All 2st WR250 Engine seizure

Hi all,
A summary of things that happened previously...
I brought my Husky (2000) to a repair shop for checking, as I don't have it yet that long and found it a bit slow.
After investigation they told me that I needed a fresh piston, because it would be really end of life.
The nicasil layer at the cylinder would be a quite thin at some area's, but just ok to run a fresh new Vertex piston.
Also the engine bearings needed replacement + a partial crank revision.
After running-in for the bearings and piston to settle, I noticed 0% more compression on the kickstarter, but a bit more pulling power.
I checked the spark plug and found that the usual NGK BR8EG plug was replaced by a NGK B9EGV and I swapped that back again for a test.
I noticed that the pulling power with the standard plug was exactly the same as before the revision.
Because I was suspecting that the carb was maybe a bit offset I put the clip one position leaner for some testing and I put a little bit more oil in the pre-mix.
After that I had some blue smoke coming out of the HTS (power valve) ventile during startup.
I ignored it, as I thought it probably would have been caused by the more oil used.
Last saturday during a ride, the throttle kept sticking when I had it fully open, and I had to jump off the bike, not to land in the woods.
The bike still ran for some 15 seconds on full throttle on it's left side and my friend (who rides KTM) was desperately pushing the red light button, as he thought it was the kill switch.
I don't know if the piston got stuck by itself or that my friend touched per accident the correct kill switch, but it stopped running in the end.
After checking the throttle cable I tried to start the Husky and the piston moved, but the compression was almost gone so it didn't run anymore.
After removing the cylinder and piston, i saw this view on the picture below.
I checked the cylinder and it has a melted aluminium trace which I could not remove completely jet.
It's a whole story, but I have no idea what caused this.
I don't know what to think anymore, but I would like to solve this mystery before ruining another piston and the cylinder...
Maybe someone could shine a light on this issue, as I spent over a 1000 Euro and it ran only 3 hours.

Thanks.

damn mate that is brutal! give it time, the masses will chime in soon.

That is sad to see. There are a couple of possible things that caused this.

1. Jetting. You leaned out the jetting and tried to compensate with more oil. If your settings were way too lean, more oil would not prevent engine damage. That said, you only leaned it out one clip position on the needle, so that wasn't a big change. However, adding more oil actually leans the fuel mixture further, so the two may have combined to make the jetting too lean.

2. Nikasil plating. If your plating was damaged enough that the bare aluminum of the cylinder was exposed, this could be the result.

I think you are looking at having the cylinder repaired and replated in addition to new piston and rings. You may be able to clean the bottom end/crankshaft with kerosene well enough that you won't need to split the cases. When it's back together, set your jetting to the rich side for break-in, then adjust leaner as needed. Always make small adjustments, and be consistent with your gas/oil mix.

Is it possible that your "sticking throttle" was actually an air leak? An air leak will cause a 2 stroke to rev very high with no load and the throttle closed.

Did you measure the cylinder to make sure you have the correct piston? Not sure about Husky's, but I remember that they offer 5 different sizes for a Honda so you can get exactly the right size (a big piston in a small cylinder is not going to work very well). Also, you should measure the out of round, etc. on the cylinder to make sure everything is in spec.

Air leak, interesting.
The sticking throttle is really something to solve too, I want to keep on living..
Is it possible that an engine can go berserk with this?
I noticed that the throttle cable had resistance for a brief moment after picking up the bike.

I don't know if those exhaust fumes coming out of the ventile, had something to do with an airleak.
Perhaps that could explain also my complaints about power.
For the cylinder, I don't have a good measuring tool for that, maybe later this week.

If you ever have a 2t bike running wide open and want to stop it cover the tail pipe with your gloved hand. 2ts can run away on detonation and even a good kill switch wont stop it.

Correct...you have to plug the pipe to stop it. Even pulling off the spark plug lead will not do it (don't ask me how I know, I still have the shakes!)

Like Dave says one clip leaner should not seize it like that but an air leak will not only seize it but make the engine run away like that too though with reduced power. I'm guessing air leak.

Was your throttle stuck open of was it running away with a closed throttle?

I rode 1/2 and enduro with a sticking throttle that was caused by a bent carb needle. bike likewise had just been returned from the dealer for motor work.

The TMX carb has a big heavy chrome slide that if dropped will bend the needle......instant stuck throttle.

The sad thing is you paid someone to do this work. It sounds like the bike wasn't quite right when you got it back. If the sticking throttle was a new situation you should have brought it back to them. Likewise for the lack of power. It is possible the powervalve was not setup right, among other things, hence the smoke out of that area. I agree with everyone else on the air leak. Something seriously leaned the bike out to cause that damage, thats not just from it falling over and running full throttle. You learned an important lesson here - that if a shop does work on your bike and its not quite right, bring it back ASAP. You can't go back to them now with the melted piston. Also as everyone here mentioned, you will need to have the cylinder replated.

That piston crown does not look lean for a new piston....it also looks like there was some blow-by too.

How does the plug look?

Do you have a screen type spark arrestor? I had part of my muffler come apart and plug the screen up and burn up my engine. It looked just like yours. Something to check anyways.

@R_Little,
Some good feedback here, this gives me some more idea's
During the event, I gave full throttle to lift my front wheel over a bump and tried to close the throttle right after that, but I didn't feel anything special in the throttle during the closing actually.
After the seizure when I checked the throttle, it was in the normal closed postion but it had some resistance (like something is moving over an edge) for a moment and it operatied perfectly when I jerked it.
This story about a bent needle makes me thinking, as I changed the clip from the carb myself and was fiddling more than usual with the needle.
It didn't co-operate so easily during assembling, I need to get into that also.
For the plug, it looked ok, just dark brown.

@Kevin Sorce,
It would be more logical to go to the shop immediately, but it was said that there would be no guarantee on this.
I can go back there and make problems, but it will only in a typical "yes" and "no" game.
These guys are experiencing hard times in terms of money, they will probably blame me and say "you didn't gave it time to settle during riding-in.
The power valve is interesting but I still have to get into this technology, I'm from the air cooled era.
However, the smoke out of the HTS system is haunting me a bit now.
What does that indicate regarding abnormalities?
Or is it just something that happens once a while, as it has a ventile/outlet?

@Lincolnlock,
For the spark arrestor, they are practically not used here in North-West Europe.

@Dirtdame, the repairs shop told me that they saw some scratches in the cylinder wall and they would clear them before putting the new piston.
Maybe the cyliinder wall is oval or something, I'll try to check this next weekend.

Some good news today!
We removed the aluminium traces from the cylinder wall and now we could have a good look at it.
It seems there is plenty of nicasil layer on it, at least we can't see any color differences.
We cleaned up the piston also a bit and fitted it, it was exactly right measured by the repairshop.
At least now I can order a new piston, instead of spending money to expensive drilling, nicasil etc.

Did they run a dingle berry hone through the cylinder? How did they clean it up?

I called a specialist regarding cylinders and pistons.
He told me nicasil is so hard, it can't be honed.
he said, if your cylinder wall is damaged, we need to drill a size bigger and put a new nicasil layer...
So we thought, if nicasil can't be honed for it's harshness, we can't hurt it...+ they need to drill it anyway.
We decided to take a couple of shots by running a dingle berry hone through it ourselves.
Appearantly it's possible to get a nice smooth surface, provided there is enough nicasil and as long the aluminium traces are on top of the wall (not grooved into it).
Probably a repairshop or specialist wouldn't have this procedure in their phrasebook, they want $$...

It's try and error learning for me.
The piston has been ordered and I will know if this works probably next week.
Another thing, the HTS..
I discovered a half inch hole in the HTS gasket..also ordered.

Check to make sure your throttle cable isn't hanging up somewhere. If the cable housing gets pulled up it can get caught at the fitting where it goes into the top of the carb and hold it wide open. Make sure when you turn the handlebars each way it isn't catching anywhere.

What are your jetting specs? pilot, main, needle and clip position. It shouldn't sieze like that running wide open for 15 seconds.

Man that's no good Deanevo,

How did the old piston look? Do you have a pic of it at all? How long was it running before it went into the shop to start with?

It sounds like the issue may not have been the original piston but rather a minor air leak somewhere which finally gave way when you were riding it last.

I agree with R-little the piston crown looks good so I would guess that the jetting is close to where it should be. Especially if the plug is dark brown.

This looks like a heat seizure issue, possibly from poor cooling or probably from running too lean. For the engine to run on like it did I would be looking for an air leak.
Check your reed spacer & gaskets, carby gaskets on the engine side, HTS gaskets, base gaskets for cracks & your crank seal on the stator side as air can be sucked in there as well.
You should also check to see if your coolant galleries through the cylinder are not blocked & that the water pump is working.

If the plating is ok you can clean the alloy deposits from the cylinder wall with a cotton tip & Muriatic acid, try your local pool shop for it, it is used to clean calcium deposits from the walls of pools. Its very corrosive so do take the recommended safety precautions.

If it is damaged a replate will be necessary. Make sure you send the new piston & the cylinder to the platers so they can match the finished bore size to the piston.

Good luck mate.