Transition slot length

[novadude]

I have a 17085213 baseplate on a carb I built.  Everything is working fine, and I have no issues to "correct".  Just an observation:

This particular baseplate has a transition slot that is much longer than I am accustomed to seeing.  Does anyone know why Rochester lengthened the T-slot on some carbs?  relative to the throttle blade (when fully closed), about 1/2 of the added length (~1/32") seems to be at the bottom, and the other half of the added length is at the top. 

Will this affect part throttle fuel economy?  I am thinking the engine vacuum will only pull as much fuel as the IDCR will allow through the t-slot and idle screw holes at cruise, since the IDCR area is smaller than the combined area of the slot and idle discharge holes.  In this case, T-slot length probably doesn't matter.  Is this correct thinking?

[Cliff Ruggles]

Common especially on truck engines with tiny cams and high vacuum at idle speed. 

In high performance applications that much exposed slot can add a LOT of fuel at idle and make idle tuning difficult.  I've had them idle fine with the mixture screw closed or nearly closed on some set-ups......Cliff

[novadude]

Thanks, Cliff.  This is an early-80s truck baseplate.  In my case, with 0.086" idle discharge holes and the fine-pitch metric mixture screws, I am about 3 turns out for best idle.  I can screw the mixture screws in until they seat, but the engine goes so lean it is barely able to stay running.  Can't kill the engine with the mixture screws, but it definitely goes too lean beyond the current ~3-turns-out setting. 

I was wondering if these large transfer slots kill MPG at all under light throttle ~30-35 mph cruise.  Does it have any impact, or is this more a function of the idle tube and IDCR?  Using a 0.035" idle tube and a 0.050" IDCR on this combo and it seems to run fine, but I don't want to leave fuel mileage on the table.

[Cliff Ruggles]

They will not have a negative effect on fuel economy off idle, assuming the carburetor is correctly calibrated for the application right to start with.....Cliff