Seizure at the end of a very long straight

I am guessing that our KX65 mini roadracer that has a few hours on it has seized up and lost compression this weekend at NOLA. The temperature was 20 degrees lower than we had run it the two weekend prior in but it was foggy so I figured the additional water would make the mixture richer. We run a 200 size main and a stock expansion chamber. Question: is there any add on to the carb that enriches it on decel? Like a backwards accelerator pump? apparently it happened at the end of the long straight, or is there another method of tuning for this situation other than I should have played with the jets? Thanks, Kevin

 

Piston or crank seize?

Did it seize before or after throttle release?


I always had my slow circuit running rich for throttle release condition.

 

What oil ratio are you running? When we mini-raced a 65 I was mixing 20 to 1.

I don't have an answer on aftermarket enrichment.

 

I want to say Wiseco offers 'road race' pistons for many of the two stroke minis. I haven't had one in my hand but I want to say the windowing on the pistons are different to help account the different use pattern? I'd like to know more on this front as well in case I road race my WR250R.

 

If I remember correctly, temperature has a bigger effect on relative air density (and jetting) than humidity for a given barometric pressure. So, with a 20 degree temperature change you might have been running a little leaner than you were expecting.

As others have noted, the type of seizure might give you some more clues. Did the bike show any symptoms of running lean? Did it occur on the throttle or off (i.e., when downshifting)?

 

Power jet in your carb? It's always trickling with some airflow.......:up:

 

It was a top only seizure, Kevin had shut it down hard at the end of the straight apparently. He has been made to take apart the bike (we love 2 strokes) and sent to bed without riding. I will call Wiseco today about road race pistons and post if there is any. Hoping the return to normal scorching hot temperatures will return the jetting to what felt pretty darn close at the last two rides. He loves riding it. Now we just need a stockpile of cylinders and pistons!

 

Maybe a bigger pilot jet and crank the throttle stop up a little to ensure some top end lubrication on closed throttle?

 

If it's on top of crown, generally it's lean & overheated. The weakest point when there's a lack of lubrication is (generally again) the small end.

I'd rather go for a richer pilot over an open throttle, but done correctly, both avenues work.

In reference to another post, I doubt there's a powerjet in that lowly carb. Anyway, it would close and lean things at high rpm.

 

Also, you may want to be careful just putting a Wiseco in it. Cast pistons and forged pistons expand differently, so in some applications you need to run more clearance to account for it. I am not an expert, but it doesn't sounds more like jetting than a piston to me. What does the plug look like?

 

After a seizure, plugs are pretty hard to read....

 

This plug was unreadable, The seizure was down the piston a bit, the ring area. Kevin rode it another lap to apparently see if it would self heal, it did not. Thanks guys, Ill check the pilot.

 

Oh, and humidity is pretty low on the scale of importance on the tune by weather-o-meter.

 

I'm a big fan of OEM pistons in two strokes, especially for road racing. I don't even want to think about all the horrible things we put out KTM 65 through, but I will say we got our money's worth out of the original piston.

 

But it can certainly tell you if there was detonation, which is what I was getting at.

 

If you're lucky and the rider is quick on the switch....

 

i have a CR125R motor set up for road race duty in an Aprilia chassis... I did what my dad recommended, which is what he did for his ama flat tracker that would give him hell on the mile tracks.

close the air screw up more or completely, run a larger pilot jet, close down the slide idle height screw so the bike doesn't want to idle. when you close the throttle you will get a lot more fuel and lubrication than you did before. this should keep it from getting tight with the throttle closed.

if you have a power jet solenoid, have it turn on at high rpm zero throttle conditions, that will help also, but kx65's don't have one from the factory i don't think?

i am routinely seeing 560-590 egt numbers while on full rip and have not had any issues when shutting down

it might be that your ring was just a little tired and some blowby got the varnish and smear started?

I almost forgot, check for air leaks also, air leaks are the kings of causing seizes

 

Most likely this was just due to not jetting for the temp change. 20 deg is definitely enough to need a main jet change. Main jets are cheap, buy several! Dirtbike jetting is definitely leaner than roadracing jetting, usually throughout all of the circuits. Very different demands off road than on pavement. The radiator isn't the only thing cooling off the motor in a 2 stroke.

Fattening up pilot circuit and losing idle function as described in the above post is a good idea as well. (though it won't make up or "fix" poor jetting or an airleak) It dumps unburned fuel/oil into the cyl even on closed throttle, helping to keep things cool and lubed. Keep going up on the pilot until the bike gets "lazy" on the blip (slight bog/hesitation when you blip the throttle) and then go back to one size leaner.