what did I stuff up here?

[80-85 husky]

another tip frosty dont go to harrow with a fresh engine.... get at least two tanks of fuel on it or more i rekon...used to see this at the bp sea lake rally back in the 80's. half the field would be  once a year racers and half would "do up the bike"just before the event. that half only made it half way down the first open salt flat

if you sand the alloy off the barrell or get a light hone, it should clean up for a new ring and a bit of a wet and dry polish on the piston. check the gap is ok. put it back together and jet it up a tad and get a tank or more in general light running on it just easing into the band at 1/2 or more throttle. after say two tanks start giving it a bit more letting it work up into the band a bit more. now you can look at cleaning the jetting up by leaning the main (if its blubbing a bit or dragging on full throttle) then look at the 1/4 to 1/2 throttle area ( Needle jet) and feel how its going as you roll the throttle thru that range...if it drags on opening the throttle then takes off on the main jet, teh needle is rich so drop it a notch see how it goes. if its sharp and responsive like the fuel is running out your too lean, let more juice thru. look for the middle of that range.
the pilot should see you needing choke to start and a nice idle with teh air screw 1.5 turns out. if the air screw is unresponsive then the pilot is wrong. (or you have an air leak) check your manifold isnt split, they can hide in the ridge lines and open up when you hit a bump. i have fallen for that chestnut before.
a massive ping and over rev when you land from a jump is also a giveaway.

hope this helps...PS There are much better people than me at jetting also is the muffler clean and well packed??

cheers

[tony27]

Going on how scuffed the piston is both front & rear I'd replace it, will rattle otherwise.
Can you feel any of the scratches on the bore with a fingernail, that will be the deciding factor on whether a quick hone is all that is needed for the cylinder

[Glen08]

I had a 250 that looked similar. To get the most out of it I cleaned it up respectfully & did the main seals. It ran strong for half a day..I'd take piston off & put it on the bench with barrel & some measuring tools

[micks]

could the pipe be helping with this problem, restrictive muffler throwing heat back into the head

[Slakewell]

Air cooled bikes = mineral oil
I will never put anything but mineral oil in my old bikes.

[Lozza]

if you sand the alloy off the barrell or get a light hone, it should clean up for a new ring and a bit of a wet and dry polish on the piston.

Can't sand aluminium off a bore needs to be 'fizzled' off with hydrochoric acid. The piston is forked it trapped the ring .

I was wrong ring locater is still intact so possibly may only need a new ring?

Its not a lean seizure its from lack of lubrication as above mineral oil will stop lubricating with enough heat, new ring new piston. Run it in by idling on the stand for 10 min then 10min cool down and then nail it pussy footing round does the piston no favours

[sleepy]

What was piston to bore clearance? Looks to me like it was a bit on the tight side. I have honed cylinders that looked like that out to the correct clearance and given them new pistons with good success.

[frostype400]

Thanks guys I'll order another piston and get the aluminium off the cylinder I'm also going to do some plug chops when I get the chance and look at getting my jetting spot on pipe and muffler is new and unrestrictive I was happy with how the bike handled my forks and shocks worked a treat I was a bit rushed in preparing for the event so I'll make sure it's well and truely sorted for next time.

[Dewy123]

I'm feeling your pain Frosty. I've now seized our 1982 WR430 Husky 3 farken times at harrow. Over 3 years 
The first time was with a new Wossner piston and running at 40:1 with Motorex Oil. It seized on the first big open strait.
Second time it had another new Wossner piston with slightly bigger clearances and runing at 25:1 on Motorex and seized on the track to the fuel dump.
This year we were out to 0.005" piston to bore clearance as it's on 3rd oversize and there's no fourth oversize. Also worked out it had a stuffed crank seal on the fly wheel side  replaced that, we thought the seal was the reason for seizing it the previous 2 times.
We ran it in at 25:1 on Motul 800 this time and had a temperature prob in the exhaust to read how hot the gas temps were getting.
It was working well and I put about 60km in on it running it in. Getting it hot switch it off and cool right down, did that 3 times not even riding it.
Than road it for 5-10 minutes at a time short shifting it for 30km.
We did the practice loop on the Saturday at Harrow and it wouldn't rev out. So went playing with the needle and main jet.
With a 460 main (used to run a 440) and the needle on the bottom clip (Also changed the needle, I can't remember the needle that was in it but i was running a thinner needle to keep the mid throttle gas temps down) and it went the best yet.
I than got greedy and thought I'd try 30:1 and went for a ride and it felt even better up top. I was riding back to the camp and thought I'd check that it wasn't too hot at half throttle and glanced at the gas temp and saw 600 degrees celsius. And it seized again 
I pulled the exhaust pipe off to have a look in the barrel and it's seized the inlet side of the piston.
Which by rights is the cool side of the piston.
So everyone's theory that the fuel has more of a cooling effect than the oil isn't correct as the engine didn't seize at 25:1 but did at 30:1 with a bigger main jet.
Anyway after that everyone at Harrow has a different theory why it seized-

1- Piston to bore clearance was too much not allowing the piston to transfer it's heat to the barrel and cool?
2- To bigger squish height, creating extra heat above the piston. It has been running a copper ring head gasket for years on Mahle Pistons and never gave any trouble.
3-  Something's wrong with the ignition timing or curve and it's running to hot with retarded ignition timing.
4- It has another vacuum/crank case leak somewhere?

Sorry to hijack your thread but I don't want you to be in the same situation we are in, 3 years in a row.
Harrow is a very hard environment on old bikes and every year we seem to be out of time and throwing our bike together last minute.
So our plan is to get the barrel re-sleeved back to a standard piston and correct clearances.
Get the squish height right.
Get the ignition curve checked.
Pressure test the crank case to 10PSI and make sure there's no leaks.
And than spend a lot of time running it in and than even more time getting the jetting correct which might be a lot easier on a dyno as after seizing it at half throttle I don't believe anyone can work out how hot the engine would be running at that speed and load by just reading the spark plug colour.
It happened that quickly.

Cheers Ryan.
 

[Glen08]

I'm interested to know what piston your using (single ring) if you don't mind sharing..?
Cheers.

[Slakewell]

I have seen this more than few times , You have to use an oil with wick action ( to help cooling ) if you choose not to then when you really labour the engine In mud/sand this can be the result. Most modern oils dont have wick action.

[tony27]

I'm interested to know what piston your using (single ring) if you don't mind sharing..?
Cheers.

Markings on top & lack of teflon coated skirt would suggest Wiseco

[Glen08]

KX500 piston mod I keep hearing about..?

[Dewy123]

I'm interested to know what piston your using (single ring) if you don't mind sharing..?
Cheers.

The Wossner Piston is single ring with Teflon coated skirts.

I have seen this more than few times , You have to use an oil with wick action ( to help cooling ) if you choose not to then when you really labour the engine In mud/sand this can be the result. Most modern oils dont have wick action.

I'm starting to wonder the same that the Motul 800 isn't up to it? But a lot of people I've talked to run there air cooled big bore 2 strokes on it up to 40:1?
When I was racing my 1974 CR125 in the 1990's it had a problem seizing the piston on the exhaust side. The fix then was to get the piston Teflon coated and we ran Silkolene Pro Comp 2 I think it was? In a metal tin.

[80-85 husky]

i used silkolene pro comp years ago and it was good. that crank pressure test will be interesting.... i wondered why you were on a pe