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I was under the assumption that race gas (C12) would always make a bike run rich due to having a lower specific gravity. It can suck more fuel through the same sized jet at the same vacuum than pump gas will with a higher specific gravity.
C12 specific gravity is .7175
Pump gas is typically .74 to .76 IIRC
YZ250’s are notorious for having advanced timing from the factory. 1-3 degrees advanced is common.
A crank case leak could also be plausible as others have mentioned.
Either way, given stock jetting. I don’t think C12 would make it lean enough to cause the damage in the pictures.
I’ve had friends run the same jetting in 80 degrees and 30 degrees and never melt an engine.
Also, if it was jetting related I think it would have grenaded sooner.
my guess is either the locator pin for the rings or an ignition side crank case seal went.
Probably dreaming on acid or something
And some useful information, pistons under extreme heat will have the ring locator pin (little pin in the piston) get "sucked" into the piston. It will appear as if it got pushed further into the piston, and the ring can spin. Remove that piston and it might be inside the piston now.
But from the pic, it doesn't appear to be the ring end gap you are looking at. 2 stroke ring end gaps are notched. Four stroke ring end gaps (at least on modern 4 stk engines) have flap or 90 degree cuts. Looks to me like a broken ring.
I would post a pic of a 2stk ring end gap but you can google it yourself, bruh.
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You said it was 30 some degrees on race day, and I'm assuming the video was shot on a warmer day. If you're jetted close to the edge on a 45 degree day and then drop down to 35 degree day, well there's your smoking gun. Check your timing, check your squish and definitely do a leakdown after she's rebuilt before it goes in the bike. Then reevaluate your jetting and maybe have a couple specs based on your temperature variance if you ride in varying conditions. But the exhaust side melted off is excess chamber temp, jetting, timing, fuel, compression and/or even your pipe can each and all contribute. Bummed for ya though, never a good feeling.
BRC YZM500 kit
https://brcracing.ca/yzm-500-my99-21/
Bikes always run greater than normal before going BOOM
Stock Jetting, stock compression running C12 on a cold 30 degree day. YOU are the reason that motor blew. Motor was beyond lean, out of time because of the straight C12 and detonating so bad it probably was detonating at idle on the stand.
C12 has completely different bouyancy charcteristics that require jetting changes. Let alone the change of how fast the fuel burns compared to other fuels.
Those craters on the piston are detonation pits and NOTHING was bouncing around in the motor. It would have flown out of the port into the pipe maybe hitting 10 or so times, it doesnt magically stay in the cyclinder and cause a million pits.
Dumb question for you but has the bike possibly been sitting for a while?
Oof
I was at Mammoth in '87 running race gas for the first time and my bike was getting slower - but no pinging like with pump gas. I was able to pull off before it fully locked up but I was not used to lean without detonation - I had gone too lean for the cold morning practice - I richened it up a bit for the race and luckily didn't wreck the engine. Perhaps that is related to your race gas issue - though you said it was running good before it blew. IDK.
Pit Row
Post a reply to: pay your respects - 23' YZ250 engine failure