Do you want the long answer or the short one Paul?
The short answer is "Set the primary transfer slot properly and then close the secondary to reduce idle, provided that the secondary is still set on the adjuster and not the bore of the carburetor."
The long answer includes the why.....
Don't call the transfer slot "the transfer circuit" it's confusing until you fully grok it all, and how it plays with everything else.
The idle circuit IS the idle mixture screw and also the transfer slot.
The total fuel is restricted by the Idle Feed Restriction. As it heads out of the idle well it is mixed with Idle air bleed air and proceeds down toward the bottom of the metering block.
From there it is in one slot..... the metering block slot that goes to the transfer slot hole in the main body and eventually down to the throttle body transfer slot.
When you adjust the idle mixture screw you are allowing fuel/air mixture into that area of the metering block and into the main body down to the idle feed in the throttle body.
You can adjust the idle mixture screw..... but the ONLY way to adjust the transfer slot fuel is with the throttle butterfly.
The idle feed under the throttle plates is constant feed.
The transfer slot is kinda like a fuel bleed AND an air bleed. (follow me?)
The transfer slot above the throttle plate is seeing air rushing into the engine, the slot below the throttle butterfly is seeing nothing but vacuum.
As you move the throttle the ratio of air bleed to fuel bleed changes...... as the throttle opens up more fuel bleed is exposed, this makes sense because air is rushing around the butterfly into the intake manifold.
If your idle air bleed and main bleed are appropriately sized the main boosters should come online at a point when the transfer slot cannot keep up with the fuel requirements of the engine.
This is why if you expose too much of the transfer slot you lose idle mixture screw sensitivity.
Ok..... so proper setup of the transfer slot.
Technically for good idle and good cruise the transfer slot doesn't *need* to be visible at all.
(technically)
But once you open the throttle a tiny bit, you need more fuel NOW, and you need that transfer slot to be functioning..... but liquids move slower than air.
Easiest way to assure this is to make sure a tiny bit of the transfer slot is showing and thus *seeing* intake vacuum in the primaries.
The secondaries in this case are not as important because at small throttle usage the secondary butterflies aren't moving. When the secondaries do function they are opening in a metered way (with a Vacuum secondary setup) so the fuel can catch up to the air movement. In a double pumper it isn't as important because the secondaries have a pump shot.
So this is the reason for the primary Transfer slot to be set properly..... if too little is visible there will be a stumble, if too much is visible you'll over fuel and lose idle mixture screw usage and the engine will be overly rich.
Many people when tuning do not understand this relationship and will do things like choke down the idle screws to limit fuel, or they will increase pump shot to try to mask the lean spot if the transfer slot isn't exposed enough...... don't be like those people.
Drew
