Rear Wheel Lockup at the Track

[svgarage]

So I finally had a chance to take my RD400 to the track. It's been modified a little- original heads with reshaped dome and a light skim, mildly ported cylinders, TDR reeds on a hogged out stock reed cage, reed spacers, VM28 carbs with Dave F mods (P-4 needle jets, 240 main jets), Y-boot with UniFilter foam filter, custom chambers probably equivalent to Wicked pipes. Installed BR9ES plugs last week. For fuel, I was running ethanol-free 94 octane. Timing is 1.9mm BTDC with HPI CDI. I'm still using the stock oil injector shimmed to 32:1 with Yamalube 2S. I squirted another 20cc's of 2R in the tank for good measure.

Motor was running perfectly. Took the bike to Thunderhill Raceway in N. California. It was my third session and I was ripping down the front straightaway at 90+ and went well into the redline. I let off as I headed into Turn 1, then suddenly I ran out of power and the rear tire locked up!! I was able to skid it to the outside of the turn and stop. I was thinking that I seized the motor. A minute later, an instructor came over to check on me. He asked me if the bike will start and I said I didn't know. I kicked the motor and it wasn't locked up. It started again on the second kick. What the heck??

I gently and cautiously rode the bike around the track and pitted in to my paddock space. I checked out the bike and everything seemed ok. I did finish the subsequent sessions, but I stayed away from the redline.

Any ideas what happened? First I was thinking I need to bump up a size on the main jets, but I wasn't at WOT. So I next thought maybe I need to raise the needle a slot. I had considered that maybe I was in the redline too long and maybe I didn't get enough fuel, since no port timing had been done. But now, I'm just wondering if my bowls were empty because the petcock couldn't keep up with the consumption.

The bike is home safe and sound. It seems to be okay, but I'm still scratching my head about what happened and would like to address the problem, so it doesn't happen again.

Any insights would be appreciated. TIA

[m in sc]

did you shut down the throttle? and on premix?

[svgarage]

Running oil injector, plus I added additional oil (2R) in the tank and gave the tank a shake. No, the throttle was still opened about halfway.

[1976RD400C]

You better at least take off the pipes and look at the piston skirts.

[m in sc]

maybe. me id up the pilots one size and adjust the air screw to compensate, even at half throttle. John Aylor tells a story of locking up a triple doing the exact same thing and turned out it was the pilots. even on partial throttle shut down it will still suck oil/fuel thru the pilot on decel   

[svgarage]

maybe. me id up the pilots one size and adjust the air screw to compensate, even at half throttle. John Aylor tells a story of locking up a triple doing the exact same thing and turned out it was the pilots. even on partial throttle shut down it will still suck oil/fuel thru the pilot on decel   

Cool. I'll do that. I'm still thinking about getting a Pingel petcock to rule out the fuel starvation. Do you think that's a good idea? or a waste of money?

[m in sc]

honestly on a race bike id def go pingel. i've starved a motor before but had more to do with needle and seat size vs petock and it wasn't fun. you'll feel a starved motor just go flat like a revlimiter. mine would do it only going into 6th gear wide open, totally repeatable. bumped up to 3.0 seats and problem resolved, but this also has (2) aftermarket petocks so that was never an issue. (fwiw)

[svgarage]

Thanks for the suggestion. Sounds like cheap insurance. I'll look into new seats.

[Hawaii-Mike]

Are you sure it wasn't the back brake locking up?

[svgarage]

Are you sure it wasn't the back brake locking up?

I hadn't thought about that. But there would've been resistance leading up to the lockup and there was no smell associated to the event. I've reviewed it on the GoPro video and there was definitely a loss of power before the lockup. Additionally, I ran another four sessions after that with no incident, since I stayed out of the redline.

[Yamanatic]

Didn't you pull in the clutch when it locked up? That would have told you if it was the motor or gearbox/brake.

Complete top-end lockups usually leave plenty of evidence (like aluminum) in the bore. Another danger if the lockup was hard enough is the chance of ovaling the rod. Doesn't happen if the brake locks.

I have had rear brakes lock up on track (well once), and it comes on fast. I fixed the problem by not using the rear brake at all unless experiencing off-tarmac excursions.

At Moroso riding an RD400 I broke a rearset on like lap-2, which disabled the rear brake but the peg was fine; my wife always timed my laps, and suddenly my times dropped 1~2 seconds, and I actually had better control going into corners. From then on, I quit using the rear brake altogether and permanently placed 1 or 2 positions higher. Try it!     

[svgarage]

Yes, I did pull the clutch. Hmmm...that's an interesting point. Didn't really think about that. Everything happened so fast.

So I just reviewed the video again and after skidding and hopping, the motor does spin again as I'm slowly approaching the rumble strip and the skidding is no longer audible. Maybe it *was* the rear brake. I recently switched my rear setup from disk to drum. But it's still curious to me why I didn't experience more hiccups for the rest of the day. And could the brakes cool that fast to allow me to ride again just 2 minutes later? 

[m in sc]

they definitely can. weird. hopefully that was it 

[svgarage]

Here's the video clip of what happened. I'm still a bit baffled. If it was a rear brake lockup from overheating, why did the engine die? I think I might take off the top end and just check things out for yucks.

[m in sc]

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.