Re: 1978 90hp Merc fuel oil mixing ratio
I was really interested in this so I posed this topic on an Inline forum, this comes from a guy who worked and raced these back in their prime.......
"Your engine spits more unburnt fuel out the exhaust ports than can ever
be used. You could run 10 to one and still have enough fuel. Would run
like crap but still not lean. When racing even with stock jets, we used
25 to 1 @7K. The extra oil gives better sealing of the engine and better
lube. Pre-ignition and detonation come from any number of other things and oil ratio is not one of them."
I'm sure there will be those of you who don't agree with this but I think it makes sense.
O.K. I guess we'll have to agree to disagree.
This is a short version of what happens in a stock (non race modified) two stroke engine -
When a 2 cycle piston begins it's cycle, the cylinder begins to fill with atomized fuel but at the same time the exhuast port is wide open. As the piston continues to rise there is a substantial quantity of unburnt fuel passing over the top of the piston and exiting out of the exhaust. This design allows the escaping fuel to cool the piston top and helps to cleanse the residual coke from the previous ignition. 2 stroke oil is a lubricant and does not have adequate cooling or cleansing properties to support this process. Once the piston height is to the point where both the exhaust and intake ports are sealed off, compression of the remaining fuel/oil mix begins.
Depending upon timing, the cylinder fires at some point shortly before or shortly after TDC. The amount of fuel, not oil, determines that quality of the flame, the duration of the flame and the amount of pressure produced by the rapidly expanding gasses. Much of the oil is forced off the top of the piston and acts as a sealant for the rings.
In a oil rich environment, the quality of the flame is reduced as well as the other desirable features. This leaves an inordinate amount of oil on the piston top that is burnt and turned to coke. Since the incomming fuel volume is less than required to clean the residue off it remains and continues to build with each subsequent firing of the pistion. Since cooling is also reduced and the coke does not dissapate heat like aluminum does, the top of the piston becomes overly heated and detonation can occur.
If the carb jetting is inadequate fuel and oil will be inadequate at the time of ignition. This will result in reduced sealing properties, reduced cooling and reduced coke. The result is a clean or very lightly tanned piston which will be excessively hot, can pre-ignite the incomming fuel and will eventually becomed damaged.
Both conditions are not healthy for a stock engine but in a 7000-10000 RPM draq motor with very little back pressure to hold fuel and oil in the cylinder, additonal oil can help to weight the fuel and aid in retention. Of course not only are we adding oil but also larger quantities of mixture with bigger jets, modified intake ports, (adding finger ports, etc.), opening up rod slots, stuffing crankcases, etc.
