bajardine
New member
I have a chart that I have used in the past for finding main jet sizes for my powered parachute ultralight aircraft. It is powered by a Rotax 583. Granted it uses a differnt brand of carbs and the application is different but the chart is still applicable to snowmobiles. The chart shows air density in percentages as compared to a starting point of sea level at 70F degrees.
This chart can be used to guesstimate a starting point in main jet sizes for the altitude and temperature range you will be riding in. To use this chart you need to know the stock main jet size for your sled. To use the chart find the temperature and altitude you plan to be at and find the corresponding decimal value. Multiply the decimal value times the stock main jet size to find the new main jet size.
Example #1: Your sled’s stock main jets are size 320. You plan to be riding at 7500 ft elevation and 14 deg F. The decimal value is .95. Multiply your sled’s stock jet size of 320 x .95 = 304. If your jets only come in increments of 5 or 10, round up to the next increment to be on the safe side. In this case, 305 or 310.
Example #2: More than likely you won’t be riding at these exact ranges and temperatures. Let’s say you will be riding at 5000 ft altitude and 20 deg F. Look at the altitudes and temperatures above and below where you will be and find the highest decimal equivalent. In this case it is .98. Again multiply this by the stock main jet size – 320 x .98 = 313.6. Again it would be safest to round up to the next available jet size, 315 or 320.
If you have a specific jet size in mind that you want to use, divide it by the stock main jet size to find your decimal equivalent. In example #2 if you wanted to use a set of 310’s – 310 divided by 320 = .969 This falls to the high side of your range of .95 to .98, more that likely right where you would want to be. You might want to use the 310’s instead of going up to the next size.
Note:
This chart is only a guide to get you in the ball range of where you need to be. Engine modifications will affect whether you need to fatten the jet or slim it down. It is most important to test and check your burn when you get to the area you are going to be riding in.
Before operating the engine again at lower altitudes, reinstallation of the original jetting is necessary or engine damage or failure may occur!
This chart can be used to guesstimate a starting point in main jet sizes for the altitude and temperature range you will be riding in. To use this chart you need to know the stock main jet size for your sled. To use the chart find the temperature and altitude you plan to be at and find the corresponding decimal value. Multiply the decimal value times the stock main jet size to find the new main jet size.
Example #1: Your sled’s stock main jets are size 320. You plan to be riding at 7500 ft elevation and 14 deg F. The decimal value is .95. Multiply your sled’s stock jet size of 320 x .95 = 304. If your jets only come in increments of 5 or 10, round up to the next increment to be on the safe side. In this case, 305 or 310.
Example #2: More than likely you won’t be riding at these exact ranges and temperatures. Let’s say you will be riding at 5000 ft altitude and 20 deg F. Look at the altitudes and temperatures above and below where you will be and find the highest decimal equivalent. In this case it is .98. Again multiply this by the stock main jet size – 320 x .98 = 313.6. Again it would be safest to round up to the next available jet size, 315 or 320.
If you have a specific jet size in mind that you want to use, divide it by the stock main jet size to find your decimal equivalent. In example #2 if you wanted to use a set of 310’s – 310 divided by 320 = .969 This falls to the high side of your range of .95 to .98, more that likely right where you would want to be. You might want to use the 310’s instead of going up to the next size.
Note:
This chart is only a guide to get you in the ball range of where you need to be. Engine modifications will affect whether you need to fatten the jet or slim it down. It is most important to test and check your burn when you get to the area you are going to be riding in.
Before operating the engine again at lower altitudes, reinstallation of the original jetting is necessary or engine damage or failure may occur!
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shortstop20
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- Mar 6, 2005
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Way too rich for my sled, but nonetheless, a nice guide. Thanks.