Today's Whodunnit ...

Is the plug you showed a few posted back the result of a wide open throttle test or just a partial throttle run?

That's as it was when it seized. Not wide open, just over 1/2 throttle. If I get the time in the next week I will test my theory into why it may have done it. It did it again on the new piston in the second video. No damage this time though.

Bore is too tight. When one of my 390's seized when it sucked in a soggy base gasket when I bored it I left it at
005" while a break in run it seized. I did the crank bearing, seals, leak down test
She was perfect I opened the bore up larger .006+" she was fine. I hammer my 390's. No problems after opening the bore up.

Heres a service bulletin from HVA regarding the modification of a 1980 390 to a 420. It says that the recommended piston clearance is 3.5 to 4 thousandths (about .09mm). Based on this I would think that Grouty is within an acceptable tolerance at .10mm especially since the seize occurs while at a less than aggressive throttle. But then theres the possibility that I'm overlooking something.

Hard to tell from the video's but were all the problems uphill ? First video bike stalled shortly after getting on trail but your bike still turned over a couple of times when it died just like you ran out of fuel . Then when it seized it looks like at the top of a hill . Second video it also stopped on a climb . Fuel starvation ? In second video , is it the same hill you noted ? 1st time up seemed shorter than the 2nd time when it stopped and sounded like you were on the gas more the second climb when it stopped . Float level ? Fuel restriction ? Venting ?

The hill climb was the same one. First time up I was on the gas, I let off at the top to make sure the other two lads were following. Second time up I felt it tightening up a couple of seconds before it died. No other problems during the day.
I am convinced it is fuel related. As I took both 390's that day, and looking at the difference between the motor builds, one thing struck me.....
I need to prove that theory. Simple though it is.

I have finally had time to take the Auto out for a run this evening. No dirt involved, just road work. I know it was happy at lower revs, and wanted to test my theory with the higher revs.

All went very well. So .... my theory ! Stupid though it may sound - the inline fuel filter ! I noticed that after I put this latest piston in and had it running outside the workshop door, when I brought the revs up to a certain level, the fuel filter vibrated causing bubbling of the fuel. After looking at the difference between this 390 and my other one (which has been brilliant), the only difference was the filter. My thought train surmised that at higher revs the harmonics kick in (vibration) and aerate the fuel. If this aerated fuel passes to the float bowl it will cause a lean mixture with the tiny air bubbles in the fuel. That may explain why it was fine at lower revs, and seized at higher revs. I was going to try and rig up my gopro to film the float bowl (Lectrons have a clear bowl) as I was riding to see if the fuel showed signs of air. But I got bored trying to mount the camera, and just took the filter out of line. So now, with just a rubber pipe from tap to carb, there should be no air in there to aerate, and a smaller mass should move the point at which the harmonics would affect it much higher in the rev range ... i.e. waaay past the rev limit of the motor.

I guess many people run an inline filter without issue. I just guess I was unlucky with mine producing a harmonic where it did. My other 390 used to do a similar thing which unscrewed the engine mounting nuts ! I cured that by putting the bolts in the other way so the nuts were on the other side. That worked a treat.

I hope I am right with this ...... I really don't want to be back in a few months with another tale of woe and scrap pistons

I forgot to mention. I took some temperature readings when I got back. After stopping the motor I took readings from the fins. Bearing in mind while it was running there were no "windows" showing on my temp strips stuck to the motor. The windows start at 160 deg C. By the time I took a reading with the laser toy one window was lit (about 30 seconds or so after the motor had been switched off). The top outside fins on the head were showing 162 deg C. The lower barrel was 90 deg C. Actual running temperature I would guess to be much less than the 160 deg C.

Ha i had it in my 360 when i first got it completely forgot i removed it!
It wasnt sufficient to supply the 38mm at full chat.

Good read hope thats cured your issues.

i had to go to bigger line and a bigger filter on the 500 . guess the previous owner had never run it wide open through 3 gears before .

Although the Lectron has a 1/4" (6mm) inlet stub, I use a 7mm i.d. fuel line (it fits the carb perfectly tight) and a genuine Karcoma tap with a 5/16" (8mm) stub. Flow should never have been an issue. I an convinced that the fuel aeration was the issue. Only time will tell.
It would be nice to know if anyone has the WFO fuel requirement for the 390. Either in litres/minute or gallons/minute.

Fuel aeration in filter is a no big deal, but in the float bowl is a problem, reason to put a screen around main jet.
Later George

there was a cr 500 Honda pinging thread on ozvmx where the pilot jet was extended deeper into the float bowl and fixed the issue. it was a known fix for other vibey big bores in the day where vibration was atomising fuel in the bowl