2 Stroke Mix

[rineholio]

The "more oil in the fuel affects your jetting" argument has been rehashed for years.

So long ago I can't remember, some dirtbike magazine loaded up a bunch of 250 mx bikes with 20:1, 32:1, 40:1...

Bottom line: All bikes ran fine without a jetting change. They dyno'd with the most power on richer oil settings. Who knows? Maybe it was because the extra oil created a leaner fuel mixture, or maybe it lubricated better and sealed rings. I don't know. Neither did they. Neither do we. It wasn't enough HP to matter anyhow.

Think statistics with this. Is the difference between 32:1 and 80:1 enough to significantly affect the air-fuel ratio enough to require different jetting?

Carburator jetting affects your burn/exhaust temperature. Rich jetting runs colder because their is more fuel than the available air can burn. Lean jetting runs hotter. There is no excess fuel to absorb heat.

Oil "vaporizes" at certain temperatures. 2stroke oil is formulated to "vaporize" in the temperature range a two-stroke exhaust should be operating. In the ideal world, a bike should be jetted so that (because of its operating temperature) nearly all the oil that ends up in the exploding mixture is vaporized and run out the pipe. The oil touching the piston sides, rings, wall... will stay liquid and lubricate. Assuming mechanicals are in good shape, a properly jetted bike won't foul many plugs. There is no way to avoid running cold in trials though. Extensive idling is required. Hence the balance and full throttle to heat the exhaust and clear the plug.

Mixture ratio is determined by what the engineers who designed the engine say it should be. I defer to them.

I believe all the above is on the Eric Gorr website. I'm just a mouthpiece. It's worked for me over the years.

My two cents:

1. Use any quality motorcycle 2-stroke oil and mix it like owner's manual specifies. I mix a hair richer to account for those special times when I use the old "looks about right" method.

2. If you mix in that range and your bike is fouling plugs, a little trouble shooting is necessary. Right plug in good shape? Clean fuel supply? Compression OK? REEDS? I don't know about trials bikes, but when reeds start going on my CR500 it really messes up the jetting.

Last thing you'd do is fiddle with the carburator. My 315 has never needed more than a tweak of the low speed mixture screw to make it run great. Last plug ran two seasons and was still in good shape (but I don't get to ride much). The next year should be interesting because I've moved North and will be riding in temperatures from hot to as cold as I feel like.

Jeff

70:1 or so, Montesa recommended spark plug, stock jetting because bike runs great and plugs last a long time.

P.S. What was this thread about, anyhow?