The air screws regulate how much air is allowed into pilot circuit. The more you turn them out, the leaner you are in that low-to-mid range.
If you do a 1-mile WOT blast, then kill the sled at the end of it while still pinning the throttle, then let off throttle, then brake to a stop, you can pull the plugs and examine your fueling through the main jet. Cardboard color is good. Darker is richer, lighter is leaner.
You can do the same test across a 1/2 mile at the "problem" area. Get the sled going about 35 and running like shit, hold it there for a 1/2 mile, then kill - let go of throttle - brake to stop. Check plugs, see what you have for fueling in the low-to-mid range.
I suspect 2 turns out for you low elevation guys is too lean. If it's too lean, it'll bog coming out of the mid range.
On a 2-circuit carb, you have pilot and main circuits. With no throttle, the sled idles through the pilot. It mainly runs on the pilot up to 1/4 throttle. Vacuum is the primary determinant of air/fuel delivery through the pilot. The harder the motor sucks, the more a/f mix it can sip through the pilot. This works fine up to about 1/4 throttle.
As throttle is increased beyond 1/4ish, the needle mechanically starts to pull out of the main jet, allowing more fuel to be mixed with the ever-increasing amount of air being sucked in by the motor. This is where the needle clip/washer is a factor. If there are 5 settings on the needle, with #1 being farthest from the needle point and #5 being closest to the needle point, then #1 is leanest and #5 is richest. If you use #1, the needle taper is lower in the main, therefore the needle must be pulled farther before it's taper begins passing through the main allowing more fuel to get past the needle. At WOT, the needle is either barely in the jet, or totally clear of it, allowing maximum fuel flow through the main jet.
Remember a three simple ideas for optimal tuning.
1 - the motor is an air pump. The faster it runs, the harder it will suck air in and push air out, and also the higher the volume of air it will try to move through itself. Your first job it to make sure the intake and exhaust tracts are relatively unrestricted and perhaps even properly "tuned" to optimize laminar air flow through the pump. Yamaha engineers did most of this work for you. If you modify intake or exhaust for even less restricted airflow, the sled will want more fuel because it is getting more air. But, flow may be more turbulent after the mods, which robs power. This is the catch-22 of mods: Anyone can give it more air, but can you make it smooth (effective) air? This is why Daman could be considered to be right when he sez Stock Rocks.
2 - once a clear air path and volume has been established, the carbs regulate how much fuel is mixed with that air. Too lean = low power, poor fuel economy, & excessive engine wear. Perfect lambda = optimum power and fuel economy and normal engine wear, but difficult to achieve perfectly. Way too rich = low power and poor fuel economy, but dranatically reduced engine wear. Just barely too rich (staying conservative) = good power and fuel economy and reduced engine wear. Since no one can be perfect, always try to err to the rich side.
3 - a fueling curve is a transition, especially as the sled moves into the mid-to-higher rpm ranges. If you have a dead range, it's because of a problem with your transition. Fuel is always flowing through the pilot, so be sure that the pilot circuit is not too rich or lean. Main size and needle clip determine how soon fuel starts flowing through the main and also how much fuel flows through the main. At WOT, the main is ALMOST all that matters, because so much fuel is flowing through it that the pilot is hardly of concern, and the needle is totally clear of the main, so clip setting doesn't matter. But the holy grail is a nice transition from 1/8 all the way to 5/8 throttle. This is where the 3 critical variables must be optimized: air screw, needle setting, and main size.
Only really high riders (always 8000+) should consider reducing pilot jet size.
Start by getting a proven expert recommendation for pilot and main jet sizes and needle clip location for your sled setup. This will vary according to elevation and engine mods and exhaust.
Using those recommended jets, set the air screws 1.5 turns out and the needle at #3. Ride for a while, do the mid and top range plug check.
Then use the air screws to try to eliminate any bogs as she's transitioning from pilot to main circuits. If you are unable to make it sound and feel right with the air screws, then move the needle clip 1 position in the indicated direction. This will change the "timing" of when the main circuit becomes a factor, and will require that you sort of "start over" with the air screws to get everything just right.
Your tools:
- plug checks at both mid and high ranges
- how does it start when warm? If you are using recommended jets and it's hard to start when warm, you are likely too lean on the air screws, turn them in a little.
You can never get it perfect everywhere, especially with a 2-circ carb. but you can get close.
The reply button sez "Post Quick Reply". That buttons obviously isn't working today.
,, ,should be a Sticky!!