Got a question for the group here: would making up a special batch of 20:1 oil/fuel mix specifically for really cold weather help to avoid the leaning out problem? Or even 16:1? I understand that rejetting is probably the best answer; I just wanted to know if adjusting the mix would actually work.
ABSOLUTELY NO!
The problem will become even worse. I'll explain:
But first, what does a lean fuel mixture do?
At perfect mixtures gasoline and air can burn at over 3800f which more than enough to melt steel and aluminum, and at perfect mixtures can explode with more power than dynamite. So how do we keep this from happening in our engine?
Well, we have to avoid perfect mixtures. By keeping the fuel mixture slightly rich (and moving and large droplet size and other tricks) we keep the fuel from exploding and burning up our engine. If we let the fuel mixture go too lean, we are into the explode and burn territory. This is not theoretical. Been there, done that, you don't need to.
No amount of oil can eliminate or solve the effects of the high temperatures or detonation from a lean oil mixture. Think of an acetylene torch and hammer on the top of your piston. What will more oil help?
So why do cold temps make lean mixtures?
From 2 main mechanisms:
1) cold air is more dense, more oxygen per unit of volume flowing into the engine.
2) cold fuel and oil is more viscous. Less of it flows through the carb jets.
Gasoline's viscosity is much less affected by temperatures than oils.
The oil however, especially premix oil, is greatly thickened by cold temps, however at thin mixtures like 50:1 it is almost a negligible effect. The main difference between PREMIX and INJECTOR oil is the viscosity index. Injector oil is thinner and stays thinner at colder temps. Injector oil will work, but premix is usually better oil, all other things being equal.
So, when you add more oil to the fuel you are displacing fuel from what can flow through the carb jets, AND you are thickening the fuel viscosity, further reducing the fuel flow through the carb jets AND this viscosity and leaning out will get worse as temperatures drop. Your engine will starve for fuel, lean out and burn up the piston.
The only answer? Richen the fuel mixture (not the oil mixture) at cold temps.
How? Jet change, needle height, or possibly a float level change can get you through.
Many guys just leave the carb jetted for the coldest possible situation, but you are giving up performance in normal temperatures. More oil is NOT the answer. It actually makes the problem worse.
Steve
Hmmm, coffee at the back woods motel. Like my pants?