Re: 40:1 or 50:1 gas mix

Posted by docmirror on June 14, 2018 at 09:40:20 from (107.213.165.255):

In Reply to: 40:1 or 50:1 gas mix posted by old560 on June 14, 2018 at 07:10:19:

Lots of good comments from long-time 2 stroke users here. The switch to higher ratios had a lot of things at work. Much better 2 stoke oils, change to mostly synthetic(although the base stock is really not a true syn, it is a dino base), better metalurgy for 2 stroke pistons, cyl and rings, and of course - EPA regs.

Older 2 strokes used more fuel as well, with stoichiometric ratios running as rich as 11:1, where a perfect fuel air mix is about 13.2:1. As the engine runs leaner, it also gets less lubrication. The modern oils and modern metals can handle much lower oil mixtures at leaner settings, and the older engines seem to survive ok mostly due to the higher stoich of air/fuel. The carbonizing deposits are a mix of oil, and unburned fuel. However, don't try to run an older engine leaner, or if one does, then one must increase the oil mixture to insure adequate lube of the upper cyl. A 20-30 YO saw, or other 2 stroke will NOT run well with a lean mixture of 13:1 with a 50:1 oil mix. It will overheat, it will seize(eventually).

Injection-lube in jet skis, and snowmobiles came about because engines were blowing up with a pre-mix of 50:1 as those engines are beat to death by kids who go out and run full throttle constantly. The EPA got into it as well with new regs on emissions and the mfg tried to meet the new regs. Water cooled engines have a different requirement than air cooled, but they can work pretty close.

I would(and do) feel comfy running almost anything around 40:1 pre-mix. I don't like 50:1 unless I know the engine is not going to be worked real hard, or I know I can change the stoich mixture with a screw at the carb. Most small 2 strokes no longer allow the mixture screw to be moved, but some have it, and it's covered by a plastic snap cap. The other countries allow the mixture to be set, but in the US that feature is disabled(unless one removes the plastic snap cap).

If you have a mixture screw, note that it may be for the high RPM, or it may be for low RPM. Adjust with caution and read the plug after running for less than an hour. A white, or super clean plug indicates too lean, and that will lead to damage. It should be nice and tan, or a light chocolate for best stoich.

I run some 2 stroke aviation engines, and we are super-critical on oil, mixture, stoich, and general condition of the plugs each 5-10 hours of run time. Mistakes in aviation engine running can be very costly, or painful.

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