Today's Whodunnit ...

Plug colour looks ok. There was still plenty of oil around the piston when I took it all apart.
Just reckon I was unlucky.

Bore clearance too tight. I think my bore clearance on my 39 0 was .006".

Did you lap the head to the cylinder?

Was the head torqued evenly?

Is it jetted to the pre mix?

I was in agreement with the previous post that suggested the wrist pin keeper came out but you said that wasn't the problem. From the looks of the extreme damage in such a small area of the piston leads me to guess that something came apart inside the carb or the reed cage assembly before ending up between the piston and cylinder. I've seen seized pistons from lean conditions that didn't do near as much damage as this. Looks like damage from metal debris.

No, checked the reeds very carefully tonight. They are still perfect. As are all the parts from the reed cage.

Yes Bill, head torqued correctly, and checked again sometime later. Head was lapped. Mixture has been spot on since it was first run.

Stated bore clearance from Wossner is .08mm. This is, as far as I am concerned, too tight. My 390WR 6 speed has just been done to .09mm. The Auto was bored to .10mm over as they run so hot.

.10mm = .0039" .09mm = .0035" you need at least .0055/.006""????

I bored my first 390cr to .004" she wasn't out of round with no taper.
She seezed like yours. The piston wasn't bad at all. I cleaned the bore to .0055/.006" and she was fine.

Back in the day the husky service mgr used to set up the tolerances quite loose on the Baja winning bikes because they ran them so hard......

I've just put a new piston and ring in the manual box 390WR after 900 miles. The ring gap with the old ring was .5mm, gap with the new ring is .3mm. It did not need a rebore (just as well as it is 84mm anyway). I repacked the silencer and did a few miles on it this afternoon. At least with a manual box I can keep the rev's down and keep it all sensible. It is a bit harder with the Auto as I can't short shift to keep the rev's down.
The Auto will be up and running as soon as my Kawasaki small end bearing turns up on Tuesday.

Richen up your main jet.Go up 10 to 15 and work back.make sure the needle is still clipped on.I had a 390 WR seize up on me too. Same conditions,not really being hammered.Upped the main jet,good after that.What I see the auto looks good.

I'm using Lectrons on both my 390's. I will go richer 1/4 turn on the metering rod, and richen the power jet by at least 1/2 turn or more. If it starts to blubber higher up the revs I can come down on that.
The Kawasaki small end turned up tonight, so if I get time later this evening I will put it back together. Running it in before the 28th might involve some extra effort.

I'm noted for doing midnight runs through the neighborhood with new engines. I rebuilt husqvarna bikes every superbowl sunday and fired them up for a rip after the game.
It's something I tear up about now because my son tells this story it's something he will always remember about me and him and the husqvarna bikes.

I'm a well known husqvarna legend in these parts (in my own mind) with these magnificent Swedish bikes I'm one of the only ones who stuck with the evolution bikes. It's that Swedish fin ring that gets my juices flowing. I can distinguish the sound from all the other bikes.
I rode husqvarna bikes when huskys weren't cool.

I do think about those Swedish blondes........

What happens on the 28th? Just sent of the documents to register my 250, keeping my fingers and toes crossed!
In the end with your seizure are we thinking it was not quite enough clearance round the piston?

No, clearance around the piston was good Steve. A little on the generous side. I just think I was unlucky.

Anyhooo .... Had an hour and a bit tonight, so the motor is all back together, and the carb is richened. Probably a bit too rich, but I can work back from that.

No chance of finishing it tomorrow as it's track recce night at Harberton. Friday evening before the pub may be the best time to finish it and run it down the road. Sunday is Dorset Classic.

Sunday the 28th is a TRF funday at the Harberton track Steve. We will use the scrambles track and all the woods and valley over the back. If you are a TRF member then no issue. If not, and you want to come down, we are allowed one guest each. All bikes must be taxed, insured and MOT'd. Proof will be required on the day.

Grouty, I don't have any experience with the Lectron carbs but every time I ride my autos or any of my other Huskys I always check the weather conditions/stats for barometric pressure, temp, and humidity (this will give me corrected altitude and ADR) and change the main jet to the current weather. If I start riding early in the day the when it is cooler then I know later in the day as it warms up I know I will stay safely on the rich side. My main jet size requirement changes day to day and sometimes the needle clip position, but not as often. For instance on my 81 420 auto I will use anywhere from a 420 to 360 main. I would suspect you probably had the carb tuned correctly from previous days riding and the day the piston seized was a day that the weather conditions required much richer jetting for that given day. I don't think many people realize how much the weather plays a part on the correct jetting. Just food for thought.

Marty

Not saying it is the Lectron at all. My point is weather plays a bigger part of carb tuning than any other factor such a the elevation. You can be at 3500 ft and with the weather conditions such as high barometric pressure, cool temps and low humidity the "corrected" altitude could actually be at below sea level! If he had his carb tuned on the lean side to begin with and the weather required much richer jetting then that would be the perfect recipe to seize the motor.

Marty

Think about this a hair on your head measures .002"/.0025". You have .0039" clearance in your engine that's .002" on a side. That's tight.

Marty may have a point. It may have been borderline lean before, and a warm day and a long run may have just tipped it over the edge.

All running tonight. Did a few miles around the local roads .... gently !

It started first kick after the rebuild.

Runs nice, ticks over smooth.

But still seems to be running hot. Maybe I will advance the timing a smidget to see if that keeps it cooler.

My 390 was/is very sensitive carb/fuel wise. once the fuel went lead free it suffered from some pinking late in motos, where the 250s seem to run fine on the new 'crappy' fuel......

I use a mix of 20% 115 octane leaded with super unleaded. I found out too that the 390's hated normal unleaded. No idea why though.