I made this thread purely out of curiousity. I found this pic of whats supposedly a factory Qjet, originally from a Hi-Perf Pontiac from the '60s.

Whats up with the big slots in the air valves? Whats the benefit/reasoning behind this? Is there any performance gain from this?

Looks to me like it would make a pretty nasty lean condition when you yanked the secondaries open...?

 

I believe it increases the signal to the secondary fuel feed. At least, that's what I recall hearing. Not completely clear on how it does it, though.

 

Yep we went over similar on the Buick board.
Some of the Eddy performer versions of the Qjet have similar done but with round holes. There is something special about it that aids performance. Can't think of it now as it was a long while ago. Bought time the bike building shows got started as I noticed them nifty stepped drill bits would be able to make a clean round hole.

Think its something like soon as the bottom plate starts to open, There is enough air to either start or mix the fuel better, till the vacuum thingy lets the bigger ones open. I dunno, but if you look, you will find lots of carbs with similar like holes. Though think the eddy ones are in a tad different spot more towarda the center.....

 

The slots seems to be at the secondary accelerating-well port position. The Doug Roe Rochester Carb book makes no mention (in its mod and tuning sections) of making any slots or holes in the secondary air valves, but it mentions the function of the ports. Quote:" As the air valve begins to open, its upper edge passes the accelerating-well ports, exposing them to manifold vacuum. The ports immediately feed fuel from the wells and continue to do so until they are depleted. The accelerating ports prevent momentary leanness as the valve opens and the secondary nozzles begin to feed fuel. Some Q-jets do not have accelerating wells." The wells needs about 15 seconds to refill.
So it seems that making slots there exposes the ports to vacuum, pulling fuel even before the air valves start to open.

 

Yea, i've seen that but with a narrower slot then that.

Key point to note is the position - right above the secondary fueling tubes. I guess one would want to increase pump shot when doing this, since you'll get a bit more air on a sudden throttle snap. But once air starts moving through the secondaries it'd pull fuel through the secondary fueling tubes quickly.

I *thought* I read about this in the doug roe book, but if fullsizewagon says it's not in there, then i'll trust him. I must have read about this somewhere online then. I think it's another trick to get the car onto the secondaries a bit faster, make it a bit closer to a double pumper

 

Which sounds to me like it would make it possible to open the AV a bit faster/sooner without the bog...?

 

bingo

 

I took an old set of air valve doors and cut notches in them like shown above. I made them a little smaller tho. I like the results. My primary to seconday transition feels better on a fast roll-on. I'm not sure if this would help on an N/A motor...