YZ250-Extreme Pitting Problems

Arkmx wrote:

. Thanks for the info. We ran our 06 on the 50/50 blend at 32:1 with no problems. I would not do any less than a...

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Thanks for the info.

We ran our 06 on the 50/50 blend at 32:1 with no problems.

I would not do any less than a 50/50 mix on the stock YZ 250, but that's me for an safety / insurance piece of mind.

My oldest stepson Blake used some of the VP U4 or similar (on a friends advice) back in 05 on his new YZ 250 and it did the same thing as the above photos! Do not run straight oxygenated race fuel in a two stroke! He learned the hard was in just a few hours...

Cheers!

FGR01 wrote:

Also, with the U4, not only the oxygenation, but weren't some of the earlier versions actually rather low octane? in that case, you would get the...

Also, with the U4, not only the oxygenation, but weren't some of the earlier versions actually rather low octane? in that case, you would get the worst of both worlds in a YZ250. The low octane and the high oxygen which would effectively lean your jetting if you did not re-jet to compensate.

CamP, I know they slightly lowered the compression in 2011 (it was actually to make the bike 1-world standard, same as the Euro, AUS bikes, etc along with the stupid long muffler), but even my 2006 will run great on 91 pump gas provided I jet for it. But then I will get a slight bit of spooge and slightly less crisp throttle response and the combustion chamber and power valve get more gunked up. So, I just stick with the 100 and it runs great.

FGR01 wrote:

Also, with the U4, not only the oxygenation, but weren't some of the earlier versions actually rather low octane? in that case, you would get the...

Also, with the U4, not only the oxygenation, but weren't some of the earlier versions actually rather low octane? in that case, you would get the worst of both worlds in a YZ250. The low octane and the high oxygen which would effectively lean your jetting if you did not re-jet to compensate.

CamP, I know they slightly lowered the compression in 2011 (it was actually to make the bike 1-world standard, same as the Euro, AUS bikes, etc along with the stupid long muffler), but even my 2006 will run great on 91 pump gas provided I jet for it. But then I will get a slight bit of spooge and slightly less crisp throttle response and the combustion chamber and power valve get more gunked up. So, I just stick with the 100 and it runs great.

CamP wrote:

There are a number of factors to consider. We know the YZ's compression ratio is on the ragged edge end of detonation with pump premium gas...

There are a number of factors to consider. We know the YZ's compression ratio is on the ragged edge end of detonation with pump premium gas. This makes fuel quality and elevation critical. If you are at high elevation, you won't need as much octane as someone at sea level. Fuel quality varies from city to city and state to state. 91 in CA isn't the same as 91 in TX, and the refineries change their formulas in winter/summer, which leaves you chasing your jetting tail several times a year. Also, if you are pumping fuel from a station that uses one hoses for all fuel grades, you'll end up with some 87 octane fuel in your jug as it clears the pump/hose. That means if you buy only 2 gallons of premium, you could actually end up with something closer to 89 octane in your jug.

You can throw fuel at the problem, but as you stated, you end up over-rich. The best option is to add enough good fuel to allow crisp jetting without detonation. My personal solution is to run 100LL avgas in all my stock compression 2-strokes and 110 race gas in my high compression bikes. That way I have enough safety factor to jet crisp, and my baseline never changes.

For the guys that are bound to pump gas, I'd suggest that they have the head milled to add volume, and reduce compression. That way they can have the peace of mind that their bikes will never burn down due to insufficient octane.

simmons97 wrote:

I would agree on that except for two things. First, milling the head is going to increase compression. You're essentially making the combustion chamber smaller, so...

I would agree on that except for two things. First, milling the head is going to increase compression. You're essentially making the combustion chamber smaller, so the same amount of air inside the cylinder must be compressed to an even smaller area. So milling the head will make your bike require I higher octane fuel. Second is more of a recommendation, if I were you, I'd be totally comfortable running 100LL in a mod bike. I actually run it in my mod 125 and have easy access to it as I'm also a pilot 100LL (because it is leaded) actually has the octane quality of a 110-130 octane fuel. This is confusing to some people and they say "well that must be crappy fuel because it's so inconsistent" false, in aircraft we are constantly changing the "jetting" (we call it mixture because there are no jets) I'm flight. At a rich setting (used for full power takeoff) it is 130 octane. But at a very very lean setting (cruise flight) it is about 110 octane. You can generalize to about 120 for a good jetting on a motorcycle.

In conclusion, 100LL is a great fuel to use in any race application given: you can run leaded fuel if you're in a race, and you jet slightly richer due to AvGas' lower specific gravity. It is better than most VP fuels and I'm pretty sure that it worked out to be slightly better than VP's C12.
Feel free to ask questions about AvGas because it is a highly misunderstood topic...

CamP wrote:

Sorry for the confusion. When I said to mill the head for more volume, I more specifically meant to mill the head's dome, not the gasket...

Sorry for the confusion. When I said to mill the head for more volume, I more specifically meant to mill the head's dome, not the gasket surface.

I've been running 100LL for 35 years. It is great 2-stroke fuel, but it does not have enough octane required for the ultra-high compression in my mod CR. When measured like auto fuel, 100LL has an R+M/2 closer to 99 octane, as told to me by a fuel engineer. The R+M/2 octane requirement for my CR is closer to 104, so I have been mixing 100LL and VP110 50:50 to keep that engine healthy.

Dragoo168 wrote:

That's because standard AvGas isn't formulated for performance as much as to be a clean standard for the elevations at which planes fly,..it's the fuel quality...

That's because standard AvGas isn't formulated for performance as much as to be a clean standard for the elevations at which planes fly,..it's the fuel quality, more than the octane (That too, depending what type you buy) that has been the main advantage for motorctcles.

Micahdogg wrote:

Regardless, that spotting pattern on the piston and head, are caused from detonation. That is liquid metal. Increasing your octane is a good start - especially...

Regardless, that spotting pattern on the piston and head, are caused from detonation. That is liquid metal.

Increasing your octane is a good start - especially since your jetting appeared to be reasonable. You guys also have to remember, every pump in the country that says 93 on the handle isn't going to have the same fuel. There are bad batches and there are tanks out there with all kinds of contamination. For all you know, it could have been 87 octane coming out of the handle. Running at the ragged edge is also going to leave you no room for bad gas, for a jetting error, for operating the bike in too cold of a temperature, for a mechanical issue (powervalve binding). It is asking for trouble.

In theory, that bike should be able to run on 93 octane. I guess you could say it hasn't blown up yet, so it's "working." My first start would be to do a 50/50 mix of your most reasonable race gas in the area. If it's a crank seal leak, you'll figure that out soon enough if it keeps acting up.

ridinbeast932 wrote:

I think I will throw a 50 and 180 in the carb and mix premix 40:1. That should give me a safe starting point. Then do...

I think I will throw a 50 and 180 in the carb and mix premix 40:1. That should give me a safe starting point. Then do a pressure test to determine if something other than jetting (crank seals?) was contributing to my lean condition. I'll have to decide if I want to try starting with straight pump or not...keep in mind powerseal is cutting .010" out of the squish. I'm not sure what that equates to in the grand scheme of things but that should definitely help. Also ordered a dial guage so I can get my timing spot on.

Anyone see any problems with my "game plan"?

Thanks again for the replies!

simmons97 wrote:

Agreed entirely. I've always had a personal mental block with mixing fuels though. You never know if its TRULY 50/50. But in a case like this...

Agreed entirely. I've always had a personal mental block with mixing fuels though. You never know if its TRULY 50/50. But in a case like this I'm sure it wouldn't matter. However it may be cheaper to use Avgas (mixing it or not) as it's only $5.50 a gallon where I'm at. Only trouble is getting your hands on it. Generally airports don't let people off the street into the airport. Too much liability. On top of that, if its a full service fueler, they probably won't pump into a can for you and it would be illegal for them to put it into your car because it doesn't have road taxes on it. My recommendation if you wanted to use Avgas would be to go to a local municipal airport and get a hold of someone there and explain your intentions and they'll probably help you out.
Anyways, Micahdogg is right, start with higher fuel grade, then check other things if problems persist.

Dragoo168 wrote:

That's because standard AvGas isn't formulated for performance as much as to be a clean standard for the elevations at which planes fly,..it's the fuel quality...

That's because standard AvGas isn't formulated for performance as much as to be a clean standard for the elevations at which planes fly,..it's the fuel quality, more than the octane (That too, depending what type you buy) that has been the main advantage for motorctcles.

simmons97 wrote:

False, aircraft engines have higher compression ratios than a lot of stock bikes. The helicopter I fly has a compression ratio of 10.8:1 far more than...

False, aircraft engines have higher compression ratios than a lot of stock bikes. The helicopter I fly has a compression ratio of 10.8:1 far more than the YZ250's 8.9:1 at higher rpm and still slightly more than its 10.6:1 at low rpm (due to the power valves.)
It has nothing to do with elevation, when we go up in altitude in the helicopter or airplane, we just simply lean the fuel-air ratio.

Dragoo168 wrote:

That's because standard AvGas isn't formulated for performance as much as to be a clean standard for the elevations at which planes fly,..it's the fuel quality...

That's because standard AvGas isn't formulated for performance as much as to be a clean standard for the elevations at which planes fly,..it's the fuel quality, more than the octane (That too, depending what type you buy) that has been the main advantage for motorctcles.

simmons97 wrote:

False, aircraft engines have higher compression ratios than a lot of stock bikes. The helicopter I fly has a compression ratio of 10.8:1 far more than...

False, aircraft engines have higher compression ratios than a lot of stock bikes. The helicopter I fly has a compression ratio of 10.8:1 far more than the YZ250's 8.9:1 at higher rpm and still slightly more than its 10.6:1 at low rpm (due to the power valves.)
It has nothing to do with elevation, when we go up in altitude in the helicopter or airplane, we just simply lean the fuel-air ratio.

Dragoo168 wrote:

Not False, but I'm not going to get into a pissing match over aviation knowledge, or Motorcycle engine knowledge, but I was referring to standard (lower)...

Not False, but I'm not going to get into a pissing match over aviation knowledge, or Motorcycle engine knowledge, but I was referring to standard (lower) grade AvGas, which caters to most (Light) aircraft that ARE low compression engines. Once upon a time, airports used to stock different octane levels of avgas - they dyed the fuel different colors so you could visually identify them.

Most of the lower power, normally aspirated planes, used 80 octane fuel, which was red. Larger and turbocharged engines required higher octane, leaded 100/130 was green. You could not run the 100/130 consistently in the smaller planes, as the amount of lead tended to foul spark plugs. When 100LL (blue) was introduced, it was used as a replacement for both types. Many small planes do not really need 100 octane, but lower octane avgas is not produced, there is not enough of a market to justify it.

Many small planes run fine on automotive fuel, but it does require an Supplemental Type Certificate (STC) and formal approval. In most cases the only modification required is changing the placards on the fuel caps. One problem with current mogas is that in many states it contains a large among of ethanol, which is a problem if it is to be used in a plane. Another is that few airports stock auto fuel, for logistical and liability reasons: it requires separate tanks and pumps, and if they were to pump mogas into the wrong plane and there were any issues, they would be sued.

A side note: many old military piston engines were designed to run on 115/145 (purple). Since it is no longer commonly available, the examples you see at airshows have to limit throttle settings to allow the engines to survive on 100LL.

Another characteristic is in how it is refined, to aid in preventing vaporlock!

Regardless, I was merely supporting what CamP was saying in that AvGas has been a great fuel in 2-strokes for a long time, and personally Reiterating the emphasis on the benefits of fuel quality as well as octane. While many should learn to utilize density readings to tune, the simplest place to start is with the suggestion of a basic idea to use a fuel (In any bike), that will provide consistency in performance.

CamP wrote:

I get my 100LL from a couple of smaller airports with self serve pumps. They both have open pedestrian access to the pumps through unlocked gates...

I get my 100LL from a couple of smaller airports with self serve pumps. They both have open pedestrian access to the pumps through unlocked gates. Running $4.40/gal right now.

ridinbeast932 wrote:

Got the engine back together and it won't start, I feel like such a newb! Granted, its been 3+ yrs since I've done my last rebuild...

Got the engine back together and it won't start, I feel like such a newb! Granted, its been 3+ yrs since I've done my last rebuild. Almost seems like it's not getting any spark, like it won't even try to start.

To summarize, cleaned carb and installed 180 main and 50 pilot. New wiseco piston, crank, bearings, seals, etc. Set timing to stock at .007". Going to start troubleshooting after work but figured I'd see if anyone had any obvious ideas.