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1969 Mach 1 - 428 CJ
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Discussion Starter · #1 ·
I have a 19691Mach 1 428 CJ with a fresh restoration. Fresh rebuild on an original CJ Block bored 30 over with an Edelbrock Aluminum top-end and camshaft. Kit #Performer RPM Cylinder Heads, part # EDL-2044.

The kit came with:
Edelbrock Performer Aluminum RPM Cylinder Heads - 350-60069
RPM Intake Manifold - 350-7105
Edelbrock Performer RPM Hydraulic Camshaft - 350-7106 (.050 inch lift/296 duration)
MSD 8594 Distributor
Topped off with a Holley 750 Double Pumper Part # 4779-10 3129 (mechanical secondary).

I have less than 1000 miles on this build.

A local engine builder put everything together for me and it runs great, but I'm having difficulty dialing in the carburetor.

It seems to run very rich, especially at idle. I can't find any vacuum leaks. The engine seems to idle fine at 850 RPMs.

I have tried adjusting the fuel mixture with a vacuum gauge and the max vacuum is about 10 in Hg at about 1/2 turn on the idle mixture screws, the engine is not very drivable at this setting. It leans out a little better but seems to stall at low RPMs under load. It runs better at 3/4 turns on the idle screws, but the carburetor runs rich (i.e. smokes at idle, smell fuel, black smoke). When I fatten the mixture back out to 1 or 1.5 turns, Hg vacuum is around 8 and the engine seems to like it, but the fuel mix is very rich and smokes even worse at idle.

The engine seems to like 3/4 turn on the idle circuit, but the driver doesn't like the black smoke at idle.

Initial timing is set at 14 deg TDC at 850 RPMs and 33 deg @ 2200 RPMs.

I have reviewed comments in multiple forums but haven't seen anything that has helped.

I saw one video published by Norm at Summit Racing. He has a video on Holley Carburetor Performance Tuning, suggesting drilling small holes in the primary throttle plates or adjusting the secondary throat plate position.

It seems Norm likes black smoke.


I want something more streetable. I'm not looking for a race tune right now.

I like old school, but I'm not happy with this tune.

I have been HotRodding for around 40 yrs. I have never seen this kind of problem before.

EFI may be looming in the future.

In this videoNorm suggested, drilling 3/32 holes in the throttle plates adjusting the secondary plate open to lean out the fuel mixture?

I could try the secondary plate adjustment.

I have another old-school Holley on the shelf if I'm going to start butchering one by drilling holes in the primaries.

Any suggestions or comments are welcome.
 

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The 750 is fine on that big a engine. Get a secondary on top of the base screw adjustment thing from Holley(see below). First thing is check the float levels. Are you aware that the 4779 has 4 corner idle ? You have 4 idle screws to adjust. Try it with all 4 about 3/4 turn out. Most of my 4 corner idle Holleys have 1/2 to 3/4 turn out. That 4779 is a great carb and easy to get right.

 

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1966 GT 4spd
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supershifter's question is valid, have you been adjusting all 4 mixture screws?

Start the engine and look down the throttle bores. If you have fuel dripping from the boosters, your float levels are too high. Some Holleys can benefit from lowering them about 1/8" below the center of the sight glass center, or just below the bottom of the level plug if it has those.

You may need to square up the transition slots, and add the adjuster bracket to the secondaries to use for idle adjustment. I'm not a fan of drilling throttle plates, you don't have enough cam for that.

Please ignore the "smaller carb" and "you need a VS carb for an automatic" peanut gallery.
 

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Lowering the float level by 3-6 flats is a proven way to lean the idle and off idle fuel mixture.

The rich idle is typical on Holley’s.

Z

PS I usually do this in concert with raising the secondary throttle plates. The kit for externally adjustable secondary throttle plates, ie without having to remove the carburetor, is a must have for a Holley. It is rather cheap of Holley to send out these carbs without the kit as standard equipment.
 

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Jrod428, Nice car
Damn straight that is a lot of carb
Too much carb in my opinion
However I do like the CFM's
Jet it down & adjust the mechanical secondaries as to not hit until later
Even though your problem is at idle do the above
And replace the power valve and retest (one good backfire can kill the powervalve in a Holley or a Motorcraft carb)
That is why in the end Ford put a vacuum port and line to the bottom of the power valve cover (84 2.8 Bronco II) Et al.
Have a manual emissions test done and report back the CO and HC numbers at Idle and 2500
Drilling holes in the plate is not the answer
 

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I'm reasonably sure the older O-4779 does NOT have "4-corner" idle mixture adjustment... at least my slightly smaller O-4777 doesn't... if your DOES then great. That said, with 10"hg of idle vacuum I suspect either the Power Valve may be contributing to your issue (I'd swap to a #55 anyway) or in order to get a decent idle your primary throttle plates are open far enough to expose too much of the transition slot and you are not running solely on the idle circuit (my best guess). You can try resetting the throttle opening so the exposed transition slot is "square" and increasing the SECONDARY idle adjustment to try and compensate. If those changes don't work, and the car won't idle with the correct transition slot exposed, then you have two choices... reduce the amount of fuel flowing through the transition circuit by placing a restriction in the metering block, or allowing more air to bypass the throttle plates by drilling a 1/16-inch hole in each "butterfly" about 3/8-inch toward the center from the idle discharge ports.
 

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You need an air fuel ratio meter, PERIOD!!! Working blind without one. I ran that carb for 13 years on a 390 w/street strip cam. Great carb. Instant acceleration. From the factory the linkage did not fully open the secondaries. Might want to check that.
 

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I ran a 4779 on my 70 429 and it ran fine with it.

That low(ish) rpm stumble under load is probably the power valve opening too soon due to your low idle vacuum. It is easy enough to check that. If you haven't already changed it the stock power valve is a 6.5 and it probably needs a 4.5.
 

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I had to go to the larger 50cc accel pump with the biggest pump cam on the seconds, and smaller on the primary pump. 4.5 power valve. Way bigger on the secondary main jets, bit bigger on primaries. Ran like a raped ape!
 

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1969 Mach 1 - 428 CJ
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Discussion Starter · #14 ·
The 750 is fine on that big a engine. Get a secondary on top of the base screw adjustment thing from Holley(see below). First thing is check the float levels. Are you aware that the 4779 has 4 corner idle ? You have 4 idle screws to adjust. Try it with all 4 about 3/4 turn out. Most of my 4 corner idle Holleys have 1/2 to 3/4 turn out. That 4779 is a great carb and easy to get right.

Thanks for the info. Currently, I have all four Idle screws set at about 3/4 turn. It runs great, just rich idle. Float levels are dead on in the sight glass. Fuel visible with a slight shake, always at the crest of the sight glass. Where is the secondary throttle adjustment? I have a Holley 4779-10 3129, four idle screws, manual choke, and secondaries. I plan on putting on an O2 meter to check the AFR and go from there. I may need smaller jets. As stated, it runs great, just rich idle. Best vac at about .5 turns on screws (10 hG) but starves the carb, stalls sometimes.
 

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Supershifter is right. The carb is great CFM wise and yes the jets are most likely 72's from the factory. You should definitely jet it down. The old 460 F250 trucks and the GT mustangs that had 600 CFM Holleys also all had 4 mixture screws. You may need to shut the rear ones down. Hell shut them all down and see how rich it still is. I was an I/M inspector for 20 plus years and had to fight to get those carbed ones to pass every year. !982 Escorts with 1.6 had an altitude compensator that was nothing more than a calibrated vacuum leak. They has a screw in them to adjust just how much air you were giving it. Possibly look into finding one. You need more air without fuel. Real common with a big carb on a small motor. 69 Z-28 with 302 had 780 holley vac secondary with a rear metering plate with jets. It ran fine because of the 11 to ones and the 2.02 valves which gave it plenty of air. Having an I/M machine at your disposal is what you need if you are dealing with carburetors. Have someone give you a printout by running a manual mode test on their I/M machine. Carbs run best around 3 percent CO at my altitude. Used to have to get those Escorts down to 1.2 % CO and it was difficult at best
 

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Have you tried lowering the float level so it’s at the very bottom of the sight glass yet ?

the secondary adjuster is only visible with the carb removed and turned upside down. Holley sells a gizmo to adjust the secondary plates with the carb installed. It should be standard equipment but they are cheap

Z
 

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Thanks for the info. Currently, I have all four Idle screws set at about 3/4 turn. It runs great, just rich idle. Float levels are dead on in the sight glass. Fuel visible with a slight shake, always at the crest of the sight glass. Where is the secondary throttle adjustment? I have a Holley 4779-10 3129, four idle screws, manual choke, and secondaries. I plan on putting on an O2 meter to check the AFR and go from there. I may need smaller jets. As stated, it runs great, just rich idle. Best vac at about .5 turns on screws (10 hG) but starves the carb, stalls sometimes.
The idle air bleeds on top of the ventris might need to be changed. those are press in but they can be drilled and tapped for screw in air bleeds. Different sizes will lean and richen the idle.









 

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Damn straight do all those adjustments
Especially the one that adjusts when the secondaries come in
You'll need to play with that meaning
Take it out and run it
I always lower the float level to at or near the bottom of the range
So for the Holley if sight glass style then just barely able to see any fuel in there would be sufficient
Usually just pull the plug and look in there
If you are going racing raise the level and jet for altitude
 
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