I will throw a few thoughts out that hopefully illustrate some things.
There is an "ideal" mixture of air and gasoline that, when ignited, totally uses up all of the gas molecules and all of the Oxygen molecules in the mixture. This ratio is very close to 14. 5 pounds of air to 1 pound of gasoline...it varies a slight amount depending on different engine types. Modern fuel injected cars and sleds, with all of their electronic controls, do an amazing job of hitting that perfect target.
The problem comes for carbureted engines because the density of air varies a great deal due to temperature changes and elevation changes from sea level up to 5000 or 10,000 feet.
The density of air tooling around on your sled on a 0 degree night is 8 percent more than it was earlier that day when it was 40 degrees F.
If you go from sea level to the mile high city, Denver, the air density drops 18 percent.
Theoretically, if your sled was jetted perfectly to start with, you would have needed to change the jetting by either 8 % or 18 % in those two examples above.
Two terms you probably already are familiar with;
Lean.....More air is in the mixture than is needed to combust all of the gasoline.
Rich.....More gasoline is in the mixture than is neede to combust all of the oxygen in the air.
Rich mixtures burn cooler, since the excess gas acts as a coolant..and the fuel economy, MPG, will suffer. Really rich will foul plugs and carbon up the motor and pipes.
Lean mixtures burn hotter,.. if you get too lean the heat of combustion is high enough to start melting pistons
Your fan cooled Drifter is actually a fairly mild-mannered motor, as far as sled motors go. The same could be said about other fan cooled motors from other manufacturers that typically rev up to 6500 or 7000 RPM.
This air density change, and need to adjust the fuel jetting to compensate, becomes a lot more critical as you run motors at much higher RPMs.
Yamaha came out with a brand new SRX for the 1981 model year, with a new 440 liquid motor. It cranked out 88 HP at 9000 RPM. It had some other problems, but a key one was how finicky it was to have the proper jetting to keep it running well. It was more than the average "Joe Sledder" was able or willing to do. Yamaha made some changes, but wound up cancelling the release of the '82 SRX. They finally got it right for the '83 model year when they released the 540 cc Vmax, partly because it revved to a 8200 power peak, which was much more forgiving to having the jetting "absolutely perfect".
People that race sleds will be looking at Horsepower figures for a 440 motor of over 100 at 10,000 RPM. You can bet they are paying close attention to the air temperature, altitude, etc.. If their jetting is off, they could be easily giving up 10 HP to the competition.
Your Drifter...not so much. If it's jetted perfectly at 0 degrees, and it warms up to 40, you might give up a couple horsepower. The milder powerband is much more foregiving than a peaky motor.
I think that many sled manufacturers tried to install jetting from the factory that was good at 0 degrees F. If you are riding on a 40 degree day, are there gains that could be made by reducing the carb jet by 8% ? Yes, but on a motor revving to 6500 RPM, those gains are minimal. The sled might feel slightly down on power, because it is. If it's bad enough, you could start fouling spark plugs. Then, you have to decide if you want to drop the jetting down to compensate.
But, in the case of your Drifter, leaving the stock jetting in place would make sense. The stock jetting is very forgiving, and covers a very broad band of conditions.
The exception would be if you live near sea level and you want to ride in the mountains. Then, as with any carbureted motor, there is such an air density change that you do need to re-jet for those conditions.
Hope this made some sense...
Topic: Why Different Carburetor Jets (Read 6953 times)