300xc Engine racing uncontrollably….

Got an issue….last fall I lost my piston skirt due to a severe overheat during a very long, very hard technical climb. My fan didn't come on, I knew I had an issue with the thermocouple, but rode anyways. Got to the top she was blowin' steam, during a long muddy descent she came apart.

 

I replaced the piston, inspected and cleaned out the bottom end. All looked good.

Then had this motor racing issue. Like a big air-leak. Huge air-leak...

It races so bad I have to put it in gear and stall it, it won't even shut off by the kill switch.

 

Found a crack in the case from the crank smashing the piston pieces into the bottom of the case, found it by some (pre-mix) oil leaking out of the minuscule crack.

Didn't see it until the oil made it visible.

Tried to fix it with some good ole JB Weld. Still racing uncontrollable.

 

When it was upside down fixing the crack, some tranny oil came out of the spark plug hole, just a drop. Figured there was another crack, like between the bottom end and the tranny. So I ended up replacing both case halves. Inspected the old cases, no crack are apparent.

Just got it finished. Still have this uncontrollable racing issue, although not quite as bad, but still pretty severe...

 

What next? Could it be one (or both) of the seals on either side of the crank? Maybe they got ruined during the over-heat? They 'looked' fine during the case replacement.

Head seems flat, o-ring head gaskets look good. Cylinder cracked? Wouldn't I see steam or loss of coolant, don't see that.

 

I'm thinking those crank seals…any input?

Lean jetting???

You could try doing a leak down test. I was battling an air leak on my bike not to long ago. I did a top end and replaced all the gaskets and it went away. I also replace the inner clutch cover gasket. Slavens did a video about them going bad and causing a lean condition. But if you already replaced both case halves I'm sure you already replaced all the gaskets

 

Here's a video I found on YouTube of a two stroke leak down test when I was trying to figure out how I was going to deal with my own air leak issue. Hopefully this helps. 

 

Throttle cable not seated in the carb?

sounds like you re used your crank seals? I wouldn't they are cheap ,

a leek down test is the way to go after any 2t motor surgery- it can save a little time finding problems and give numbers that matter.

throttle cable not seated is a good call, throttle cable pinched by the tank? or kinked?

Did you really re use crank seals?

Lean jetting???

Nothing to do with jetting. Carb wasn't touched before or after the piston came apart.

Throttle cable not seated in the carb?

Checked that. Not a carb issue.

Did you really re use crank seals?

Yep. Engine ain't that old. New seals didn't come in the gasket kit. I thought, and inspected, they were good. Maybe my bad, that's why I suspect them. I never thought it would be an issue.

You could try doing a leak down test. I was battling an air leak on my bike not to long ago. I did a top end and replaced all the gaskets and it went away. I also replace the inner clutch cover gasket. Slavens did a video about them going bad and causing a lean condition. But if you already replaced both case halves I'm sure you already replaced all the gaskets

 

Here's a video I found on YouTube of a two stroke leak down test when I was trying to figure out how I was going to deal with my own air leak issue. Hopefully this helps. 

 

Nice!

Yep, all gaskets changed, but not the crank seals.

sounds like you re used your crank seals? I wouldn't they are cheap ,

a leek down test is the way to go after any 2t motor surgery- it can save a little time finding problems and give numbers that matter.

throttle cable not seated is a good call, throttle cable pinched by the tank? or kinked?

Yep, I'm leaning toward the seals. Just would have never thought it was the seals...until now, that everything else has been changed out.

New seals ordered.

Will do a leak down test in the mean-time. Maybe even when the sides are off to actually see if it is the seals.

Did you check the nut holding the handle bars to the foot pegs? I've wondered if yours had a few screws loose sometimes...

Edited by pre-mixed

Did you check the nut holding the handle bars to the foot pegs? I've wondered if yours had a few screws loose sometimes...

I knew you would show up sooner or later Jake..... :p

Well I'm just hoping you get it fixed before I show up at your house again

I know you said its not the carb, but the tiny tab that's on the floats can sometimes get bent up for one reason or another, and it lets JUST enough fuel into the carb to run the bike but not enough to fill the bowl... It make the bike run super rich and Revs the hell out of the poor thing.

 

I literally just went through my entire 250sx to find that it was just that.

 

Hope all goes well and its something small / cheap!!!!

I know you said its not the carb, but the tiny tab that's on the floats can sometimes get bent up for one reason or another, and it lets JUST enough fuel into the carb to run the bike but not enough to fill the bowl... It make the bike run super rich and Revs the hell out of the poor thing.

 

I literally just went through my entire 250sx to find that it was just that.

 

Hope all goes well and its something small / cheap!!!!

Yep, I understand. But it's not the carb. After I installed the piston, it raced, Pre-mixed and I went over the carb and all was good, that was our initial thinkerizing as well, ruined a great ride that day.... That's when I noticed the crack on the bottom and realized I had to change the block.

Maybe that crack on the bottom wasn't really that bad and I have been fighting a bad seal all along...?

 

Anyways, going on a short vacation and going to tackle this when I get back (and the seals arrive).

 

 

Talking about seals always reminds me of a good joke:

What does a walrus and tupperware have in common?

 

 

 

 

 

 

 

They both like a tight seal.....

Yep, the crank seal on the dry (magneto) side.

 

Replaced the clutch side first, since I saw oil in the chamber when I had it turned over. Figured I'd start with that side. Still raced (actually seems to be getting worse and worse).

So then I did the magneto side. All fixed!

In fact, she throttles up more sharply than before...think maybe that seal had been going bad for some time now.

 

I did the Slaven's ATF-in-the-starter mod years back when I first got this bike (it's the mod where you seal off the magneto cover, put in a small vent hose and about an ounce of ATF fluid for a lubricant).

I'd bet the farm that the seal didn't like that ATF fluid and that's what started the seal to go bad!

 

Anywho....she's all fixed now.  :ride:

You think it was the ATF that took the seal out? Not getting it super hot? Like hot enough to melt your piston?

Haha what kind of oil did you put in this time? I use 5 weight fork oil in my wet mod.

When are you going to be able to ride down your way?

Overheating will not cause a piston skirt to break. Too much piston to bore clearance causes skirts to break. I hope you had your cylinder measured up before rebuilding so that it does not happen again

Overheating doesn't cause the piston to 'melt'. It gets a little out-of-shape and can crack and breaks into pieces, especially under load (no fat jokes...)

Yes, everything was measured and in spec., including barreling.

So in simple terms, yes, overheating can cause a skirt to break. Seen it, been there, many other guys (that were there when it happened and afterwards) will contest to that.

This wasn't a 'simple' or slight over-heating either, it was a pretty tremendous overheating. Go ahead and drain the water out of your bike and ride it real hard and see what goes first, I bet it's the piston.

Overheating causes engines to seize, breaking a skirt is basically a seizure (then part of the seizing parts break apart).

 

And, yes, I do think the ATF fluid ruined the integrity of the seal, heat probably just accelerated the failure.

This should be taken in consideration for those people who use ATF fluid in place of oil in there trannys.

This time I left it out, although I did leave it 'wet' to offer some sort of lube for the time being.

 

Pre-mixed/Jake- as soon as the mud and snow is off the trails....my hand is doing good and I'm up for an easy ride at the least. BTW- did you know Johnny-O moved up here? Got a place over in Mendon.

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