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This happened to me a few weeks ago while I was racing at finke. It must have been running damn lean to cause this I guess but the wierd thing is it ran a a peach on the 230klm run down and started fine the next day for the run back but only lasted 6klm. It all seemed to happen in about 5 seconds, I was fairly ringing its neck in top and then it just died with basically no warning. I had the jetting done on the dyno a week before and you can see the AFR's on the dyno sheet. The piston is a wiesco and I have a 172 main jet in it. Its currently bored 90.5mm and the damage is pretty deep up near the top so I think its gonna need a sleeve or new jug. Was told by the dyno joint that 13:1 is stoichometric for a 2 stroke so it should have been a little ritch under power. Can anyone let me know if this is true? Had 40:1 fuel mix on the dyno but brought it back to 35:1 for the race. Could this have leaned it up enough to cause this much damage? .Im keen to hear your thoughts, Cheers

i wouldnt trust a dyno to try and set up real world jetting, simple plug check works everytime

I'm no jetting expert but 172 seems a little lean to be WFO for 230km?

I cant fathom how shit gets this bad on a single cylinder engine...

Its obviously too lean, looking at the exhaust port it's taller than stock.
Here's a good rule of thumb for anyone with any aftermarket or stock reeds and carb with non-oxygenated fuel....
The exhaust duration in degrees is almost exactly what main jet you need. Ex: 178 duration is stock, if you ride it at the top end of the motor it needs a 178.

Your porting, with the wide ears and the tall exhaust I'd start at 188 and try to sneak down. You could do lube holes where the exhaust almost overlaps the intake also, you're building major heat there and the exhaust has a good starting point to creep around the piston at that angle.

What cc is the head? Should be down in the 58-60cc area for that much exhaust too, if it is and the squish is 55-60 and the main is 188 or close to then run a colder plug also.

First thing that happened here is the ring end gap closed, on a port that wide go .022-.025" gap.
After the rings butted the dome sunk, it got so hot after that it had to stick
What piston clearence was it? I'd have gave that motor .004"

I'd also be looking for air leaks, intake boot, crank seals, base gasket

I don't see any exh bridge relief either???????

Thanks for the replys everyone, the 2 stroke tuning / timing thing is all new to me so I appreciate the info. Unfortunately I dont have many answers for you as I dont know anything about 2 stroke port timing etc. I bought the engine second hand and the porting was already done. I picked up a new piston and had it bored to suit so I am unsure what clearence they went with. The head has ML3N E-1 cast into it and as far as I know it is stock 89 model. This bike will be mostly used for desert so should I just stick to a stock porting for reliability as most of it will be in the higher rev range?? Thought I was doing the right thing paying a shop to sort the jetting on the dyno but it has kind of bit me in the ass. Heres a few more pics of the jug. One last thing, can someone explain the exhaust bridge relief??

Cheers

Hey mate
That must have been you I saw parked up in the weeds just outside of finke. Would love to have stopped for a chat but had a spike to collect back in Alice
Sorry to hear about the motor, looks a bit nasty. If its any help to you next time i was running the pwk39 with a 48 pilot, 180 main on a DGL needle in the middle clip all on 40:1. If your staying in the upper reaches of the rpm a lot try a 185.
I ran with Roosty's porting and a boyesen RAD valve to top it off. If your looking for someone in Australia for the machining try Bruce Woodley at power flow in Melbourne, i really like his porting as well. Bruce also machined my head to run better on unleaded to (now prefers 98 octane from the pump but will run on 95 as well).
The other thing I did was put close ratio 5th gear in to stay on the pipe a bit easier in the sand. Final gearing was 15/40 which was more than enough for me to round up the others when needed.

Yeah mate, that was me alright. I think with the setup I had I was lucky to make it down. I wont be making this mistake again next year.

that is some very aggressive porting, look at the exhaust width. I had a jug that wasn't even close to that wide on the exhaust and probably similar duration and when I was on a pwk, I want to say I was around a 190 main, but then I switched to a mikuni TM with a powerjet.