The basic's, to run right the engine needs the correct amount of fuel added to the incoming air, so the correct size jet must be used. Now what screws up this simple plan. What were really after is the oxy content of the outside air, this varies by both the temp of the air and the elevation you are at. Your choices are to change jets to compensate, or to use the adjustable main jet
Now, in reality, it will run to some degree on either the rich side or the lean side of optimal jetting but not making full power, how much your off depends on, as the temp changes, how far from perfect does it go. The adjustable jet allows you to get back to correct as the weather changes

 

Okay, I get that. The colder/denser the air, the leaner it would run with the factory jet, and richer with warmer/less dense air, correct?

How far off from making full power would it (the factory jet) be? Are we talking marginally or significantly? Significant enough that, in an Improved Stock (which must retain the stock airbox, muffler and valve springs) snocross race, it would be helpful or are we really only talking about the Modified classes with engines tuned to the nines that would realize a benefit?

How/where does it install anyway?

 

What rules are you going off of. The way I read the ISOC rules is that setup is for stock class. Adjustable main jet is way worth it.

 

Any tips for how to 'tune' using the adjustable carb? I bought a sled that has one on it but I really don't know how to use it ...... Is it like the old idle screw on a mikuni? Twist it closed then back off one turn for below zero? turn and a half for 10 to 0 ??? two complete turns for 30 to 10 ?? etc......

 

You turn it in as temps get warmer. The real benny here is being able to balance the low speed jet (factory screw) against the main for a smooth transition between them These carbs are unfortunately the same as on Generators or pumps, their meant to either run at idle or at the 3600 governor speed on equipment, not raced or trail ridden
These tend to develop a bog as the carb moves from the low speed jet to the hi speed system as you open it up, balancing these to adjustments helps get past that

 

AK: these would be ISR rules.

Bob: how do you adjust the low speed to balance and reduce the bog? And forgive the newb question, but which is the low speed?


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On the right side of the carb closer to the head is a small screw that adjusts the low speed fuel mix. If you have the adjustable main set up, you keep going back and forth between the two to find the sweet spot where theres not hint of a bog as it changes over from the low side jet to the high side.
Do you have the Gov. bypassed on your sled?

 

I'm going to be removing it next weekend. ISR legal to do so (according to tech director). I ordered the case gasket on Friday, should be here tues. might get to it during the week, actually.

As far as tuning... is the goal to lean out the low speed and enrich the high speed? Curious what's going on in the process.


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