Two-Cycle Motor Oiling

| December/January 2002

  • SmokStak

  • SmokStak

A recent topic on the Engine Ads SmokStak bulletin board at caught my interest when an antique Maytag oil mixture discussion gave way to some high-tech two-cycle oiling discussion. As ever, various individuals started, commented on and concluded the following bulletin board thread.

I have a little single-cylinder 250 cc two-stroke motorcycle engine. I replaced the piston and liner twice before it was even warm enough to ride this spring. After finally getting everything adjusted right, and spending nearly $400, I started using AMSOIL two-stroke oil at a recommended mix of 100:1. It ran very well all summer - until last week.

I had all my firewood cut for the season and had about a quart of 40:1 chainsaw gas left over, so I dumped it into the motorcycle, which still had about a gallon of the 100:1 mix in it. I had to ride about 12 miles on an errand later that day, and on the way back I noticed the motorcycle steadily slowing down as I tried to give it more throttle. All of a sudden, at about 30 or 35 mph, the back tire locked up and I went into a skid. I squeezed the clutch and coasted to a stop. The engine had locked up, locking up the back wheel. I tried turning the engine over and it was tight and hot as hell.

The following morning I started it up and it made some god-awful noises, so I shut it right back off. I know the piston and the sleeve are probably shot. Logic says it locked up due to over heating due to a lack of oil. I can't understand why adding 40:1 mix with 100:1 mix would cause that. I think there is something very important that I should learn from this, and I need your help doing so. - Marty

Marty, a person could get a degree in the science of oil. I didn't, but I have worked on two-strokes for 24 plus years, 16 of them in a dealership working on saws, trimmers and mowers.

Every factory school goes into detail on what oil to mix and how to mix it. And every school says the same thing about too much and too little oil in the mix. Cut the oil too thin and it runs lean. Add that extra little bit left in the quart (if a little is good, then more is better, right?) and you have another lean mixture. Heavy on oil, but lean on gas.


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