carburetion

Jay,

Give me a call some time .I have a Pantera shop in Maryland . Maybe I can help you out with your issue? 410-596-3170

Ron McCall

Holley 4150 carbs are decent once they're dialed in for a motor. The trick is setting-up the ignition properly and then setting-up the carburetor properly.

Holley's double pumpers are set-up rich out of the box in the idle circuits because they're set-up for a motor with a high overlap cam and therefore lower than stock idle vacuum. Mount a carburetor jetted that way on a motor that has a mild cam (high intake manifold vacuum) and it draws too much fuel at idle. Of course the carb can also be set-up wrong, i.e. the primary or secondary butterflies may be open or closed too far exposing too much or too little of the transfer slots, or the idle jets are improperly adjusted. And then there's the setting of the fuel bowl floats and the vacuum rating of the power valves.

How have you set-up the motor's ignition timing? Is the motor equipped with a single plane or a dual plane intake manifold? What's the intake manifold vacuum at idle? Have you replaced the cam ... if so what's the new cam's specs? Was the carb new or used? If used did the previous owner modify it? Does the carb have copper or black phenolic floats? Phenolic floats will float less and less with age. What's the list number of the carburetor? If the carb is new are you using it jetted as it came out of the box?

There are Holley 4150 style carbs that are jetted properly for a street engine out of the box (Quick Fuel Technology). And there are shops who specialize in tuning carburetors for specific engines. There's several ways to resolve the issue.

-G

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.

Double-pumper racecar Holleys are problematic on the street unless you are a carb expert. I've played with them for years and finally bought a tuner-reworked Holley 700 DP. The carb guy said there were 35(!) different areas modified in my brand-new carb to set it up for a 351-C in a street Pantera! FWIW, it increased power and torque across the board and returned 21 mpg on a 1000-mile open road run. So I suggest you take Ron up on his offer. Or get used to the smell of gas on your hands at supper.

quote:

Originally posted by jayb:
... Looking for thoughts re: different type of carb, efi, etc.
Thanks. Jay

I'll put in plug for the F.A.S.T throttle body EZ EFI. It bolts to a Holley square bore bolt pattern manifold. Fantastic throttle response, Instant cold start and smooth idle. Easy tuning with a hand held pendant. You can set idle, cruise, WOT air fuel ratios, and the computer tunes itself over time using a wide band O2 sensor. You can also tune the "pump shot" parameter, as well as control electric fans. No need to even open the decklid. It is impervious to heat realted issues like fuel boiling in the bowls on a hot day. Over carburation is not an issue. My TB flows 1000 CFM.

That is the good. Now the negative. EFI is expensive. $2500 for a system. EFI is complicated. You need to plumb new fuel lines and new electrical connections. It would be best to pull the motor and do the plumbing and wiring right. EFI needs no leaks whatsoever in the intake or exhaust. It is technological. If you are not a do it your selfer, or engineer type that refuses to be defeated by technology, then EFI is not for you.

Otherwise, EFI can solve all drawbacks that the metered leak has.

I am running a Holley 4150 (List #4306 - 735 CFM, Vacuum Secondaries).

With a lot of help from a local expert, my carb is "dialed in" and runs, idles, and behaves great. I just got 16 MPG on a recent trip to / from Phoenix.

The car has been set-up for about a year now, and after it was set up, I haven't had to do aanything at all to keep it running right.

Note that my initial problems really seem to have been electrical (ignition), vs. a carb issue.

If you have access to an expert, the 4150 series carbs can work really well.

Rocky