Timing could have shifted but it looks like it went lean to me. Petcock is a possibility... I had a similar failure (desert racing) when I ran out of gas. It happens quick, especially if you're on the throttle.

It's very helpful to do a pressure and vacuum test before teardown as I suggested a couple of times. If an air leak developed, you may never know now.

DT360 seals are not that easy to find anymore and some ebay sellers are somewhat deceptive... several show pictures of seals with Yamaha packaging in the photo but what they are selling are inferior single lip seals that may not hold pressure AND vacuum or may fail prematurely. And even OEM seals do fail or are defective.

it is indeed an aftermarket petcock
thanks

Easy enough to test the petcock. Pull the feed line from the carb and measure how much fuel it flows in 30 seconds or so.

I don't know if there is a specification for this but if you have a problem with fuel delivery it should be apparent. I would expect at least 200cc's of fuel in 30 seconds.

thanks all

Not ignoring advice to pressure test before disassembly, but right or wrong I decided to tear down fully to properly clean and rebuilt back, pressure testing on bench once complete.

Anyone have a line on where I can get oversized piston/rings? 1st oversize will clean up bore, but given scarcity I would got to 2nd if necessary to get parts. Also, where do I get most reliable as in not crap crank seals? If anyone knows/has please let me know so we can do a deal.

Thanks

Another thing that can happen... happened to mdscott on his DT360 when we were out riding Cal Dreaming a few years ago... is the cap vent can get restricted. He had just replaced the cap seals with new. We took cap apart and modified and it was fine after that. His would actually run out of gas... he got pretty good at "key off and out, unlock cap and open, key back in and on, cap closed" while riding back to camp... usually got it done before coming to a stop and this was out on the trails!

Your crank seals might be fine.... I'm being cautious from what I've seen and experienced. You might not need a bore either. Cylinder doesn't look hurt. Might be worth having professionally measured.

Enduronut should have all the parts you're looking for.

thanks
first stop is a machine shop to measure bore/taper
Interestingly enough I replaced the gas cap seals just before taking this ride....but I did pay attention to the vent labyrinth and determined that parts can only go one way and used a WD40 straw to make sure that I could blow air through the little hole, confirming that it was venting. But then again, the motor did melt down

I am not a fan of the "flip top" caps from my experience with them as a kid... so I have avoided those models as an adult. But could have been the cause for going lean.

I vaguely remember there were some slight differences in the Enduro cap seal designs. Mark (mdscott) will probably remember better but his did look like it should be fine but after gas got to the new seal a while it wouldn't vent. Even after the problem was identified as the cap we took it apart and I remember thinking it should be fine. But it wasn't. If I remember correctly it was the smaller rubber disk that seals over the labyrinth we modified by putting a notch or hole right in line with the small vent hole? (basically bypassing the labyrinth) I'd have to see it again to maybe remember or Mark might. I think it was on his 74 DT250a.. Might have been the DT360a. I've wondered if there are a couple of years that had a slightly different seal design?

Look in the manual for how to measure the piston and take the old piston with you. If bore looks good and machinist says old piston would still have correct bore clearance then you should be fine with a new standard piston.

Just checked fuel flow on mine, it took about ten seconds to deliver 200cc's of fuel. The tank is nearly full, it might not flow so much with less fuel in the tank. Still, even half that would be plenty.

Thanks, that was very generous of you

I emptied >gallon via petcock and same hose and filter leading to carb in a couple minutes so piston didn’t incinerate from starving carb

When I checked cdi timing at .114” btdc the marks were spot on. Cyl base gasket didn’t leak when tested and was nice at tight so I guess I will check seals next. Any ideas appreciated, thx

Just a few points...

** Impossible to know for sure if an air leak was contributory since engine was not pressure and vacuum tested just before teardown. Vacuum test is especially critical to test crank seals. A seal can hold pressure perfectly and not hold any vacuum. More likely if aftermarket seals are used that may not be "dual lip" like OEM. Not the end of the world. Just need to check carefully upon assembly. I've seen a few engines leak significant air through strange places like a front motor mount... last one was a DT360.

** "Main jet was clear" usually means little. Where were you holding throttle when meltdown happened? Doubtful it was at full throttle where the main jet would be the carb circuit in use. Important to check the pilot jet and the needle and needle jet to make sure they are original Mikuni and in good condition. Was needle clip in correct groove? See Mikuni tuning manual for description of carb circuits.

**Unfortunately, "Timing marks aligned" also means little. CDI's can fail where somehow the timing is way off. Important to check the timing with an automotive strobe timing light to verify all is good.