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Old 11-20-2017, 08:25 AM
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Tom Vaught Tom Vaught is offline
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Join Date: Dec 2001
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A good friend of mine, TUNER, from the Washington State area posts a lot about carbs on the different websites.

He has posted on Speedtalk several times.
A friend of his Mark Whitener (goes by 'jmarkaudio' and Tuner have helped a lot of people on the different boards. Like I help the Boost Guys on this board and as a Moderator on the Turbo Forums board.

So Tuner was having a discussion with some guys on Speedtalk and a lot of misinformation was being posted by people who did not really understand how Holley Carb Circuits work.
I will include a picture from the thread that shows the drillings in the typical carb mainbody and what they do and the circuit they control.

Today's discussion is about the transfer slot, the Transition Circuit, the idle circuit, and a little bit about the main (booster) circuit.

First off, one of the guys in the thread attached
https://speedtalk.com/forum/viewtopi...43705&start=45
thought that the Power Valve Circuit would affect the idle circuit.(Blown Power Valve deal).
As was mentioned in the thread, by Tuner and others, the Power Valve circuit feeds the Main Circuit with extra fuel for Maximum Power needed by the engine. The Main Circuit is used for the vehicle "CRUISE Mode" (Main Jets).

Normally the Primary's Blade Angle of the carbs throttle blades will travel about 22 degrees before the Main System starts flowing. The Throttle blades already have some angle to them so the total travel to WOT is not 90 degrees but about 82 degrees. So 22 degrees is roughly 1/4th of the blade travel (Throttle Opening) before the main circuit starts working completely. Before that point you are in the Transfer Slot circuit which is the discussion for today.

As was said, the Blade angle on the carb should have the Transition Slot covered except for about a .020"-.040" tall window for the slot to be exposed. Most of the fuel is being fed to the idle circuit and its discharge hole in the base of the carb. Between the Idle Feed Restriction and the Idle Air Bleed and the Idle Mixture Screw you can dial the air fuel ratio to a nice 13.5 to 14.5 air fuel ratio at idle. The engine on a street car should idle about 600-700 rpm smoothly and a engine with more power should idle about 1000 rpm (CLEANLY) with a bigger camshaft installed in the engine.

If you remove the Accelerator Pump Lever on the Primary side of the carb and carefully open the throttle blades you can track on your air fuel meter exactly when the main circuit starts to take over from the Transition Circuit. (What the rpm point is, what the air/fuel ratio is, what the air/fuel ratio is when the main circuit starts. These are all little tests that a Holley Carb Engineer does on an actual running engine.
Reinstall the Accelerator Pump lever and you can note what the air/fuel ratio is under a light "Tip-In" of the gas pedal is. Do this a bit and you can tune the Pump Shooters using a air/fuel meter.
Just enough fuel for clean transition between the idle circuit and the main circuit.

So back on the post on Speedtalk.
One of the people there thought that that if the power valve failed it would affect the idle of the engine. As was posted in the thread two completely different circuits. Tuner posted that you could totally remove the main jets (which are much larger vs the Power Valve Channel Restrictions and the engine would still idle fine.
I personally have not done that test as I already know that they are on different circuits.
But read the thread on the Speed Talk link and learn from some real good carb guys like Tuner, jmarkaudio, and others. Mark Whitener is very good with Dominator Carbs. Somewhere is a link to a carb business he runs on the side. Shaker455 also does a nice job on carbs.

So there are literally millions of thread posts out there on how a Holley carb works, some are fact and some are confused mistakes, but on the whole if you look long enough and talk to the right people you can get some good Holley information. That being said, and I posted this before in other threads, at one time Holley bought carbs thru secondary sources in the 70s of about 30 carb modifiers. Of those 30 modifiers, 3 actually made some slight improvements to the carbs for a given application. The other 27 modifiers stuff was for the most part, SMOKE & MIRRORS. That being said, this is not the 70s, we have the internet and some great people passing along accurate info of stuff that works on a carb system.

Have a great day.

Tom V.
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Last edited by Tom Vaught; 11-20-2017 at 08:31 AM.