Is this the right way?

This subject really deserves to be a full blown article, but here's a few highlights.

The common thought with RAD is a 5% change in air density = 5% jet number change.

That’s where we run into the first issue. You can't add or reduce fuel flow in a one to one relationship with density changes because carbs respond to the square root of air densityl. So for example if you had a 9% reduction in air density you can't reduce fuel flow by 9% due to the square root relationship. If we look at the square root it would be a 3% reduction in fuel flow, but experience has shown me that this would be too rich and something closer to about a 5% reduction would be dead on. These fudge factors above the square root seem to vary from bike to bike a bit but more so as the corrected density altitude changes. Higher density altitudes require smaller increases above the square root than do lower density altitudes.

Fixed orifice jets (pilots, mains, and needle jets) tend to react consistently to external changes. Air fuel ratio is based on weight of fuel and air, but jets essentially meter by volume. So we really need to know the weight of the fuel flowing through a jet to understand all this.

As altitude and barometric pressure change so does the head pressure on the fuel.

So even though the air density is lower which would require less fuel, the reduction in head pressure will lower the amount of fuel passing through the jets in a self-compensating manner.

Problems arise from the fact that air density (weight of the air) is changing at the same time, and you get the extra fun of changes to the fuel's evaporation temperatures as the pressure/altitude changes. It's possible to calculate jet changes based on changes in air density, but it still takes tuning to get it spot on. Even Honda with all their brain power has to tune at the track .

Best power comes in the vicinity where detonation is most likely to occur so it can be dangerous territory. Slightly rich of best power tends to have a minimal impact on power but power usually falls off fairly quickly once you got to the lean side of this point.

Mikuni hex head jets are numbered by bulk flow, while Mikuni round head jets and Keihn jets are numbered by inner diameter in mm.
So a 175 Keihn main jet is 1.75mm inner diameter. You can't use this inner diameter number for flow calculations you have to use the inside area of the jet orifice.

The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) . Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" (I'll let those so inclined do the math to determine the flow difference) .

Now to further confuse the issue keep in mind that air/fuel ratio is based on weight of fuel and weight of air, but jets essentially meter by volume. So we really need to know the weight of the fuel flowing through a jet to understand all this. In simplest terms fuel weight is a function of the area of the jet multiplied by the value of the square root of the fuel head pressure multiplied by the density of the fuel.

It looks like this:
weight of fuel taken in = jet area * ( SQR Root (head pressure * fuel density) )

Don’t worry too much about the specifics of the equation as much as the variables themselves.

If we assume our jets and fuel stay the same then the only thing that will influence how much fuel makes it into the engine is the head pressure. For the purposes of clarity we’ll define the head pressure as nothing more than atmospheric pressure applied to the fuel by the carb vent hoses.

OK, so we go up 2000 feet in altitude and the air density drops. Common thinking would be that we would need less fuel due to the lower density air being pulled into the engine.

None of the above takes into consideration the changes in fuel vaporization characteristics as you change altitude, which can have a profound impact on the final air/fuel ratio available in the combustion chamber ( the only place a/f ratio is really significant anyway) when the sparkplug fires.

As fun as it might be to look at jetting this way, the sad fact is the cross sectional area of two jets marked with the same number can vary FAR MORE than the difference in flow due to small changes in density . Good jets can vary as much as 5% cheap jets can be closer to 10% variance.


It's been my experience that the quality jets are marked pretty accurately in terms of the physical dimensions, the differences in bulk flow can just as often come from the manufacturing process. With flow rates this small it doesn't take much in the way of an internal flaw to effect the variances we are talking about. Not to mention the fact that it is pretty difficult for most of us to accurately measure a hole this small. If you can, then you should be able to identify the obviously BAD jets and eliminate them, which should help get you a bit closer to your goal here.

The best advice I can offer is to do some testing with a specific set of jets, make accurate records of the differences as you compare the changes in the bike's behavior while you try out the math based approach, note any unusual variances, and then always use those same jets when attempting to pre-jet for conditions. It's not a perfect approach but I think you'll find it will be pretty consistent for a specific bike.

Personally I've found that once you get a feel for a specific engine and how it reacts to changes you can pretty much predict what changes you'll need for specific conditions just by reviewing your notes. With the variety of fuels available today you can even cover a lot of conditions just by changing fuels rather than changing brass.


If you want to play with the more technical aspects of fluid flow through an orifice here's a program you can use to play "What If" :
http://home.swipnet.se/controlengin...flowcalceng.htm


Air Density and Density Altitude Calculations
http://wahiduddin.net/calc/density_altitude.htm


I hope this helps some. ;)