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Old 02-22-2010, 01:12 AM   #111
Got some screws loose!
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Joined: Sep 2006
Location: In the Valley of Bomb Runs...
Oddometer: 2,366
Originally Posted by MeterPig
I had never heard this outside general "let the bike warm up". How are two strokes more prone to problems with cold running than four strokes?

My thought is that when cold-the cylinder and piston are cold and therefore there is more "space" between two during cold running. Under acceleration the motor would warm up quickly right?

Just curious your thoughts on this.
You have dissimilar metals for the piston, rings and cylinder and they are designed to fit proper at operating temperature. They have different expansion rates so you need to let the temps increase gradually. If you have a tight piston with new rings and you load the motor when cold, you risk having the piston expand faster than the cylinder can tolerate. The oil in your pre-mix is also designed to work optimally at a certain temperature, too cold and it cannot migrate fast enough to properly lubricate the cylinder walls. (imagine pouring some olive oil in a cold frying pan vs a hot one; see how it flows quickly in the hot pan? Its time in the cylinder is very limited and it must migrate quickly before it is combusted/exhausted) Too much ring pressure from the expanding piston scrapes all the oil off the cylinder and the conditions become favorable for a cold seizure. Even if a cold seizure does not occur, accelerated wear is happening. Do this often enough and you will shorten the lifespan of the moving parts.

This happens less to four-strokes because it only fires half as often for any given RPM and modern engines squirt oil on the bottom of the piston.(older motors still had the crank splashing oil around) All this combines to prevent the piston from expanding too rapidly. Incidentally, this is also why most four-strokes need choke for longer when cold. They just take more time to warm up because of all the idle time between ignition strokes.

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