2 stroke experts...what happened??!!

[Derrick Adams]

I bought my 83 250R and it ran, but when it warmed up it would hardly take off. Took off the head and the cylinder and piston were trashed. I had the cylinder bored and installed a new piston assembly.

After around 3 hours of riding, the bike died and wouldn't restart. I pulled the head and this is what I found.

So what it causing this? If I didn't know better i'd swear it was from running gas with no premix, but that's impossible. Could a bad rod bearing cause this?

I'm not sure what to do. I bought another cylinder, but I don't want to have it bored and install another piston only to have it happen again.

[deathman53]

crank seals?

[250RNUT]

could be from air leaks causing a lean condition,running very hot, piston expanding more than the bore(improper bore/cylinder tolerences when bored).just my .02

[Dammit!]

Really hard to see from the pic but, are your rings lined up right? Doesn't look like it.

[Huffa]

could be from air leaks causing a lean condition,running very hot, piston expanding more than the bore(improper bore/cylinder tolerences when bored).just my .02


I was thinking the same thing.

How did it idle after rebuild?

[Derrick Adams]

Rings were lined up fine. Idled perfect. No issues at all, until it died.

[deathman53]

I had the crank seals go on my atc250r and caused my 330 kit to do the same thing., bad crank seals will cause a very lean condition

[KASEY]

wow you couldn't hear the detination??? looks like 5 more minutes and you would have burnt a hole in the piston. the BIGGEST problem i see is people don't know what a LEAN running engine sound like . it was srceaming for a long time before the melt down,,,,,,,,,,,,, :D :D :D

[TimSr]

Im having trouble telling from the picture, whether its begun eating the top center of the piston from a lean condition, of if its just heavily carboned with an uncoated spot in the middle. Is the bare spot in the center smooth, or does it look rough like it was sand blasted? If its etching the piston its definately lean, from several possibilities. If its craboning up that bad, and just hasnt coated the center yet, Im more inclined to think it was an incorrect bore tolerance, possibly bored for a cast piston instead of a forged one.

[Troll 2]

Looks like a lack of oil problem, I found a read and I go by it that basicly says theres a oil level in a two-stroke. By the looks of your pics there should be oil present on the crank. At leasty if you rotate it around. I'll post the original artical.
Then another one with a different view point. Any good luck........



Here's a come-back to the two-stroke oil theory;
Original post;
I found it here;
http://www.motocross.com/forum/topic.asp?TOPIC_ID=21626
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Pre-mix 101

OK, looks like it's time for a little pre-mix 101. I don't usually get into ratio discussions, because mix ratios are like religions to most people, and they tend to be closed-minded on the subject, but I'll put in my $.02 here anyway.

There is a prevailing myth that less oil is better, and that the oil in the fuel is what lubricates the engine. Both are wrong. The engine is lubricated by the residual oil that builds up in the crankcase. All the oil in the fuel does is replenish this oil.

The best way to determine if you are running enough oil is to check the level of the residual oil in the crankcase. If the ratio you run leaves enough residual oil in the crankcase to cover about 1/8" of the bottom of the crank wheels, then you are fine. If you don't have that much residual oil in your crankcase when you pull the top-end off, you aren't running enough oil for your riding style and conditions.
With that said, to have that amount of residual oil in the crankcase at 50:1 (a ratio made popular by magazines and oil bottles), you can't be riding very hard, or your bike is jetted richer than necessary simply to deliver enough oil. I arrived at 26:1 for my bike with my riding style because that is the amount that gives me the proper amount of residual build-up. Small-bore engines require greater oil concentrations than larger engines to achieve the proper amount of residual build-up, because they rev higher and have higher intake velocities. Along the same lines, someone that pushes the engine harder, and keeps the revs higher, also needs to use higher oil concentrations to achieve the proper residual build-up.

To understand why the residual oil is so important, you have to understand what happens to the oil in your fuel when it goes into the engine. While the oil is still suspended in the liquid gasoline, it can not lubricate anything. It has about as much lubricity at that point as straight gasoline. When the gasoline enters the engine, it evaporates, dropping the oil out of suspension. Now that the oil is free, it can lubricate the engine, but it must get to the parts to lubricate them. The way it gets to the bearings and onto the cylinder is by being thrown around by the spinning crankshaft. Some of the oil eventually makes it into the combustion chamber, where it is either burned, or passes out the exhaust. If the combustion chamber temps are too low, such as in an engine that is jetted too rich, the oil doesn't burn completely. Instead, some of it hardens into deposits in the combustion chamber, on the piston, and on the power valve assembly. The rest becomes the dreaded "spooge". The key to all of this working in harmony is to jet the bike lean enough to achieve a high enough combustion chamber temperature to burn the oil, but also still be able to supply enough oil to protect the engine. If you use enough oil, you can jet the bike at it's optimum without starving the engine of oil, and have excellent power, with minimal deposits and spooge. At 50:1, you simply can't jet very lean without risking a seized engine due to oil starvation.

With the high oil concentrations that I use, I tend to get far more life from my cranks and rings than most of my friends that run leaner oil ratios. The high oil content also produces better ring sealing, so more of the combustion pressure is retained.

One small point. No one ever broke an engine by using too much oil.

__________________________________________________ ____________________________________

Pre-mix Ratios and Horsepower Production

I have run Dyno tests on this subject. We used a Dynojet dynamometer, and used a fresh, broken in top-end for each test. We used specially calibrated jets to ensure the fuel flow was identical with each different ratio, and warmed the engine at 3000 rpm for 3 minutes before each run. Our tests were performed in the rpm range of 2500 to 9000 rpm, with the power peak of our test bike (an '86 YZ 250) occuring at 8750 rpm. We tested at 76 degrees F, at 65% relative humidity. We started at 10:1, and went to 100:1. Our results showed that a two-stroke engine makes its best power at 18:1. Any more oil than that, and the engine ran poorly, because we didn't have any jets rich enough to compensate for that much oil in the fuel. The power loss from 18:1 to 32:1 was approximately 2 percent. The loss from 18:1 to 50:1 was nearly 9 percent. On a modern 250, that can be as much as 4 horsepower. The loss from 18:1 to 100:1 was nearly 18 percent. The reason for the difference in output is simple. More oil provides a better seal between the ring and the cylinder wall.

Now, I realize that 18:1 is impractical unless you ride your engine all-out, keeping it pinned at all times. But running reasonable ratios no less than 32:1 will produce more power, and give your engine better protection, thus making it perform better for longer.




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Another way of thinking found here; http://www.macdizzy.com/oil2gas.htm



If you get the opportunity to take apart an engine very often you may notice that the bottom end of a two stroke almost always has a substantial amount of oil laying in the cases. Even in engines that have been run at leaner oil to gas ratios (50:1 to 100:1) there is plenty of oil. The other internal parts of the engine also are well coated wet with oil. It doesn't matter if the motor is air cooled or liquid cooled - the internals of the engine are still coated. Much of this oil may accumulate at idle and during periods of low rpm running. Once you get the motor spinning faster and it is under load, that extra oil in the case may finally have a chance to become suspended again in the fuel and air mixture and perhaps be burned.

Many people will rev their engine to clear that oil (that plume of smoke they get at startup or after idle) and refer to it as "cleaning out" the engine. And that is exactly what is happening. That oil - which has become separated from the fuel has gathered in the cases and is standing by waiting for some serious turbulence to get it up the transfer ports and into the cylinder where it can finally be burned. Too much oil in the gas can lead to additional problems like carbon deposits on the piston crown and cylinder head, sticky rings, fouled plugs and wet drippy black gunk (unburned oil) coming out of the joints of the exhaust system.

It is probably best to avoid sustained periods of idle, or very low rpm running under no load. It is also probably better to choose a lower gear (for instance 3rd at 5000 rpm instead of 4th at 3800 rpm) and let the motor spin faster when driving at slower speeds since there in less likely a chance of the oil not finding its way to the combustion chamber. It is my opinion that if you use top quality oils (synthetics are the best) you are only throwing away money and making more smoke by running more oil through the engine than it needs. I have had good luck by always using the same oil to gas ratio and the same oil brand.


Anyway it's interesting.........

[Quickonstep]

1. you mixed your gas wrong, 2, your running SUPER lean, whats the plug look like? 3. you got a bad cooling issue..

im leaning away from the bad cooling.. just because it was ran for 3 hours.. if it was ran for 10 minutes it would be different.. but im gonna say your super lean, eaither via the jetting or a air leak someplace, which could be a number of things from your carb boot to carb to crank seals, or even an exhuast leak.. if you posted a picture of the plug, it would elminate alot of things and tell people what was goin on.. the plug will tell you exactly whats up with a motor at any given time

Jesse

[Derrick Adams]

Top of the piston looks brand new. Just has carbon around the edges. The sparkplug looked great as well. Golden tan! That's what's so disturbing. It never acted as if it had a lean condition. Never idled crazy or anything, that I know of. I never actually rode the bike much. Friend of mine was riding it, but I didn't notice anything out of the ordinary with it. I did get it together and rode it enough to set the carb and check jetting/ break in rings. Then I handed it over.

I think at this point, i'm going to take it to the local cycle shop and have them check it out. I sure don't want to waste another top-end.

[hondaATCman]

You can check the rod bearing by moving it around. A little side-to-side play is alright but there should be no up and down movement at all. Is there any burrs or rough places around the ports in the cylinder?

[TimSr]

I think at this point, i'm going to take it to the local cycle shop and have them check it out. I sure don't want to waste another top-end.

Id have them measure it see if its been bored with enough clearance for a forged piston. Ill never bore with anybody other than Wiseco again!

[Derrick Adams]

Here's what I found out after having it checked out. We pulled the flywheel and inspected the crank seal, he said it looked like brand new and not damp. No signs of that being bad. Checked the rod, said that was fine. Lower base gasket looked great, same with the head gasket.

He determined it was caused by one of two things, lack of oil/poor oil, or holding it wide open until the piston got hot and seized.

I'm sure that the oil suppy was good, because our fuel is mixed at the pump and all the bikes got gas from the same can. As well as lots of residual oil still in the case and pulling up on the crank. There was also substantial carbon build up on the outer area of the piston top, which seems to be that it had plenty of lubrication.

Oh well, I guess i'll put a new top-end in it and see what happens.

[4cylinders]

hey, just from experience, break in the engine before you let anyone else ride the trike.

[AZ250R]

Them "air-foolers" CAN'T FOOL WOT conditions for too long.... Been there, done that, more than once! :rolleyes: Then bought a "water donk", dang Ausie Slang is catchy, but don't run flat out for 5+ miles anymore eithere. :beer

[MTS]

im leaning towards what tim said, not having it bored correctly, shouldnt matter if you have it WOT for long periods of time, as long as your going fast, or moveing for that matter-

[kando]

My son had his 200xl blow up like that after just he overhauled it and the problem if you can believe it was an oval piston (manufacturing defect). They paid for everything to be replaced. You don't see that too often its always somebody elses fault.