13° F (-11° C) On Long Island This Morning

Be careful, cold weather leans them out. Years ago I blew a couple closely jetted snowmobiles within minutes of each other on a wonderfully crisp -25c morning...


Steve

I have found that these engines love the cold air. Could be cause they go lean? I dunno. I run 4 oz synth per gal year round never had an issue or a self destruct I could blame on a lean condition due to cold air temp.
Thanks for the heads-up sbest.

If you're jetted right on the ragged edge of lean fuel/air-wise, a large drop in temp can make them too lean.
Also, cold seizure is more likely. For those that don't know, that's when the piston warms up faster than the jug and can cause scuffing of the bore and worse. It doesn't always 'seize' the engine though.

Warm 'em up for a couple before running off when it's cold outside!

Excellent info! Good to Know. Thanks joe.

I'm waiting for the rain to stop so I can trail ride with 40 F and lower temps. Just camping at upper 20's F in the am, it not so easy. More power and not like the 95 to 100 F NorCal in late spring 2015. Possible not to need to be ice, just use snow if it remains around.

Crossing streams and crunch the thin ice over the shallow current!

MT

Bikeguy Joe said:

If you're jetted right on the ragged edge of lean fuel/air-wise, a large drop in temp can make them too lean.
Also, cold seizure is more likely. For those that don't know, that's when the piston warms up faster than the jug and can cause scuffing of the bore and worse. It doesn't always 'seize' the engine though.

Warm 'em up for a couple before running off when it's cold outside!

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I don't care what the rest of you guys say about him, Bikeguy Joe knows his schitt!

Joe is exactly right. If you are jetted perfectly right for 20c, you will blow an engine at 0c.
If you are really rich at 20c, you bike will run better and better as temps drop.
So the goal is to jet for the coldest temps you expect to run, and be aware if temperature ever drops below that.

Cold seizure can happen at any temperature, but cold temps do make it more likely.
It is just as Joe said, most often from working a cold engine too hard and MOST often with forged pistons in watercooled engines. I don't foresee it as a great problem with our China Girls (I love that name) but I have had it happen with aircooled motors before.

Cold seizure is often called a "4-corner or 4-square sieze" or some similar name because of the 4 cornered scuff marks it leaves on the piston or in the bore. Here are some picts:

Engine on the right is a cold or 4-corner sieze. Left is lack of oil I believe.

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allen standley said:

I have found that these engines love the cold air. Could be cause they go lean? I dunno. I run 4 oz synth per gal year round never had an issue or a self destruct I could blame on a lean condition due to cold air temp.
Thanks for the heads-up sbest.

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Allen, you are jetted rich if this is happening. Not a bad thing if you are not fouling plugs or using too much gas, but I can tell you like the better performance leaning out could bring!

This has nothing to do with oil mixture. Your 32:1 is fine, but leaning out in fuel mixture can cause overheating and detonation and piston or bearing destruction no matter what the oil mixture is. They are 2 different things.

Steve

OHhhh. Very good to know. Thank you Steve. kioshk, I bet you never would have thought a lesson would come from such an off the cuff statement about the weather. I love this forum.

The only time I ran a small gas engine in winter freezing temps, was when it was my chore to run the Briggs mower 4 stroke engine once a week in the winter for 5-10 minutes instead of emptying all the gas and doing the other cold storage tips.

The auto choke with the spring affected by cold to adjust the start up mixture but not running mixture seemed to always not work well. I usually just opened the fuel mixture needle valve a turn to start it up, then closed it back to what ever made the range of engine speed work optimally by ear.

I know there is no load in that scenario. I might see about if I ride in 40 F weather me on a 160 lb bike doing some trails, shall I see what there is to know about fuel air mixture so as to not have damage as you have shown?

I do recall but it was a long time ago I did run a motor bike and mini bike in cold weather trail riding, but didn't have any issue.

MT

Thanks Steve! Lol.

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.

Allen_Wrench said:

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.

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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?

What he said.

Yes. The difference between oil mix and lean/rich fuel mixture is sometimes hard for people to get, isn't it?

Maybe another way of saying it is that the oil mix is the ratio of oil/gasoline in your fuel while the lean/rich mix is the mix of fuel and air coming out of your carburetor and into your engine. ( "lean" being more air and less fuel. "Rich" being more fuel and less air.)

What do you think? Is that clear?

Or is it clear as mud?


Oh......one other note. I've ridden definitely as low as 7 deg F and, I think, a few degrees colder. My engine ran. It didn't run real well. But it made it.

Is this really mostly only a problem for two stroke engines, now that I hear about viscosity difference between fuel with and without added two stroke oil?

It is more common with 2 strokes, but highly tuned 4 strokes suffer from this also.

Low performance engines like our China Girls are less bothered by cold leaning.
Oil injected 2 strokes are less bothered by it too.

Yes Bluegoatwoods, you've got it right.

Steve

Thanks Sbest, for explaining that so well. I'm glad I never felt like I needed to test that theory. I probably should have figured that out myself. Mine runs fine in colder weather but I'm often in not much of a mood to take it out in winter (having old joints that dislike temperatures below a certain point). So far I've never really had to face a need to rejet either, she's harder to start in the cold but runs well when given time to warm up so it's never really been a big deal.

-26 here this morning. I'm with 2door. I don't mind this weather much so long as I'm indoors still in bed or at least inside with a cuppa coffee & a scone. Being part bear I believe in hibernation. Only four more months til the fishing opener, melting icicles and motorbike weather... woohoo!
SB

This is embarassing.
Went out to do some filter testing yesterday at -7c.
Didn't bother to richen up the carb, mainly laziness.
Several hours of running and testing Filter housings, found one that really worked.
Pulled the plug out and it was a nice grey colour, running GOOD. Right on the edge.
Took it for a good long run, reveling in the new found power.

Pulled the engine apart at the end of the day, found the exhaust side of the piston overheated and a piece of the piston stuck to the cylinder. Oops. too lean.

And I should know better...

Steve