Re: Too much oil ?
In a premix engine, the carburetor main jet will pass a mixture of gas and oil proportional to amount of air going past the venturi. This amount of mixture does not change for a given amount of air. So, for a given amount of mixture (lets say one ounce worth), at 2% there is more gas than oil than a 5% mixture of the same volume, (one ounce). The lesser amount of gas in this 5% mix amount leads to a lean (gas to air) mixture, risking a heat seize as the gas provides cooling for the 2-stroke engine. The oil provides lubrication, but not as much of a cooling effect like the gasoline does.
Why is it that it is dangerous to exceed the 2% ratio in a 'modern' motor when the piston-ported ones ran so well on two and a half times as much ?
You can run a 'modern' engine on 5% mix, or 10% mix or what ever you want, but you will have to re-jet accordingly (bigger main jet) to make up for the diminished amount of gas in that heavier-oil mix, to prevent the risk of heat seizure.
Example is the p200 with autolube runs a 116 main jet. The pre-mix p200 (europe) runs a 118 main jet. 2 sizes bigger to provide a similar amount of gas, which has been displaced by the oil, on a pre-mix model.
I hope this makes sense.
You are correct. The piston-ported motors also had the design where the incoming fuel/oil mix charge is introduced at the back of the piston, after which the oil had to 'work it's way' to the main bearings and con-rod bearings. The rotary-intake motors had the incoming charge aimed right at the main bearings, and con-rod. Oil was where it needed to be due to the intake design. This allowed the oil amount to be lowered from 5% to 2% resulting in less fouled plugs, better combustion, less carbon deposits, and better efficiency.
So, the piston-ported motors are just jetted for the lower level of gas in the mix ?
There is no reason to deviate from what is recommended for your SS. Run what is specified for that engine. That is how it was designed.
My SS runs beautifully and has great power on 5% and I have no intention of deviating from that.
Interesting test article by Gordon Jennings, from 1978 about mixture ratios and how they affect power and longevity.
Vespas 1964 GS160, 1965 SS180, 1977 V9A1T, 1983 PX150E
Joined: 16 Apr 2011
This is an old horse thatís been beaten to death. Some run 5%, some 4%, some 3%. I have run 3% in my GS160 for several years and hundreds of miles with no ill effects. I currently run 4% in my SS180 as the engine in newly rebuilt. After a hundred or so miles Iíll take it down to 3%. It keeps the smoke down to an unembarrassing level.
P Series / Li / LML / Motobi
Joined: 24 Jun 2019
Location: UK - 3rd Rock From the Sun
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