73StreetRace

Most Holley carbs have a tranfer slot. It's a little rectangular hole, which can be found on the two bores of the

primary side of the carb just at the level of the throttle plates. To work correctly, only a small part of this slot

must be visible under the plate at idle ( about 0.04 in or less ). Otherwise, a flat spot can occur when you get off idle.

On most carbs, the secondary is also equipped with an idle system, and the secondary throttle plates have an adjusting screw
( adjusted about 1/2 turn from having the plates seated in the bore ).

So my question is :
Is it possible to use the secondary throttle plates adjusting screw ( and how much ) or is it better to drill small holes in the primary throttle plates to keep the transfer slot in the correct position at idle ?

MILO

Use the secondary screw adjustment first. If you don't get the expected results, then drill the holes in the primary plates. If you have a "wild" cam that you want to tame off idle then drilling may be your only option. The idea is to keep your idle circuit working through off idle. Otherwise you will be overly rich at idle because you will be pulling more gas through your jets rather than the idle circuit. The Holley book explains this and everything else you want to know about Holleys. It's worth the $20. Also once you drill, it's permanent. Play with the idle mixture and secondary adjustments first. Adjust the secondaries until the plates move just off the base.

Good luck.

norvalwilhelm

If you open the secondaries too much exposing the transfer slots fuel will be pulled through the transfer slot causing a very rich idle that playing with the idle mixture will not cure.
The transfer slots are exposed to fuel, all it takes is vacuum to start pulling this fuel out into the motor. Any descent cam will overtax the idle circuit making it necessary to drill the carb butterflies.
If you drill the carb while assembled by just turning the carb upside down you drill on the side of the butterfly that causes the carb to open. with one hand you have to hold the throttle closed and drill with the other. If you can just drill you are not on the right side of the butterfly.
Start with something like .090 in that range. All 8 of my butterflies are drilled on my twin double pumpers.

Hvymtlc5

For a BB what is the cam spec that "usually" causes you to exceed the idle circuit spec and require the butterfly drilling? I have never started the car... but if drilling is going to be required...might as well do it now.

I have a BG mighty Demon 750 on a 454 with this cam:

240/246 @ 50 .560 / .573 DUR: 300/306 LS: 112

comp

Brettmc

Use Holley's 26-137, secondary adjusting lever to enable you to adjust the secondary throttle blade position just like the primary blades. The Summit # is HLY-26-137. If this doesn't allow the proper adjustment, adjust the quantity of fuel in the idle circuit by using wires in the metering block or purchasing aftermarket metering blocks with an adjustable idle circuit. Only as a very last resort should you drill the blades as this can cause a lean tip-in condition at the low-to-mid RPM ranges. However, if you do drill and wished you hadn't, JB Weld will save your blades. Just don't forget to sand the excess after it dries completely.

Brett

Brettmc

My prediction: No drilling required.

norvalwilhelm

The wires in the idle circuit really help lean the idle out. I use a .015 copper wire about 3/4 inch long. Bend it into the letter L with each leg about 3/8th inch. Place it in the metering block and use the gasket to hole the wire in place. This makes the metering screws more sensitive. A holley is designed to idle at 1 1/2 turns out. Any big change from this is out of it's range and needs to be addressed.
The secondary throttle plates can be opened up ONLY if the rear doesn't have a idle circuit. If it is 4 corner idle openning the rear throttles uncovers the transfer slots and they dump gas into the circuit all the time.
I rutinely drill the throttle plates and never run into a problem yet.

Avette4me

interesting...looks like i'll be pulling my carb this weekend..

73StreetRace

Thanks to all of you for your comments !

b71vette

on a 383 SBC, mild solid roller cam. I am having an off idle stumble. I have set the 4 corner idle mixture screws using a vacuum gauge & tach, and get a max/steady 15psi on vacuum @800RPM. To get this the mixture screws are only out about 1/2 to 3/4 turns. The BG instructions say they should be out 1 1/2 turns, but it idles way to erratic like this.

Any recommendations on how to go about diagnosing the problem ?

Thanks for your help

b71vette

ttt

AlwaysWave

Common problem, This guy explains it better than I can.
http://chevyhiperformance.com/tech/...rs/0510ch_tune/

norvalwilhelm

For off idle stumbling I richen the pump shot. I drill out the squirter nozzle the next size bigger. The shot hasn't increased but it gets it faster. If the idle screws are only out 1/2 turn I would install the .015 wires in the metering block. The leans out the mixture allowing you to open the idle mixture screws.

68coupe

When do you get the of-idle stumble?
Let me guess, if you mash the pedal hard right off idle.
Do you get it if you just ease on the accelerator slowly? No, right?
If this is the case, you will have to increase the jets.
What carb do you have on the engine? Vacuum secondary? (I am guessing it is a mech. secondary)
Thanks,
Joe

norvalwilhelm

Jetting is about the last thing you should touch. Jetting is for a steady state running not helping off idle stumbling.
Holley carbs particularly the double pumpers are rich out of the box but it is not the jetting but the idle circuit/transfer slots.
Work with squirter size for off idle stumbling.

68coupe

I agree with what you say...
I guess my question for b71 is when do you get this stumble or hesitation?
Does it happen only when you hit the gas hard (A.K.A. wing the throttle back)? or when you slowly accelerate? or both?
Thanks,
Joe