Rear Wheel Lockup at the Track

[m in sc]

what does the underside of the piston look like? color above the wristpin?

[svgarage]

what does the underside of the piston look like? color above the wristpin?

Thank you to all for the input. I haven't taken the pistons off yet. Maybe today. Just got my bore gauge tools back.

[svgarage]

Yup. 4-corner seizure. My tuner is going to do a quick cleanup hone on the holes.

[m in sc]

oil looks a bit cooke dbut not bad. was def getting a bit warm but not ridiculous.

[teazer]

that's definitely a soft seize. you can hear it in the motor. Probably ok overall but yeah id still suggest a bit of fuel.

yep.  It nipped up.

I should have read to the end of the thread, but you can 'hear' it tighten.  I would fit larger float needle and seat plus larger pilots and better fuel tap as suggested earlier.

[svgarage]

Got 3.0 needle/seats in the mail today 

[1976RD400C]

Does the other piston look like it was getting hot too?

[Yamanatic]

Interesting anecdote and a grain of salt; for some odd reason, it's usually (almost always) the left side that seizes on ALL Yamaha 2T twins. Even back to the YDS/TD1 days, left cylinders and heads were in short supply due to detonation and seizure.

At the Yamashop back in the 1970's when RD's came in looking like this one, we would often just bore the left since the rights were generally fine. When racing, I've seized a couple 2-cylinder 2T motors, both RD and TZ, and it was always the left that was the worst. 

Even more telling was when buying a few retired race team bikes and parts over the years, there was always a shortage of left cylinders and mostly good used rights in the piles. Neither I, nor other Yama-riders and mechanics never could figure out why, and after a bad seize of a RD400 at Daytona that almost put me on my head, I jetted the left cylinder rich and ran a higher float level on that side which helped; that was the last one I stuck.

Warren 

[svgarage]

Interesting anecdote and a grain of salt; for some odd reason, it's usually (almost always) the left side that seizes on ALL Yamaha 2T twins. Even back to the YDS/TD1 days, left cylinders and heads were in short supply due to detonation and seizure.

At the Yamashop back in the 1970's when RD's came in looking like this one, we would often just bore the left since the rights were generally fine. When racing, I've seized a couple 2-cylinder 2T motors, both RD and TZ, and it was always the left that was the worst. 

Even more telling was when buying a few retired race team bikes and parts over the years, there was always a shortage of left cylinders and mostly good used rights in the piles. Neither I, nor other Yama-riders and mechanics never could figure out why, and after a bad seize of a RD400 at Daytona that almost put me on my head, I jetted the left cylinder rich and ran a higher float level on that side which helped; that was the last one I stuck.

Warren

That is indeed interesting! During normal operation, it is my right cylinder that usually runs 10-15 degrees hotter than the left. I'm getting ready to make adjustments to the carbs, but I'll wait till I get the new rings seated first. This is really a street bike and I don't plan to wind the motor up like that on a routine basis. I have two more days booked for the track in August, then it's possible that'll be it for the old girl. But it's always nice to know that the bike is 100% sat and ready for any/all situations. I'll be moving to Reno, Nevada in a few more years. Who knows? maybe Fernley will get it's act together and I'll do some track time out there?

[m in sc]

another weird anomoly, just as a point of interests, is when there's low voltage or such on a stock type ignition, the RH coil is always the one that seems to fail 1st. 

[svgarage]

another weird anomoly, just as a point of interests, is when there's low voltage or such on a stock type ignition, the RH coil is always the one that seems to fail 1st. 

Interesting idiosyncrasies 

[teazer]

When I had a shed full of RD400s, the first one I bought had the left cylinder 1mm O/s and right was stock, so I gathered up a few spare motors and most had larger left cylinders or were missing a left. I put that down to the left crank seal behind the alternator drying out and leaking, but I suppose it could also have been crankshaft whip on the side with the heavier rotor compared to the small primary drive gear.

[svgarage]

When I had a shed full of RD400s, the first one I bought had the left cylinder 1mm O/s and right was stock, so I gathered up a few spare motors and most had larger left cylinders or were missing a left. I put that down to the left crank seal behind the alternator drying out and leaking, but I suppose it could also have been crankshaft whip on the side with the heavier rotor compared to the small primary drive gear.

With all this talk about left cylinder issues, I looked back on my records to check things out. I remember than when I had the engine rebuilt, I had to shop for replacement jugs. The previous owner had left cylinder problems too! The right measured as standard bore, but the left was at 66.0mm!

[m in sc]

crank whip on the LH side is interesting theory, makes sense it can 'push' the seal out of shape. 

[Dvsrd]

crank whip on the LH side is interesting theory, makes sense it can 'push' the seal out of shape. 

If that's the root cause, then a Vape or HPI ignition should help, as the rotor is lighter, and much closer to the main bearing (so less leverage for any bending forces)
On that note, I have read somewhere that the heavy stock RD rotor can cause bending oscillations of the crank Stub at high rpm (say well above the stock redline), which in turn gave erratic timing due to the points cam being even further from the main bearing.