Jetting a snowmobile................
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Senior User
Joined: Jul 2001
Posts: 126
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From: Montana
Jetting a snowmobile................
I brought my 97 Ski Doo Formula SLS 500 to Montana from Wisconsin. It has been ballanced and the head shaved 20 thousanths. I have long tracked it but still need to jet it. What do i need to do?? I believe it will be running lean here compared to Wisconsin right?? Would a set of Dial A Jets do the trick???
Senior User
Joined: Mar 2004
Posts: 220
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From: north central illinois
if you know who did the motor work contacgt them for the jetting set up. i don't think the dial a jets will make up that big of a jetting difference. they are i think only a 2 jet swing. they are better for fine tuning. also contact ski doo and find out what the stock jets should be and that will get you close and you can probably go one or 2 sizes bigger to be safe and start there and tune from there.
Elder User
Joined: Sep 2004
Posts: 941
Likes: 4
As you go from low altitude to high altitude a carbureted engine will tend to run too rich, not too lean. The same holds for MAP style fuel injection, although some computers implement BARO correction to mitigate this. A mass-air-flow style fuel injection system automatically corrects for altitude, except for starting.
The reason for this is that carbs sense air flow, not air mass. The thinner air "weighs less" for a given volume, and thus you need to rejet with LESS fuel to achieve the same air/fuel ratio. (Remember that AFR is measured in WEIGHT, not volume).
If the operating temps are MUCH colder than in Wisconsin, there is a partial compensating effect. Colder air is denser, and will tend to lean out the mixture. But the change in altitude is most likely the much more significant effect.
The "shaved head" will partially compensate for the loss in peak combustion pressure caused by the lower intake air density. This is generally independent of the jetting-for-altitude issue.
The reason for this is that carbs sense air flow, not air mass. The thinner air "weighs less" for a given volume, and thus you need to rejet with LESS fuel to achieve the same air/fuel ratio. (Remember that AFR is measured in WEIGHT, not volume).
If the operating temps are MUCH colder than in Wisconsin, there is a partial compensating effect. Colder air is denser, and will tend to lean out the mixture. But the change in altitude is most likely the much more significant effect.
The "shaved head" will partially compensate for the loss in peak combustion pressure caused by the lower intake air density. This is generally independent of the jetting-for-altitude issue.
Laughing Gas
Joined: Apr 2003
Posts: 936
Likes: 25
From: La Ronge Saskatchewan
Some sleds have a jetting chart in their owners manuals or under the hood maybe too.
Plugs should be chocolate brown thru all your speed ranges. Temperature and altitude will affect your jetting. Dial a jet is expensive and not needed for most flat landers.
Plugs should be chocolate brown thru all your speed ranges. Temperature and altitude will affect your jetting. Dial a jet is expensive and not needed for most flat landers.
Elder User
Joined: Dec 2001
Posts: 800
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From: Ontario, canada
Take it for a ride short first and then read the plugs. Find a long straight stretch, open it up to speed, then immediately hit the kill switch and brakes. Idling before you read the plugs will distort the results. If it's a mild day, then it's ok for things to read a bit rich.
Sounds like your motor is pretty mild, so you may not need to change the jetting at all. Dial a jets are cool, but unless you plan on fiddling with the jetting alot, they arent really worth it.
In my experience, Ski-doos are generally jetted pretty good from the factory, and they will tolerate running a bit on the lean side better than other brands.
If you changed the track setup, you may want to look into clutching and/ or final gearing also.
Sounds like your motor is pretty mild, so you may not need to change the jetting at all. Dial a jets are cool, but unless you plan on fiddling with the jetting alot, they arent really worth it.
In my experience, Ski-doos are generally jetted pretty good from the factory, and they will tolerate running a bit on the lean side better than other brands.
If you changed the track setup, you may want to look into clutching and/ or final gearing also.
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