....................The basic's of it was Pump gas varies widely in specific gravity which effects jetting, from one end of the range .720-.770 to the other by up to or more than 2.5%, @ 5pts on a Keihin jet or 180-185 main. That's just fuel no air density or temp accounted for.
Going from a 180 to a 185 main jet results in a 5% change in fuel flow not 2.5%. A 180 Keihin jet has a 1.80 mm theoretical hole diameter, a 185 main jet has a 1.85mm hole and so on. For Keihin jets one has to calculate the flow area of the jet. The number on a Mikuni amin jet is a flow rate and does not require any area calculations.
Example:
Going from a 300 to a 310 Mikuni main jet increases the fuel flow by ( (310/300) - 1) * 100 = 3.3%
Looking at the specific gravity of a fuel can often be an indicator of a fuels relative heating value. The change in jetting is more related to the change in the heating value (BTU/LB) of the fuel and not the specific gravities affect on the fuel flow rate through a jet because a change in the fuels viscosity. The heating value is what affects jetting not the specific gravity.
Fuels that have high heating values tend to have lower specific gravities and typically need smaller jets to get the same air/fuel ratio than fuels with higher specific gravities.
The observed trend is: fuels with low specific gravities use small jets and fuels with high specific gravities need larger jets. Typically, fuels with specific gravities of around .700 will have heating values of 18,000 to 20,000 BTU/Lb and fuels around .750 will have heating values under 18,000 BTU/LB. I do not have a chart in front of me, but the heating values tend to go down as the specific gravity goes higher.
Additives like MTBE, ethanol, methanol, and other aromatics usually have lower heating values than gasoline and typically increases the specific gravity of the resulting mixture the fuel is made of.
Methanol has a heating value around 9000 BTU/lb and has a specific gravity of around .800 and takes roughly twice the flow area through the jet to get the same amount of heat from an engine as the same engine on gasoline. An engine processes heat and turns it into useful crankshaft rotation.
The heating value of your reference fuel divided by the heating value of the new fuel equals the required change in fuel flow.
Example for methanol:
heat value of methanol =9000
heat value of gasoline =18,000
Main jet size on gasoline = 300
(18,000 / 9,000) = 2
For methanol we need to flow approximately 2 times as much fuel, so we need a (2*300) = 600 main jet
Example for different gasolines
Heat value for gasoline #1 = 18,500
Heat value for gasoline #2 = 18,000
Main jet size for gasoline # 1 = 300
18,500 / 18,000 = 1.028 times larger
For gasoline # 2 we need to flow 1.028 times as much fuel so we need a (1.028*300) = 308.4 main jet, rounding up we would install a 310 main jet
I think I remember that Ethanol has has a heating value of around 11,500 BTU/lb and E85 around 13,000 Btu/lb. E85 requires a main jet with approximately 30% more flow area than gasoline.
Some race fuel manufactures publish the heating value of their various blends of fuel. If they publish these fuel specifications one can look at the heating values to calculate a jet size or see the trend whether you will need to go richer or leaner when changing fuel. If the fuel manufacturer does not publish the heating value, look to see if the fuel you will be using is going to have a specific gravity that is heavier or lighter than your current fuel to see if you need to go richer or leaner.
