250 SL holed its piston. What to do?

[Darrell]

For reasons unknown, so far, my 250SL holed its piston yesterday.

As I recall, back in the day two-stroke tuners used to change out holed pistons like they were socks. However, I'm concerned about molten piston crown that might have found its way to the crankcase and crank bearings.

I prefer riding to wrenching so I really don't want to hear that I should split the cases, but if I must, then I must, I guess.

PS my float valve has been sticking occasionally, dirt must be getting through the brass mesh inline filter. Could the bike have been dumping gas along the road and creating a lean condition?

Thanks in advance for your wise input.

[thrownchain]

What kind of riding were you doing when it holes the piston? And yes the only safe way to do it is total overhaul, that will give you a new motor and hopefully fix whatever problem caused your issue.

[Darrell]

What kind of riding were you doing when it holes the piston? And yes the only safe way to do it is total overhaul, that will give you a new motor and hopefully fix whatever problem caused your issue.

I was doing about 60mph along the highway.

Did a complete bottom end rebuild a couple of years/approx. 3,200 miles ago: New crankshaft assembly, bearings and seals.

I wonder why I've never seen a piston dome with a melted crater developing, either they're fine or holed, no in between. Is it always a sudden failure?

[MWL]

I only use race gas in my 250. Hodaka recommened premium gas. Not sure what octane premium was back in the mid 70's but it was probably close to 100 octane.

[thrownchain]

Possible too hot a plug ? or a running lean condition, whats the color of the plug ? nice choco brown ? Makes blue smoke while running ?

[rtboone]

Greetings Darrell

Has it been colder where you've been riding? Colder air=denser air which will result in in a lean fuel to air mixture which can be compensated by re-jetting. I go to a larger main in the winter.

Hopefully you can get by without splitting the cases. It's not the easiest Hodaka engine to install because of all the mounting spacers.

Perhaps the material from the piston was blown out the exhaust and you can skip that step. Compressed air might blow any remaining pieces out of the crank once you get the cylinder removed. More experienced mechanics might recommend splitting the cases just to be sure

Good luck---Tom

[Darrell]

Thanks for all your responses. I should have given more background circumstances about the situation.

Here goes:
94 Octane premixed 35:1 with Yamalube 2R.
Standard B8ES spark plug. Is and always has been a proper tan color, except now the electrodes are also a bit peppered with aluminum.
Stock size main jet and all other carb settings.
No mods to engine, except for knife edging the intake port bridge.
Air temperature around 45 degrees American (Fahrenheit).
I haven't pulled the barrel (just the head) but seizure damage is evident through the exhaust port.

Yep, I think I recall my fingers and knuckles getting pretty mashed up while aligning the engine mount spacers.

[thrownchain]

Oh and to answer your other question, you normally don't see a piston in the mid point of a melt down because you don't realize there is an issue till it's too late, and that means a hole.

[hodakamax]

I'll throw my two cents and remind us that a clogged jet can cause lean conditions or advanced timing and/or low octane or old fuel can cause a holy piston. As for the parts you may be lucky and they left out the exhaust. Strangely, I rarely, actually never, encountered holed pistons in my racing career because the items mentioned were checked before every race. Holed piston means too hot which is detonation or lean conditions. Anyway, things to check.
Max's opinion.

Maxie

PS--I'm with Tom concerning where the parts went. Close inspection with the cylinder off and a blow out and rotations while noting any rough bearings should be in order. Good luck!

[michael_perrett]

Lousy cylinder head (one notch thicker than a pancake) and too lean oil mixture.

Mike Perrett

[Darrell]

LATE BREAKING DEVELOPMENTS!!

OK, I think I know where the smoking gun is.

I took the float bowl off of the carb -- one of the (brass) floats had come off of the arm and was floating along for the ride.

I guess the carburetor may have developed a mind of its own -- depending on whatever position the stray float bounced around at. Typically one should expect the carb to flood because there now wasn't buoyancy to shut the float needle. That was one of the issues on my last ill-fated ride. Now, I think that the stray float might also have partially jammed under the remaining float, or float arm, and created the lean condition that holed the piston.

The coincidence and correlation between the card float deficiency and holed piston are too hard to disregard, wouldn't you think?

[racerclam]

I don't know if you guys know this but the 250s CDI box it not engineered correctly since it advances timing with RPM increase rather than retards it , that is not! desireable in a 2 stroke , when I built my RG250 I used the super combat CDI that does retard on top. Also the combustion chamber would benefit from torque grooving to improve combustion efficiency reducing possibility of detonation . Also you can make a head gasket that is the size of the head basically adding another cooling fin. I think Hodaka Dave might have these for purchase.

Rich

[Bullfrog]

With 3,200 miles in the past couple of years . . . the first guess has to be that something pretty important had changed (failed?). Glad you found it easily.

[Darrell]

I don't know if you guys know this but the 250s CDI box it not engineered correctly since it advances timing with RPM increase rather than retards it , that is not! desireable in a 2 stroke , when I built my RG250 I used the super combat CDI that does retard on top. Also the combustion chamber would benefit from torque grooving to improve combustion efficiency reducing possibility of detonation . Also you can make a head gasket that is the size of the head basically adding another cooling fin. I think Hodaka Dave might have these for purchase.

Rich

Bleh! That's not what I wanted to hear...this bike's routine involves highway stretches between off-road sections. This CDI should be factory recalled ! Who's picking up the phone at the Hodaka Complaint Department nowadays ?

I used to have a 98 Super Rat CDI box, that I traded straight across for a sealed beam (with a bad high beam) at Hodaka Days. At the time I thought I would never have a use for that CDI box; Maybe it would have been good medicine for my 250.

[Darrell]

Better split the cases. There's lots of debris in the crankcase, including some piston flakes measuring about 1/4" x 5/16".

[thrownchain]

Yes, needs to come apart,

[hodakamax]

Yikes! So much for Max's theory that all the debris went out the exhaust. Once the piston failed the engine was no longer a pump. Strangely I don't think I ever saw that in my motorcycle career. I suppose that most engines seize before they get to that point. Live and learn on my part.

Maxie

[thrownchain]

Not only the hole, but the heavy scoring on the side. What does the cylinder look like?

[Bullfrog]

When I see a photo like that, it always amazes how much damage a piston can take before it gives up the ghost.

Ed

[hodakamax]

I'm amazed how the proper (or improper) fuel air mix can generate a flame hot enough to melt aluminum. It's certainly a graphic picture of what happens when things go wrong in the mix department.

Max

[viclioce]

There is only one thing that can cause destruction on this order, and that is ignition. In this case, setting the ignition to 1976 standards and using 2016 gas. As pointed out be Rich Gagnon, the 250 CDI system advances ignition at higher rpm. If the guy was running 110 octane and set his ignition so that at its farthest advance, it was no more than optimum advance for high rpm, he could get away with running hot down the highway.

As it is, what you see in the photo is classic stuff. The erosion on the exhaust port side, toward the camera, is detonation pure and simple. That means he was running crappy gas with too much compression/ignition advance, i.e., stock settings on today's gas. It doesn't matter if he thinks he is running 94 octane. Have you ever seen 94 octane for sale anywhere in your lifetime? I have not. Then, even if he is buying so called race gas, what is used to make a higher octane rating is important. Modern 4 stroke racing bikes can tolerate modern gas. Vintage 2 strokes can't.

Then, too, today's gas will absolutely not hold its octane rating after sitting in a can or gas tank for weeks at a time. It may or may not have the rating it is given when pumped out of the ground, but it starts to evaporate immediately. Guess what evaporates first. Volatile additives that make octane. Tetra Ethyl Lead used to stay in gas indefinitely. Modern junk doesn't. Now we see everything from ethanol to MTBE or toluene used to bump octane, and none of these things stays in gas because of evaporation. Add to that the guy lives at 6500 feet where vapor pressures are reduced, and gas evaporates immediately.

Just the other day I took the carb off my Hodaka to look at the jets. There was nothing in it but oil sludge. Every bit of the 91 octane pump gas was gone. It has been sitting unused for about two months. Even when I ride it regularly, I can see gas flow to the carb every time I open the petcock. Thats because the gas that was there just the other day is gone.

Point is, this has NOTHING to do with oil mix. No one I know uses 35:1 but even if this guy does, so what. 35, 32, 40, whatever. If the oil was too lean, it would score the piston skirt, not burn a hole in the piston top.

So look closely at the photo. You will see two distinctly different, but closely related problems that occurred in sequence while running full bore down the highway. First, detonation which ate away the front of the piston and started to seize the rings, because this situation was allowed to proceed, full bore, for several seconds, the ignition advance issue raised heat to the point of pre-ignition, which is far more dangerous than detonation, but it was the detonation that ran away to the point of pre-ignition. Detonation and pre-ignition are caused by the same things: advanced timing, too high compression for the gas and/or too low octane for the compression.

This bike was set up wrong. Probably but not certainly, the gas was bad, and certainly, the guy was pushing his ride down the highway at full throttle. He should keep in mind that 1) he is riding a 40 year old bike; and, 2) he is NOT riding a highway cruiser.

The only other issue I can think of that would lead to a catastrophic failure like this would be a failed dry side crank seal. This, too, might lead from a solid ride to a failed engine in a matter of seconds, but he will have to inspect the seal to know.

His assertion that the float loose in the carb caused this is wrong. The lost float would do little to harm the running other than jamming the float so it overflows. Otherwise, it was just in there. Had nothing to do with a holed piston.

Thats my 2 cents on it, from someone who rides both two stroke off road bikes and four stroke cruisers. ; D Victor

[hodakamax]

We may never know on this one but as I always say, it got too hot, hot enough to melt aluminum. It well could have been the broken float jamming it into a lean condition (or a seal). Almost always in a disaster there can a series of events that causes the end effect. Everything that has been mentioned could be part of the disaster. Certainly overly advanced ignition could be one of the culprits along with bad fuel. Detonation can also be caused by overheated parts setting off the mixture before the spark plug does. You can call this pre-ignition and/or detonation but it's all bad. Pistons melt on the front because that's where the exhaust exits. Any one of these things we might have gotten away with but put them all together and bad things happen.

Just thinking out loud but that's my opinion.

Maxie

[Bruce Young]

Max, I agree with you completely, "stuff happens", no matter what the reason, it now needs complete overhaul. New Piston and rings, and gaskets, split the case, check and replace all items that need it, especially the seals. The carb needs a complete overhaul, and it sound like parts of the ign., need to replaced. Next and last but least, any small bore two stroke that is run out in the traffic, like its a road warrior, needs now be ridden maybe with a little more care. Change sprkts., to reduce engine turn up to hard.
Next time you gas up make sure you use good fuels, and if bike is oil injected, check all those areas as well.
It now needs an overhaul, we can make suggestions of what happened, but we where not there. It happened and now its time to fix and get back to riding, and enjoying the bike. Bruce Young

[hodakamax]

Bruce, you've summed it up nicely! That is definitely the next step.

Maxie

[viclioce]

Detonation is a post ignition phenomenon. It happens after the spark plug fires. Flame spread rate suddenly flashes instead of burning smoothly, causing a knock and a pressure spike? Left unchecked, it will erode the edge of the piston crown because that's where detonation occurs. Generally caused by not enough octane for the compression and ignition timing as set. Cure is higher octane fuel, lower compression, retarded timing, or a combination of all three.

Pre-ignition is a pre-spark phenomenon, hence pre-ignition. It is caused by something in the cylinder that is so hot that fuel begins to burn on contact before the spark plug has a chance to fire.

Pre-ignition and detonation are simply not the same thing, and not caused by the same thing.

Run away detonation can cause a hot spot that can then cause pre-ignition, but while detonation occurs generally after TDC as the flame kernel expands and travels into the squish zone, pre-ignition happens before TDC causing an pressure spike that, in combination with uncontrolled release of heat, attaches the center of the piston where it is farthest from the cylinder wall where the piston can dump heat.

It shouldn't be surprising that gasoline burning in a cylinder can melt aluminum. Internal combustion temperatures can exceed 3,000 degrees F (3880* F in a constant pressure environment). Aluminum melts at 1,220 degrees F. So in fact, the aluminum alloy piston lives in an environment where it is constantly exposed to temperatures well in excess of its technical melting point.

When temperatures and pressures spike due to uncontrolled combustion, the melting point is rapidly exceeded, and the pressure spikes blow molten alloy out of the melt zone. It all happens in seconds once conditions are met.

How do I know this. Well let me put it to you in an automotive example.....

My son in law is a mechanic, professionally. I have worked on motors for years as well. He acquired a Saab 900 Turbo and drove it for well over 2 years without issue. He then sold the car to my nephew's girlfriend, telling her over & over that the car requires high octane fuel, & running it on regular unleaded will damage the motor. She bought the car with that knowledge & understanding & chose to run it on 86 octane regular unleaded. After two weeks, she retuned the car, without paying for it, by having it towed to my son in law's house, having had my nephew pay for the tow!

The motor had not 1 but 2 holed Pistons, caused by pre-ignition from not using the high octane fuel required. Same as our friend here. It doesn't matter the kind of motor. If you set something up which causes pre-ignition, holed pistons will be a common occurrence. Whether or not you chose to do what is recommended is your choice. You don't have to do things the wrong way for long to get the anticipated results. Taking a 40 year old bike, designed in an era to run on regular which had an octane rating of 92 in the 70's and running it with advancing timing and an extended period of high rpm running is a complete disaster waiting to happen. You can disagree with the facts, but that doesn't mean they are wrong.

Bruce is also correct that now the only thing which can be done to correct the situation is a rebuild. But a rebuild without modification of what is causing the pre-ignitionwill only be successful for a short period and the problem will rear its ugly head again. Isn't that why the poster was asking for advice on what happened? ; D Victor