Freeway did her in

[Arrow]

Thanks. I don't disagree with you, it could well be the tune. If we rule other things out, the tune may still be with us.

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[rodneya]

Post some pics of the piston, particularly the crown and underside.

[Striker1423]

The spring was a new replacement as earlier on the 250 top end I had a return issue (it was cable related not the spring fyi). That was as far down as the spring would go into the pump body. I wound it as tight as it would allow the pump to open. Was a pain, but it has never let go.

As far as the pump is concerned, I will do as Arrow asks. As soon as I get footage I'll let it cool and tear the right head off and see the piston.

I should note as I hadn't mentioned before. That when I gave the bike full throttle on the freeway,  from 3/4 to the top, the bike was no longer building steady power. To me that indicates lean, but I'm sure the piston will tell the tale.

[Arrow]

Thanks for the update Striker. The pattern spring has longer ends than the Yamaha spring. It can only fit (as all pulley springs) with about 1/4~1/3 turn of tension. If not the pulley will not rotate to fully open.

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[Arrow]

Am I correct in saying, your pump has no gasket fitted between the two castings?

I would like to actually see this pump and sort it properly for you. But like most things in life, there is always a down side.......

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[Striker1423]

Am I correct in saying, your pump has no gasket fitted between the two castings?

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You mean the main body of the pump, in the middle of it? When I replaced the seals I followed the WellaRD guide on the aircooled RD forum for the pump. The rebuild kit I purchased only had the new seals, screws, and the square gasket for the pump cover.  It did not come with new check balls, or springs. This was 2 years ago so I don't remember if a gasket was on the main body of the pump or not. If it was, I believe I would have tried to save it.

Here's the new video.

[Arrow]

Thats ok.

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[Striker1423]

Ok, so the initial look revealed what looks to be a four corner seizure. Still have to disassemble the carb setup to pull cylinder. I'll also pull the left side just to be safe.

[m in sc]

Under the crown will tell the story, but yeah, i think heat. just enough.

[Striker1423]

Here's the piston crown.



EDIT:

Here's the piston sides too.

[rodneya]

Doesnt look lean, the piston is washed completely clean. But it did get hot.
Rich mixture could have been just enough to keep it cool enough to stop melting until you got off the gas.
Mark looks like he was correct with something in the ignition

[Striker1423]

I set the initial timing with the dial indicator to 1.8mm on the Vape ignition. I found an old Craftsman timing gun on Ebay and bought it. It worked well...sporadically, but it looked OK once functioning. However, I was having trouble with the ignition rotor slipping. I thought I rectified it by heating the rotor and re-seating it, but perhaps the timing moved? If the rotor did slip, wouldn't it only retard? 

Also, the left side piston and cylinder looks just fine. A bit oily as it should be, with no real discoloration. Both heads look identical too.

[m in sc]

weird it was on the right. any signs of a slight head gasket leak? inspect the gasket very closely, look for a line across it, even a thin one.
def doesn't look oil related.

Im not going with timing either, the domes would look different.  It got just hot enough just quick enough to expand that piston just enough to stick.

you know, it might even boil down to the bore clearance was just a hair tight on that side, that simple.    everything else looks correct of the headgasket checks out.

[1976RD400C]

It looks like the head gasket may have started to leak out the front side. How much run time since it was bored? Could it be that side was just a touch too tight? Is that a forged piston?

[Striker1423]

Here's a few more. I should also note, that the spark plug hole was repaired with a timesert. Well, at least a similar thread repair. I always have dark threads on the spark plugs on this side. But, never noticed any loss of compression. Perhaps it leaked there? May be the culprit.

Runtime, just over 2000 miles on it. First time on the freeway for over 10 miles. Wiseco pistons.