Air/Fuel Mixture - Chevelle Tech
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post #1 of 2 (permalink) Old Sep 15th, 19, 12:29 AM Thread Starter
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Air/Fuel Mixture

Small block 383 with L79 intake and stock Holley carburetor. Runs strong at highway RPMs but had an 'off idle hesitation'. Tried different jets (bigger) and different nozzles (now up to 37), seems better. Today I connected an air/fuel meter with an exhaust pipe fitting to make some measurements. I'm seeing an average of 12.9:1 most of the time, epically under acceleration. That is 12.9 parts of air to 1 part of fuel (by weight) as I understand it. Stoichiometric perfection is said to be 14.9:1 so I'm a bit rich but I believe in our world of automobiles erring on the rich side is preferable. With smaller jets it would stumble and cough under some conditions, hence the larger jets.
What say you? Is this an acceptable air/fuel reading? It still is not perfect but the gut-grunt is great and it looks like a stock 350HP L79 so I'm not interested in any other top end products. I can change the jets and the nozzle as easy as changing my socks, is there a better air/fuel ratio I should strive for? I can go to even larger nozzles but I'm guessing with it now at 37 that I'm squirting a lot of fuel in it, would more help it? Thanks... Al
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post #2 of 2 (permalink) Old Sep 15th, 19, 12:53 PM
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Re: Air/Fuel Mixture

FIRST to check is the idle throttle plate settings. Holley has what are called Idle Transfer Slots in their throttle bores. These slots hold fuel in suspension when the plates are sitting in the right places at idle, and discharge that fuel when the plates are opened. This fuel is first into the manifold off idle, NOT accelerator or other circuits.

The primary throttle plates should be set to allow only .020 opening of the ITS at their hot idle setting. Too far open, engine will use that fuel to idle, and there will be an off idle hesitation.

I set it up so I can see where the throttle plates are down the throttle bores, instead of having to pull the carb to see them, easy to do. With the carb off the engine, set the idle setting to have the ITS open .020, then, mark a line across the throttle bores ABOVE the plates, so yu can see the line down the bore, put the carb back on and go forward.

If the ignition system is set up correctly, and the carb is working correctly, idle should be stable at the correct throttle plate setting.

If the engine insists on having too much open throttle to idle, it indicates the engine wants more air to idle correctly. This usually is needed for bigger cams in engines. Holley counteracted this by adding bypass holes in the throttle plates, but, do not just go off drilling the plates, the holes and sizes are critical, and the very last resort in getting it right.

Check the ITS first.
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