Thread: Two Stroke Oils/Fuel

And who remembers the days when you just took your can to BP, and filled it with Zoom25...

Originally Posted by Blknight.aus

yeah but you've got to get it up to temp first and it can make starting frustrating. I thought the magic point was about 17:1.

Nope, richer than that.

Most race air cooled two strokes that rev above 15,000RPM under load use around 16:1.

Maybe oils have changed and engine technology has changed, but there used to be a simple reason why two strokes produced more power with more oil.

Extra oil made the fuel a bit thicker so that it was harder for it to flow through the main jet. That leaned out the mixture. Two strokes depended on the air/fuel mixture for part of the cooling, so needed to run a richer than optimum mixture to avoid overheating.

So adding extra oil gave a leaner mixture and more power until the engine began to lose power through overheating which made the air/fuel mixture in the crankcase less dense; the opposite effect to running an intercooler.

If pushed hard enough with too much oil, the engine would seize from overheating.

If the carby was jetted correctly for the thicker fuel mix, it would happily run the oily fuel, but if it was jetted for a mix with very little oil, adding more oil could make it seize.

Originally Posted by rick130

Nope, richer than that.

Most race air cooled two strokes that rev above 15,000RPM under load use around 16:1.

Years ago, I had a moment of brain fade as I was filling up my Hodaka 100. I realised when I noticed vehicles getting lost in the smoke cloud behind me at the traffic lights, that I had added double the required amount of oil.

Since it usually ran 20:1 and the tank was almost empty before the fill, it must have been close to 10:1.

It seemed to be running quite happily apart from the embarrassing cloud of smoke.

Originally Posted by vnx205

Maybe oils have changed and engine technology has changed, but there used to be a simple reason why two strokes produced more power with more oil.

Extra oil made the fuel a bit thicker so that it was harder for it to flow through the main jet. That leaned out the mixture. Two strokes depended on the air/fuel mixture for part of the cooling, so needed to run a richer than optimum mixture to avoid overheating.

So adding extra oil gave a leaner mixture and more power until the engine began to lose power through overheating which made the air/fuel mixture in the crankcase less dense; the opposite effect to running an intercooler.

If pushed hard enough with too much oil, the engine would seize from overheating.

If the carby was jetted correctly for the thicker fuel mix, it would happily run the oily fuel, but if it was jetted for a mix with very little oil, adding more oil could make it seize.

The main reason a richer mix works is better ring seal.

Air cooled engines have looser clearances to allow for expansion so this is where the richer mix helps (as it also did with karts spinning to 21,000RPM. Skirt seizure and little end failure was a real danger)

Water cooled engines with their tighter clearances just don't need the richer mix.

All the two strokes i've run (karts and saws) use simple adjustable carbies which makes tuning a snap (if you know what you're doing)

Like so many things, there are probably a number of factors involved.

If better ring sealing was the only effect, that would not explain why engines could seize with too much oil in the fuel.

My experience with two strokes was in the 1970s and the scramblers and short circuit bikes I was familiar with did about half the revs of the engines you are describing.