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Old 10-09-2012, 10:56 AM   #45
rxcrider
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Joined: Apr 2007
Location: N Ridgeville OH
Oddometer: 1,249
I had this concern when I picked up a 2005 300 EXC a few years ago. I only found a few people who had actually had a piston seize and they occurred when running at or near WOT and then closing the throttle. Here's why:

The engine and exhaust are hot with little excess fuel and oil like you have at lower RPM. Close the throttle and the engine is still spinning along, but the fuel and air flow just got chopped. Now, when the piston comes down, there isn't a pressurized charge in the crank case to get pushed up the intake ports. This lack of incoming pressure allows the pressure pulse from the expansion chamber to push hot, dry exhaust gas farther back than normal. (go read here http://www.702sportbikes.com/showthread.php?14147-Two-Stroke-Jetting-The-Wet-Oil-Line-Method or google wet line two stroke jetting) Normally, the pulse from the expansion chamber pushes most of the fresh, wet charge back into the cylinder leaving just enough to keep the piston, rings and power valve cool and lubed. When the pressure pulse pushes the fresh charge too far back, hot, dry exhaust gas comes in contact with the pistong and rings at the exhaust port, heating them up and burning off teh oil film. This is when the engine will seize.

The solution I heard that made the most sense was to slow down with WOT engine braking. If you push in the kill putton and twist the throttle to full open, you are pushing a fresh, wet charge through the engine, without burning it. After a couple seconds, you will have filled the expansion chamber with a wet fuel / air mix as well. The next step needs to be done in the correct order to keep from blowing the expansion chamber off the exhaust flange. Doing it backwards won't always lead to a big bang, but it can. Close the throttle, then release the kill switch and the engine will return to life. Then resume riding normally. After being at WOT with the ignition off (where you will observe more engine braking than you are used to from your two stroke) the pressure pulse in the expansion chamber can push the fresh charge as far back as it wants because gas in the expansion chamber is also cool and wet.

The issue is that a perfectly jetted two stroke at WOT won't seize, but when you close the throttle abruptly without reducing the engine speed as quickly, the jetting is no longer perfect because the pressure in the exhaust is much greater than the pressure coming from the crank case.

This can happen as easily in a long sand wash as it can on a road section.

I first learned about this subject when a friend began dual sporting a CR250R. We made him get an FMF Q because the shorty silencer someone had put on it was way too loud. What we didn't realize was that by going to a more restrictive silencer, the pressure pulse would become stronger and he would need richer jetting to accomodate it. Having only dealt with four strokes at that point, I figured a more restrictive exhaust would just make it run a bit richer and the worst that would happen is a fouled plug. I was dead wrong and he seized the piston when he closed the throttle to slow down for a turn on a road section.

EDIT: and I'm now dualsporting a 1999 Husky WR250
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rxcrider screwed with this post 11-16-2012 at 08:50 PM
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