What is the rule of thumb for jetting with temperature changes. I have heard 20degrees requires a mainjet change to achieve optimal performance.

How much does humidity affect jetting? If it is 20 degrees colder but humid, do they cancel each other out?


Machine runs really well at 70 degrees but we have a race this weekend and it is supposed to be 50 and we are trying to be proactive and jet ahead of time.

Thanks!

 

Guess nobody knows!

 

20 degrees is the rule I've always used. I've never had the humidity problem, its always humid in michigan when its hot. Humidity is the measure of water vapor in the air, but shouldnt make the air any more or less dense as temperature does, unless the water is taking up space... Come to think of it I guess I really dont know for sure, I just know that humidity hasnt traditionally been an issue when I have jetted.

A four stroke can get by a lot easier than a 2 stroke in different temps, so if its a thumper I wouldnt worry about the humidity or the 20 degree rule as much.

 

Thanks Maddog! I ended up changing the jetting to make it richer then changed my mind and put it back again! :-)

 

Go to the head of the class DritDragon. Your thinking is right on the money.
The track we race our drag car at is right on the ocean. The weather there is usually cold and one would think that we would have to really jet up. This is usually not the case because of the "fog" in the air. Water will displace air (O2). When this happens the engine will go fat.
As Dog says, if it's a 4 stroke I would not worry too much about jetting if you are close in normal temps. If it's a two stroke, you may have to play with it a bit at the track on the day you race.
Good luck with your race.

 

this is where the dial-a-jet comes in handy I think.......atleast on 2 strokes......I had one on my old 400 sport, but don't have one on my pred....