[chiphead]

I've got an out of the box QF 750 mech sec dual feed double pumper on a RPM on top of a 400 with 62 heads and summit 2802 cam. Choke, float level and fuel pressure are in the ballpark. Accel pump squirters and linkage seems to be working correctly. New R45S plugs turn black almost instantly. Black, fluffy carbon, not oil.

This carb has stock jetting of 73 primary jets and 82 secondary jets. Do I need to jet it down or look somewhere else for cause of fouling plugs?

[74Grandville]

I would start with the basics and make sure the floats are set correctly as well as the primary blades set correctly. i would think stock jetting should be close.

Do understand a double pumper loves to dump fuel when opening the secondaries.

[chiphead]

I found some posts here about how much fuel the idle circuit on a holley feeds at light cruise. Do I need to lean out the 4-corner idle first? I haven't had a chance to lean it out from the initial baseline of 1.5 turns on all 4 corners. Soon after the cam break-in, I found the loose converter, so I shut it off. I'm used to Q-jets, so some holley tuning is new to me. I did a cam break-in and one mild trip around the block, and the plugs are black as night. Mechanical secondaries or not, that can't be right.

[Formulajones]

Yes absolutely start with the idle circuits. I don't know what Quick Fuel uses for idle feed restrictors but 1 1/2 turns out is quite a bit on a 4 corner idle circuit, especially on such a mild combo.

[mgarblik]

Originally Posted by Formulajones

Yes absolutely start with the idle circuits. I don't know what Quick Fuel uses for idle feed restrictors but 1 1/2 turns out is quite a bit on a 4 corner idle circuit, especially on such a mild combo.

X2 on the idle circuit with that mild engine. Put a vacuum gage on it and idling, slowly turn the 4 screws in 1/8 turn at a time and give it a chance to stabilize. I am guessing you will see maximum vacuum at about 3/4 turn out from seated with the 4 screws. Even so, 4 corner Holley carbs tend to idle rich, which keeps them happy with larger cams. You may want to try a different spark plug, just slightly hotter if your doing allot of low RPM, in town driving and idling. I would think a R45S would be plenty hot but R46S should be available. Adjust the idle mixture first.

[Tom Vaught]

TWO-CORNER Idle Mixture Screw setting on older 750 carbs was One and a Half Turns
out. 99% of the of the 4 corner carbs should be at 3/4 turns out from seated or slightly more(7/8 turn out) on the 4 needles. Your Screw Settings are like an old 2 corner idle carb that fed WITHOUT a Secondary Idle Circuit.

Someone gave you the carb???

Secondary Jetting should be about 6 sized difference with a Power Valve Plug in the secondary PV location. Not 12 sizes difference. Race only (no secondary PV) can start with a 8 jet size difference.

Tom V.

Send the carb to Shaker455 and let him dial it in on a running engine (with the correct carb calibration).

[Kenth]

Too fat running Holley style carbs should always have the power valve condition checked before rejetting.
If age of power valve is unknown install a new, as old ones mostly are stiffened or blown leaking fuel past the jets.

[ta man]

I agree on the idle mixture being too rich. Most of the 4 corner idle mixture carbs I've tuned have wound up in the 3/4 turn range. Just beware the 4 corner carbs are more sensitive. On my Holley 950hp 1/4 turn rich will blacken my plugs.

[Tom Vaught]

VERY TRUE ta man!

Tom V.

[chiphead]

Here's what I did. I opened it up and made a few changes.

Starting Primary Jet: 72
Starting secondary jet: 82

I went down to 70 jets on the front. Since the recommended stagger is 8-10 sizes, I went with 79 jets in the rear. So that's down 2 jet sizes in the front and 3 in the back.

I opened the secondary baseplate screw 1/8 of a turn from baseline. The secondary transfer slot is NOT exposed.
The front transfer slot is a little more exposed than squared up. That was giving me about 1000RPM curb idle.
Idle mixture screws are now 1.25 turns from closed for the restart
Replaced the fiber N&S adjuster washers with the reusable plastic ones.
Lowered float height in the primary bowl a bit.

.

[Tom Vaught]

Originally Posted by chiphead

Here's what I did. I opened it up and made a few changes.

Starting Primary Jet: 72
Starting secondary jet: 82

I went down to 70 jets on the front. Since the recommended stagger is 8-10 sizes, I went with 79 jets in the rear. So that's down 2 jet sizes in the front and 3 in the back.

I opened the secondary baseplate screw 1/8 of a turn from baseline. The secondary transfer slot is NOT exposed.
The front transfer slot is a little more exposed than squared up. That was giving me about 1000RPM curb idle.
Idle mixture screws are now 1.25 turns from closed for the restart
Replaced the fiber N&S adjuster washers with the reusable plastic ones.
Lowered float height in the primary bowl a bit.

.

Jetting is right where it should be now according to the Holley Master Calibration Book.

850 RPM (unless you have a radical camshaft) is still plenty to keep the engine alive at idle. The extra rpm just wastes gas at street lights unless it it a drag only application with a high stall converter. You are almost there.

Tom V.

[JSchmitz]

Did you check to see if you have the correct size power valve? What is your idle vacuum with it warmed up (in gear if auto trans)?

[chiphead]

Either my vacuum gauge is busted or I was on a ported vacuum port. Because I didn't see any vacuum. It has a 6.5 power valve on it and the power brakes work so who knows?

I warmed it up yesterday and put a baseline tune on it. Four corner idle screws ended up at one half turn out. About 850-900 RPM is the lowest it seemed happy idling at.
I changed the oil and check to make sure it all the push rods were spinning.
Ill take it for another short run see what the plugs look like. Then I need to put the inner valve springs back in. I'm being real careful to let it get some run time to seat the lifters. I'm keeping the RPMs under 4k so I don't risk floating the valves

[chiphead]

Got a few tuning issues. The fast idle system isn't working right. I've got the elec choke adjusted so the fast idle cam and screw are maxed out but it doesn't really raise cold idle speed a lot. The primaries are opening wider when on the fast idle cam but it barely runs at 1000 RPM when cold.

What am I doing wrong?

[Tom Vaught]

1) Check Choke Blade gap to airhorn when on the high choke step.

2) Check and adjust fast idle screw under electric choke housing.

Tom V.

[chiphead]

OK, adjusted both, still doesn't really come up on a high idle. The fast idle cam is all the way up and the fast idle screw is maxed out. Choke flap adjusted so it just barely closes when cold. 15* base timing.
I'm assuming this thing is really rich with the choke on. What do I try next?

[Bruce Meyer]

Have you tried not using the choke? You should be warm enough there in SC to not need one. I just run a fast idle solenoid for cold days. Here a cold day is in the 50,s.

[chiphead]

I opened the secondary throttle plate screw about 1/2 turn. That let me close the primaries down and I rebalanced the idle mixture screws.. pulls about 13 inches of vacuum at idle. Now the high idle system works. New problem is I have a lean flat spot right off of curb idle.

If adj the primary accelerator pump system doesn't work, What do I need to check next?

[ta man]

Originally Posted by chiphead

I opened the secondary throttle plate screw about 1/2 turn. That let me close the primaries down and I rebalanced the idle mixture screws.. pulls about 13 inches of vacuum at idle. Now the high idle system works. New problem is I have a lean flat spot right off of curb idle.

If adj the primary accelerator pump system doesn't work, What do I need to check next?

My golden rule..square up the transition slots in the primaries and leave it. Use your rear blades to adjust your warm idle speed.

[chiphead]

How do you tell the difference between a rich stumble and a lean stumble? When the motor is warming up, I can wack the throttle in park and it'll respond instantly. When the motor is at 180F, it develops a bad off idle stumble. It hesitates and stumbles when I try to wack the throttle the same way. It also hesitates slightly when I take off gently from a stop. I've adjusted the accel pump linkage so I know it's right.