750 Mech Secondary IAB sizing question

[rgalajda]

[quote="SP70Nova"]I have a question on the Secondary Idle Air bleed sizing on a Mech Secondary carb. This is more of theory that the actual size of the IAB.

Based on my notes on what I've read over the years, there seems to be different schools of thought on this:
1. match the primary side IAB
2. size similar to a vacuum secondary IAB since the time spent in or transitioning through the transfer slot will be very short.
I understand the need for a smaller IAB on a vacuum sec carb since there is no accel pump shot available for the secondary's.

I'm lost on this because mechanical or vacuum secondary will usually have similar size secondary IAB these days.
Example : my 770 quick fuel vacuum secondary with 4 corner idle has .031" IAB primary and secondary.
Of course there are exceptions.

[cat herder]

Mech or vac has nothing to do with it. If it's a 2-corner idle carb and the rear idle fuel is metered only by the little piddle hole in the baseplate, it uses the much smaller IAB. If it's 4-corner and idle fuel is metered by a mixture screw and has a large discharge hole in the baseplate, it uses IAB sizes similar to the primary side.

[SP70Nova]

Thanks Cat Herder.

The carb idle configuration differences (2 corner/4 corner) is what I was missing and things makes sense now.

Scott P