is to have the squash checked and corrected.

Paw Paw

I have heard these engines can be finicky on pump. Luckily I have the cylinder and head at Powerseal USA for repair right now, I'll see what they can do with the head to fix the sqish gap.

Thanks!

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1/9/2015 9:01 AM

Even with a stock setup, mix 1 gallon of race gas with 2 gallons of 93 pump. These bikes detonate like crazy right from the factory. And, check your timing with a dial gauge. Dont trust the factory marks on the stator.

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1/9/2015 11:25 AM

HackMan162 wrote:

Even with a stock setup, mix 1 gallon of race gas with 2 gallons of 93 pump. These bikes detonate like crazy right from the factory. And, check your timing with a dial gauge. Dont trust the factory marks on the stator.

I just ordered a dial gauge so I can get the timing spot on. Powerseal also said they'll cut .010" out of the head squish. Between those and a new crank seal I should be good. Thanks!

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1/9/2015 12:33 PM

Okay here's a little lesson on fuel and how it's not the cause. I wouldn't be so sure about pump gas is the problem. The bike is designed to run on 91 octane or higher. That being said, the factory has decided that if you run 91 octane (or higher) you will have no problem with detonation, pre ignition, or pinging at factory settings. Don't waste your money on race gas, unless you have compression mods or anything that will cause higher temperatures in the engine. Also, the higher in elevation you get, the lower octane fuel you can use because the atmosphere is thinner. Thinner air, less air, less compression, lower minimum octane. I'd say if you're above 3,000' you can get away with 89 octane, however its damn near impossible to find non-ethanol 89 octane. Also remember that Ethanol will reduce the octane number. General rule of thumb is that 10% Ethanol will reduce the octane rating by 1 or 2. It's still labeled as 91 w/ 10% Ethanol, but it will act as an 89 or 90 octane, in addition to rotting any rubber components including: fuel lines, gaskets, engine seals, bearing seals, and depending on how long it sits it could even rot your plastic fuel tank.

In conclusion, the cylinder head repair should fix the problem. If not, rebuild the bottom end and check all tolerances for factory spec and follow all the factory guidelines for replacing parts. It's also likely that crank seals are bad like everyone has been saying.

Hope I could help a little bit!

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1/9/2015 1:03 PM

if you anything above a beginner you need to at least mix race fuel, the YZ's will not run on pump and live

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1/9/2015 1:34 PM

Is that a crack I see in the left side case?

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1/9/2015 6:32 PM

DoctorJD wrote:

Is that a crack I see in the left side case?

That's just leftover from the casting, they look like cracks, but are usually raised up like a casting seam.

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1/10/2015 2:53 AM

When a crank seal is suspect, the best thing to do is a basic pressure test of the engine, before any disassembly of the engine begins. Test again after reassembly. A two stroke is dependent on a well sealed assembly.

You could have a perfectly good crank seal in place , or knacker up the new one upon installation. Without a pressure check, how would one know ?

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1/10/2015 3:49 AM
Edited Date/Time: 1/10/2015 3:57 AM

you must run at least 97-98 octane for a yz250. this is specified in the manual.
anything less and your engine wont last long, luckily i can get 98 from the pump at BP here, non ethanol.

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1/10/2015 6:21 AM

run vp c12 mixed with yamalube 32:1 and you will not have this problem !!!

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when life throws you a curve throw a leg over a bike and rail that bitch

1/10/2015 8:49 AM

Paw Paw 271 wrote:

Yes a leaking crank seal on the stator side could do that. A warped head can also cause it and I see sighns of that in the photos.
Remember that those bike have a reputation of running right on the ragged edge of detonation right from the factory. In most cases the squash is not correct, or running lean or poor quality fuel will be the cause.
I have seen a lot of these over the years and the best fix is to have the squash checked and corrected.

Paw Paw

Does this apply to YZ125's? I'm referring to factory set up on ragged edge for lean/ detonation and high octane of > 93. What does squash mean? I assume it means a true seat all the way around circumference of mounting? I have a 2005 YZ125. How do you recognize leaking crank seal and is that common?

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1/10/2015 8:50 AM

Get your squish and chamber re-done and it'll have a long life on pump gas. The $80 you invest in that will save you 10 fold in having to use race fuel in the future.

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1/10/2015 10:25 AM

Looks like you had a leaky o-ring. When the water turns to steam, it will do that to the edge of the cylinder and head.

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1/11/2015 4:06 PM

Matt Fisher wrote:

Looks like you had a leaky o-ring. When the water turns to steam, it will do that to the edge of the cylinder and head.

I think the o-ring got eaten away as the pitting propogated towards it...definitely didn't help things once that started happening. Good thing I tore it down when I did instead of continuing to ride.

I'll definitely keep an eye on things once I get it back together, but getting the squish fixed and crank seal replaced should get me headed in the right direction.

Thanks for all the replies!

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1/12/2015 6:57 AM

I think you're right that the detonation eroded away at the o-ring. I think the detonation was caused by a bad combination of lean jetting and fuel selection.

You absolutely can run 91 pump in a stock YZ250 with no problems. However, you will have to keep your jetting on the rich side to prevent detonation so performance will not be ideal. It will still be a strong running bike, though.

What was your jetting when this happened? The carbon pattern on the piston and head suggest lean jetting. Post us a pic of the underside of the piston also. I've seen a lot of guys running ridiculously lean jetting on their YZ250's. Like main jets in the 160-168 range (178 is stock).

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Powerband in every gear !

1/12/2015 7:04 AM

FGR01 wrote:

I think you're right that the detonation eroded away at the o-ring. I think the detonation was caused by a bad combination of lean jetting and fuel selection.

You absolutely can run 91 pump in a stock YZ250 with no problems. However, you will have to keep your jetting on the rich side to prevent detonation so performance will not be ideal. It will still be a strong running bike, though.

What was your jetting when this happened? The carbon pattern on the piston and head suggest lean jetting. Post us a pic of the underside of the piston also. I've seen a lot of guys running ridiculously lean jetting on their YZ250's. Like main jets in the 160-168 range (178 is stock).

I'll dig into the carb and get some pics of the piston this evening. Not sure on the jets since I bought the bike used. Is there a way you can differentiate between lean jetting and lean condition caused by a leaking crank seal (based on observations of the carbon pattern)?

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1/12/2015 8:04 AM

YZ250's have been that way for 15 years..... My YZ pinged bone stock with rich factory jetting. I have to run at least 50/50 race and pump gas.

Could you not hear it pinging?

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1/12/2015 9:15 AM

simmons97 wrote:

Okay here's a little lesson on fuel and how it's not the cause. I wouldn't be so sure about pump gas is the problem. The bike is designed to run on 91 octane or higher. That being said, the factory has decided that if you run 91 octane (or higher) you will have no problem with detonation, pre ignition, or pinging at factory settings. Don't waste your money on race gas, unless you have compression mods or anything that will cause higher temperatures in the engine. Also, the higher in elevation you get, the lower octane fuel you can use because the atmosphere is thinner. Thinner air, less air, less compression, lower minimum octane. I'd say if you're above 3,000' you can get away with 89 octane, however its damn near impossible to find non-ethanol 89 octane. Also remember that Ethanol will reduce the octane number. General rule of thumb is that 10% Ethanol will reduce the octane rating by 1 or 2. It's still labeled as 91 w/ 10% Ethanol, but it will act as an 89 or 90 octane, in addition to rotting any rubber components including: fuel lines, gaskets, engine seals, bearing seals, and depending on how long it sits it could even rot your plastic fuel tank.

In conclusion, the cylinder head repair should fix the problem. If not, rebuild the bottom end and check all tolerances for factory spec and follow all the factory guidelines for replacing parts. It's also likely that crank seals are bad like everyone has been saying.

Hope I could help a little bit!

The manual calls for 100 octane.

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1/12/2015 11:47 AM

i could hear mine making all sorts of funny noises the 1 time i tried 95 pump gas, like a knocking noise right as it was getting into powerband.

put the 98 pump back in and 100hours later smile, i do have a new piston sitting there ready to go in lol.

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1/12/2015 3:04 PM

ridinbeast932 wrote:

I'll dig into the carb and get some pics of the piston this evening. Not sure on the jets since I bought the bike used. Is there a way you can differentiate between lean jetting and lean condition caused by a leaking crank seal (based on observations of the carbon pattern)?

No, the carbon pattern alone won't allow you to tell what exactly caused the lean condition. You need to analyze all factors and like others have said, a leak-down test is the ultimate answer on leaky crank seals.

Jetting is everything on 2-strokes. Never buy a used bike and just trust what is in it. One of the first things you should do is take the carb apart, clean it well, and record the jetting and then adjust from there as needed.

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Powerband in every gear !

1/12/2015 7:06 PM

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1/12/2015 10:21 PM

I say go stock on jetting, maybe go to a 180 main from the stock 178. I put a lot of hours on my yz, and what I lack in speed I make up for in jump revving and not shifting. 300+ hours since I bought it new in 08 and not a single issue, never even cracked the carb open.

I have put 6 top ends through it, it just never blew up. Lost a transmission too, but thats not really fuel related. I run 50/50 pump with 110 plus 36(ish):1 with maxima superM.

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1/13/2015 8:34 AM

ok, so your jetting is all stock except the pilot (48) is one step down from stock (50). Very reasonable jetting and likely not the cause of your lean condition. Although in colder months (and given your elevation) you could likely use a 180-182 main in the winter.

The very pronounced dark spot under the piston indicates high heat and a very lean condition. I suspect an air leak. Could be the crank seals, reed block, cylinder base, etc. Need to rebuild, reseal everything, and pressure test.

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Powerband in every gear !

1/13/2015 9:04 AM

FGR01 wrote:

ok, so your jetting is all stock except the pilot (48) is one step down from stock (50). Very reasonable jetting and likely not the cause of your lean condition. Although in colder months (and given your elevation) you could likely use a 180-182 main in the winter.

The very pronounced dark spot under the piston indicates high heat and a very lean condition. I suspect an air leak. Could be the crank seals, reed block, cylinder base, etc. Need to rebuild, reseal everything, and pressure test.

I put in a parts order last night and picked up a stock pilot, wish I would've seen this sooner and I would've got a bigger main too. Which pilot would I be better off starting with 48 or 50?

Definitely planning to reseal with all new gaskets and I found a great pressure tester DIY so I'll give that a shot.
http://www.thumpertalk.com/topic/1061686-build-your-own-diy-yz250-leak-down-tester-pics-included/

Any suggestions on premix ratio? Been running 50:1 but thinking maybe go down to 40:1?

Thanks again for all the replies, lots of useful info!

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1/13/2015 9:28 AM
Edited Date/Time: 1/13/2015 9:32 AM

Paw Paw 271 wrote:

Yes a leaking crank seal on the stator side could do that. A warped head can also cause it and I see sighns of that in the photos.
Remember that those bike have a reputation of running right on the ragged edge of detonation right from the factory. In most cases the squash is not correct, or running lean or poor quality fuel will be the cause.
I have seen a lot of these over the years and the best fix is to have the squash checked and corrected.

Paw Paw

I'm with Paw Paw, It looks the same to me,..i.e., coolant ingestion and light detonation. It's definitely a good time to take a good, close look at everything! I would also mention that while the head mods should fix the squish issue, and enable you to run pump gas, keep in mind the basis for this engine design, consider running race fuel or a percentage at least. As any decent tech will tell you, it's not just the octane, but the quality of the fuel, and the consistencies that come along with using quality products in a race machine. There's always someone that's going to boast they've been using chain saw oil and old lawn mower gas forever without problems, but just smile at them and make an educated decision for yourself, LOL! I always run 40:1 in my 2-strokes, but I use Klotz SuperTechniplate, and C12/Premium Chevron mixed

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1/13/2015 10:13 AM

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Pump gas will cause this problem on the "modern" YZ 250's every time.

50/50 pump and race gas mix and you will be fine. 32:1 is a good ratio to mix.

If a crank seal was leaking you will loose the bottom end crank bearing most of the time.

.

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Arkmx.
Jim G.

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1/13/2015 11:04 AM

Jim, not always true. One of my riding buds has a 2014. Stock jetting, 91 pump and Yamalube at 32:1. No detonation at all. Bike runs great. Like others have said, the stock timing marks are known to be suspect and some bikes come delivered with the timing actually a little advanced even though the mark is lined up.

Not that I am advocating for pump gas. I run 100 in my YZ's just because it is cleaner and crisper and allows me to jet a bit leaner and not worry about spooge or detonation.

I have a stock 50 pilot in mine and it is fine even when I ride at 3,000 feet. At 4,500 feet it gets a bit of a rich stutter but is still very rideable. So, I would start with the 50 if I was you. I would also start with a 180 or 182 and go from there. It's always best to start on the rich side versus the lean side.

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Powerband in every gear !

1/13/2015 5:02 PM

In 2011, Yamaha took 0.5cc out of the combustion chamber to lower the compression and make the YZ250 more pump gas friendly.

If you are running an older YZ on straight pump, you are playing Russian Roulette.

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