Rear Wheel Lockup at the Track

[svgarage]

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.

What's a soft seize? Not familiar. I already ordered 3.0 needle/seats. I've got 35 pilots, but I can bump to 37.5- I reviewed my spreadsheet and it ran pretty good there.

[m in sc]

when the piston will seize (swell) just enough to stop the motor but not nec hurt anything too badly. Ive done this on street bikes, have seen it at ahrma races as well. Hell, even did it on a borrowed bike at deals gap once   

other factors come into play. the needles and seats are just good insurance.

[svgarage]

So what is the remedy? That was scary as f**k! More fuel? Richer mixture? It had a ton of 2T oil in it. Did it basically run out of petrol and was just sucking air, hence the overheating?

[Hawaii-Mike]

More oil is not the same as a richer fuel mixture.

If the piston has some scuffing you might think about changing it. Maybe you can save yourself from honing or reboring the cylinder.  Muriatic acid works great for cleaning embedded aluminum from a steel cylinder bore.

[1976RD400C]

I would expect that to need a new piston. Post up pictures of everything after you take it apart to help figure out what caused it to happen. 

[Kawtriplefreak]

More oil makes for a leaner fuel mixture.

[svgarage]

More oil is not the same as a richer fuel mixture.

Yes.

More oil makes for a leaner fuel mixture.

Yes!
Wasn't even thinking about that when I added more oil.

Yeah, I figured a new top end might be best. Shopping for parts. Base gaskets already in the mail. I've been on stock wrist pin bearings, as that was part of my original rebuild, before I decided to go bigger on mods. Going to upgrade those while I'm in there. Likely to just go with Pro-X pistons.

[Striker1423]

The flashback I just had to the 35-mile freeway run is all too vivid. My RD350 stayed locked for over 5 minutes though. Still ran and got me home rattling away at 25mph. Damn Mustang cutting me off lol. 

I soft-seized my old Kaw G3SS a dozen times before I had a new bore done. It would shut off and then refire straight away. Was the weirdest thing.  Piston had what looked like a "four-corner" seizure. Somewhere on the net is a piston seizure chart that Arctic Cat had out for snowmobile techs to reference. It helps to visualize the damage from various issues.

Although that's tiny, so try this:
https://www.scribd.com/document/44454041/Two-Stroke-Piston-Diagnosis

[svgarage]

Thanks for the chart, Striker. That's helpful. I pulled the head/cylinders off the motor yesterday. Looks like the seizure occurred on the left side. Piston scuffing at outside exhaust and inside intake ports. Top of pistons look good. Haven't taken pistons off yet. Going to retrieve my bore gauge tools back today, then do some measurements.

[rodneya]

Thanks for the chart, Striker. That's helpful. I pulled the head/cylinders off the motor yesterday. Looks like the seizure occurred on the left side. Piston scuffing at outside exhaust and inside intake ports. Top of pistons look good. Haven't taken pistons off yet. Going to retrieve my bore gauge tools back today, then do some measurements.

What does the dome look like, and underside of piston crown?
Intake and exhaust side seize with normal looking dome could be no oil. If the dome is really dark it could be overheating.

post some pics of the parts.

[svgarage]

What does the dome look like, and underside of piston crown?
Intake and exhaust side seize with normal looking dome could be no oil. If the dome is really dark it could be overheating.

post some pics of the parts.

I'll pull off the pistons sometime this week. I am stuck in Jeep world for the next couple of days, getting ready for an outing. I'll get some pictures up after I do that. I hate posting pictures to this forum, it's such a PITA and I have to hunt down which ones of my accounts haven't closed yet...

[rodneya]

Just hit reply and at the bottom in the middle you can attach pics directly to your post

[svgarage]

Just hit reply and at the bottom in the middle you can attach pics directly to your post

Oh, cool. I guess I forgot about that function- think I used it once before.
Here's some pics of the left side.

[m in sc]

4 pt seizure, due to heat. i still think it could be remedied w the pilot.

[rodneya]

Almo

4 pt seizure, due to heat. i still think it could be remedied w the pilot.

Almost zero wash on the piston crown, I would go up on the mains as well.
Definitely do a leak down test after putting back together.