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Yamaha MX & Off-Road Dirt Bikes
Help! I blew up two '06 Yamaha YZ125's this past weekend
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[QUOTE="Rich Rohrich, post: 1120979, member: 16241"] That myth is older than Malcom Smith's original boots. :) The amount of oil in the mixture doesn't influence combustion temperatures to any significant degree. Without seeing the whole thing first hand we are all just guessing as to why these engines broke but I'll add my guess just to muddy things further. :whoa: Fuel requirements (jetting, octane, distillation curve) are very dependent on how much work you are asking the engine to do, in other words the [b]load[/b] being placed upon it. With smaller two-stroke engines this becomes even more significant. A good way to illustrate it would be to go up a hill in a specific gear and throttle opening that causes spark knock, and then ride back down in the same gear and throttle opening. It will tend not to knock on the downslope due to the reduced load on the engine. Small two-strokes running in sand present you with the rare situation where you can use nearly all of the power available and do it for extended periods of time. Lots of load and lots of heat, for long periods of time. If you add in how tough it is for radiators to work effectively on a sand track you get and engine that runs much hotter, is being loaded near it's maximum, and requires more fuel and better cooling (especially on the piston crown) to balance the temps and load. All of this is a perfect environment for knock (aka detonation) which is influenced by temperature, pressure and time. Raise the temps, the pressure or the time under pressure (load) and you increase the risk of abnormal combustion and eventually detonation. Pick the right fuel and it will tolerate high temps and pressures better and have a longer delay time before it crosses over into ugliness. Adding in more fuel with richer jetting [b]might[/b] cool things enough to fix things but experience has shown that if you load the engine hard enough jetting alone won't compensate for the wrong fuel. I hope this helps some. [/QUOTE]
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Yamaha MX & Off-Road Dirt Bikes
Help! I blew up two '06 Yamaha YZ125's this past weekend
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