1.5 Turns Out For The Air Screw?Where Did We Get 1.5 Turns Out? Looking at the Mikuni VM manual it does not say 1+1/2 is the goal. It says that if the air screw turns for maximum idle speed is 1+1/2 to 2, then leave it at 1+1/2 to be slightly rich.
In the Dellorto manual it doesn't even list # of turns out. It just says to choose an idle jet that gives you the best idle and the best progression of power off idle.
The Keihin manual doesn't list # of turns out for the air screw. It just says to adjust it for the highest RPM at idle.
There's only two criteria: 1) How well it idles, 2) How good the jetting is off idle. If those take you to a strange place, then so be it. We aren't adjusting to please people but to get our engine running as good as possible.
I just tried two idle jet sizes 12.5 and 20 on my Mikuni VM18 and the resultant air screw settings were 1+1/4 and 2+1/4. If I buy a 17.5 jet then the turns should be just under 2. I can get both jets to idle good but with the 12.5 jet the off idle power progression is terrible, and the #20 isn't too bad so I expect the 17.5 to be just right with almost 2 turns out.
Just to make sure everyone is up to speed here, the spacing between the needle and needle jet contributes the major portion of gasoline at idle. It's not all from the pilot circuit. Early era carbs didn't even have a pilot circuit. At idle there is still plenty of vacuum and air flow at the needle to draw gasoline up there. So if your spacing there is off then the ideal idle setting compensates for that. But then if the needle jet is wallowed out and the engine idles fine then above closed throttle the engine may run too rich.
There is no "starting point" with air screw turns. Find the peak RPM and then slowly turn the screw in till the RPM starts to drop. Then leave it there and go ride testing. Then put in the next size idle jet and repeat. Keep trying different jets till you find the one that gives the best off idle power. And to hell with the air screw position. Seriously.
Here's an unknown "standard" for idle jetting: If the idle jet is too big then the idle slide height will be too high, and vice reversa. So far I think the normal slide height is 14% of the carb size. So if you have a 28mm carb then that multiplied by .14 is 3.9mm. You can use small allen wrenches to get a "close enough" measurement of that slide height. This is mostly for engines with a reed valve intake. I'm not sure what the percentage is for piston port or rotary valve intake engines.
Here's the best comments from my facebook posting of this:
TJB: "They can't just say one and a half turns is good for everyone because some people live at different altitudes. Plus it's better to be slightly rich at idle for more lubrication."
IO: "I ignore the # of turns altogether and go for the best, reliable set I can get."
LF: "I think the idea is to get the best idle through 1/8 throttle performance"
DM: "1-3 turns, any more then 3 turns the pilot jet is too rich and will need 1-2 sizes smaller, any less then 1 turn and your pilot jet is too lean"
DB showed the YZ125 manual which did recommend 1.5 turns and I responded "Yes of course a manufacturer can recommend one setting but we should be careful not to think the exact same applies to all other carbs"
RJ showed a Ski Doo factory race manual and I replied "it's interesting that they allow a broad range from 1 to 2.5 turns out"
And to all of you who said 1.5 turns is a good starting point, you totally missed what I was saying which is that any combo of idle jet and air screw turns is OK if it gives you a good idle and good running just past closed throttle. That's basically what the three carb manuals I read said. So the idea of setting the idle jet size to match 1.5 turns out is just another fabrication by lazy and stupid people. Whatever is best for running is the best, regardless of air screw turns.