quote:Originally posted by jayb:
having carburation issues with Holley double pumper on '72 351 cleveland. transient improvement with rebuild. Hard to start and won't hold idle now. Looking for thoughts re: different type of carb, efi, etc.
Thanks. Jay
You didn't "push all the right buttons" on stating the exact nature of the problem, but even though you said that you rebuilt it, I have a sneaking suspicion that you have the power valve blues.
These power valves are so so bad now that the slightest hint of a backfire starting will blow out the diaphragm and leak into the idle vacuum chamber.
You can reduce this tendency to a great deal by installing the power valve check valve, available as a kit, into the throttle body.
The entire Holley double pump carb line all idle nasty rich and really to use them on the street you need to lean out the idle by increasing the size of the idle air bleeds.
They are really intended to be race carbs, drag race specifically, and presumed to be run with open race headers. With that set up, the idle mixture is right where it needs to be.
The number you need to change to isn't a published number by Holley but you can get an idea of what it should be by measuring the idle air bleed with numbered drill bits on a carb that is intended to run clean on the street like an 1850. A List 1850, 600cfm with vacuum secondaries.
You only need to go bigger if you intend to run at Lemans for 24 hours, flat out. Then a 750cfm like a List 3310 is what you want.
If you've never done this before than you are best off taking it to someone with experience with them like Ron.
Remember once you have gone large on the air bleed, you can't go back unless you remove that bleed and press another one into the main body.
You also need to verify that the passages are clean of debris from the procedure and you can only do that by disassembling the carb and using compressed air on it on your workbench.
So many people attempt to fix the rich idle by adjusting the idle mixture screw but just like on a Weber IDA, all this does is change the VOLUME of atomized fuel the car is idling on. Like Webers IDAs, you need to change the air/fuel ratio.
The only way you can do that on a Holley is to ad more air into that mixture and you do that by increasing the size of the idle air bleed on a Holley, or idle air holder on a Weber IDA.
The Holleys, in effect, have emulsion tubes like Webers use but they are built into the carb and not changeable at all. It is possible that you can go through all of this and the freakin' thing is still too rich. In that case, go to a known street Holley.
If you drive the car only on the street, an 1850 is all you really need. Especially if you are using a stockish hydraulic lifter cam. Jet it right and it will make the engine pull like a freight train. A freight train with about a 6800 rpm limit.
You want high engine torque to be the determining factor on the street, not ultimate horsepower.
You can also get around 20mpg with the 1850. Try about 12 or 13 with the 4779?
I ran a 4779, 750 double pumper on a Shelby intake manifold on the street for years. It was run in a Boss 351 engine configuration with much hotter than the stock Boss 351 cam, solid lifters and big tube headers. That carb should be very doable to make run right. At least acceptable. If I could do it, anyone can. Ain't no German rocket scientists here!
Best of luck with your endeavor.