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Carburetor hesitation
#1
I have a 1973 Mach 1 with a 351 Cleveland Engine, H code.
The intake is an Edelbrock intake manifold dual plane for a 4 barrel carb
The carb is a Holley 460. Its probably about 30 years old.
I have new ignition parts everywhere: Petronix II, new rotor and cap, new spark plug wires and spark plugs etc. etc.
Timing and starting the engine are fine.
No vacuum leaks. The engine idles great.

Problem:
When I initially push on the accelerator I get a slight hesitation. However when driving at 30 mph or above there is no hesitation due to the rpm of the engine and when I fast accelerate/kick down no issues.
Also before it warms up I’m getting some misfires thru the carburetor like its not getting enough fuel or something and the car slows down from the loss of power. I checked and adjusted the carb with a vacuum gage and I’m getting a reading around 46-48 on both sides. Also when I accelerate the carb by hand when its warming up I get like a backfire thru the secondary’s like I stated before. However, after its hot and running for down the road for 10 min. I don’t get the misfires but it still hesitates on acceleration. I also check the fuel pump and the pressure is fine.

Solution:
I don’t know which is best either to do a rebuild or replace the carb with:
1. Holley Carburetor 4V Electric Choke 600 CFM Polished Finish 1965-1985
a. Holley Part Number: 0-80457S
2. Holley Carburetor 4V Electric Choke 650 CFM Classic Finish 1965-1985
a. Holley Part Number: 0-80783C
3. Holley Carburetor Street Avenger 4V Electric Choke 670 CFM 1965-1985
a. Holley Part Number: 0-80670
4. 600 CFM Four Barrel Street Carburetor
a. PART #: 0-80452
5. 650 CFM Four Barrel Street Carburetor
a. PART #: 0-989`5
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#2
Sounds like an accelerator pump or power valve to me
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#3
My first guess?
Initially sounds like a lean accelerator pump issue.
Is it leaking by chance?
The diaphragm could be dry and resistant to "filling", not giving it enough squirt. It would be more noticeable when cold as described.
Also the accel pump circuit itself may be restricted somewhat, especially if it hasn't been overhauled in recent memory.
Put a gasket kit in it without fooling with any adjustments and call it a day.

Of the other carbs mentioned (even though, if yours is otherwise tight I wouldn't replace it), #080457 the "standard" carb for many years for Fords. It has the adapters required for easier hookup.

Pete - MotoArts Decals and Signs
'71 Sportsroof 351C-4V/4-speed - FINALLY under construction - no, wait, on hold again...
'90 Mustang 7-Up 5.0 ragtop, rolling beater
'66 Sunbeam Tiger Mk.IA, survivor
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#4
Initial hesitation sounds like your lean to me. Did you add headers or a freer flowing exhausts or different heads at some point.

Accelerator pump and power valves both need to be looked at.

an engine's vacuum pulls fuel through the main circuits and atomizes it. When you accelerate quickly the engine vacuum drops and the signal that draws fuel weakens or even stops-that will be felt as a "stumble" The accelerator pump sprays fuel (like a windex bottle) for a moment to help the carburetor transition from pulling fuel in with engine vacuum to operating on venturi vacuum. If it was running well and stopped then rebuilding the carb and tuning the accelerator shot would make a lot of sense.

A blown power valve allows fuel to be present in addition to the idle fuel, causing a rich mixture at idle.

Simple test is to screw in the idle mixture screws all the way and if the engine does not die, the power valve is bad. If it does die, the power valve is not leaking.

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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#5
caspianwendell;80406 Wrote:Sounds like an accelerator pump or power valve to me

{thumbs up} Any holley of that age will not have anti power valve blow out either like the new ones do...and accelerator pump..Specialy if you never rebuilt it....if you got a freind with a good carb to test on yours...i would try that..If not i would get a 650cfm or more for stock 4v..No smaller than 650 thou...The factory 4300d on my stang was 716cfm's.
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#6
check the power valve and the engine died when I screwed in the idle mixture screws. How do you check the Accelerator pumps?

Out the list of carbs above what is recommended?
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#7
sgtjd;80536 Wrote:check the power valve and the engine died when I screwed in the idle mixture screws. How do you check the Accelerator pumps?

Out the list of carbs above what is recommended?

From that list the 0-80783C gets my vote. Has adjustable, center pivot floats, a secondary metering plate that allows jet changes, and ford kick down lever. Chuck
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#8
As to replacement carbs a Holley 4150 vacuum secondary between 600-750 cfm will be fine. Electric choke is nice if you have headers or an intake change that keeps you form running hot air to the choke.

You could run a double pumper, but they are less efficient and harder to tune for good driveability in all circumstances.

Manual chokes are easy to forget to use properly and require a cable to be run to the engine compartment. Aluminum bodies and special finishes etc are all just a matter of preference.

to check the accelerator pump you should be able to remove the air cleaner and just quickly open the throttle momentarily and you should see the squirters misting and spraying fuel. Open it slowly and you should not see this.

Accelerator pumps have a rubber diaphragm that can fail and they won't squirt or will squirt with reduced volume and intensity. Cheap and easy to fix, but a critical part of having a carb perform smoothly. Adjustment is important too as the pump arm requires a small amount of clearance as specified by holley to perform properly.

A lot of people want to bolt on a carb and go. The manufacturers say it can be done, but I am less of a believer. I think every carb is going to need to be evaluated and adjusted and tweaked to give OEM or better throttle response. Most folks just put up with less than the performance they could have from their fuel systems.

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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#9
(07-17-2012, 11:54 AM)Jeff73Mach1 Wrote: As to replacement carbs a Holley 4150 vacuum secondary between 600-750 cfm will be fine. Electric choke is nice if you have headers or an intake change that keeps you form running hot air to the choke.

You could run a double pumper, but they are less efficient and harder to tune for good driveability in all circumstances.

Manual chokes are easy to forget to use properly and require a cable to be run to the engine compartment. Aluminum bodies and special finishes etc are all just a matter of preference.

to check the accelerator pump you should be able to remove the air cleaner and just quickly open the throttle momentarily and you should see the squirters misting and spraying fuel. Open it slowly and you should not see this.

Accelerator pumps have a rubber diaphragm that can fail and they won't squirt or will squirt with reduced volume and intensity. Cheap and easy to fix, but a critical part of having a carb perform smoothly. Adjustment is important too as the pump arm requires a small amount of clearance as specified by holley to perform properly.

A lot of people want to bolt on a carb and go. The manufacturers say it can be done, but I am less of a believer. I think every carb is going to need to be evaluated and adjusted and tweaked to give OEM or better throttle response. Most folks just put up with less than the performance they could have from their fuel systems.

I have to agree, I bolted mine on and it runs and all but properly to OEM efficiency I would say no. I'm just not that good at what to do or look for tuning a Carb. I will worry about it when I start driving it on the road.
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#10
had/have the same issue with similar setup.

Since you have a 73 i assume you have open chamber heads.
before 72 closed chamber was used and the compression was higher.

basically your engine is not getting enough fuel in the mixture.

What doesn't seem to help is theEdelbrock intake. for some reason this intake hates open chamber heads.

anyway....

So i have gone through this also. i've used carbs from the recommended 550CFM to 750CFM
what you will find in this case is upping the CFM will slightly reduce the problem. this is not the solution, upping the CFM will increase the fuel flow for a while but then what happens is the vacuum signal falls off and the carb runs leaner. too small a carb tends to run rich due to higher vacuum signal, too big a carb runs leans all the time, too low a vacuum signal.

your issue like mine is the overlap from the idle fuel circuit to the cruise jets circuit.
think of the carb as a complex circuit board. with 4 pieces.
Idle
Transition
cruise
power.

basically focus on the transition that is when Idle and the accelerator pump is used. basically your accelerator pump shot is too small.
options:
1) up the shooter size for the pump shot
2) change pump shot cams.
3) up the accelerator pump fuel diaphragm size.
4) advance timing 2 degrees.

#3 should be the last resort. the options for 3 are going from a 30CC diaphragm to a 50CC that leaves you going from 7 cam options to 2 cam options.
in my case i had to do all 4 of these to make the engine run 99% normal hot or cold.

your problem sounds minor.
first i would make sure the accelerator pump is shooting fuel correctly.
with the engine off, press the accelerator pump lever and make sure fuel shoots out of the shooters in the carb body.
next check the adjustment of the lever make sure the instant the throttle is touched the shooters start to go off, a lot of people adjust the curb idle speed but forget to adjust the accelerator pump shooter, if there is too much play between the throttle plate and the accelerator pump lever then the accelerator pump will not shoot the instant the throttle is hit and there will be a lag and that can cause a hesitation.

you don't want to go crazy. the pump lever should just kiss the accelerator lever.

if you checked all that and can;t find a problem start upping the shooter size go from a 27 to a 29 and keep going until either the problem goes away or you get black smoke coming out the tail pipes when you punch it. if you get black smoke, back off 2 shooter sizes like if you are at 37 back down to a 35. do not exceed 40 shooter size. if you keep going and are at shooter size 42 with the hollowed out shooter bolt then you have a bigger issue and need to take a different approach.

going up 2 or 4 sizing in the shooter might be enough to cover the lean condition.
if you find this clears up the problem 99% and there is just a little bit, then you can start messing with the accelerator pump cams, they all have different profiles and you can tailor just when you need the shot of extra gas.

now sometimes you can cover up a slight lean miss with 2-4 degrees of extra initial ignition timing.
you could try that as well.

another thing is to check the vacuum advance on the distributor. if you have an adjustable advance you can play around with it as well.but i say leave this alone.

the best is to test the car with the vacuum advance disconnected and plugged for the vacuum leak. you run on just mechanical for testing and see how the car responds to accelerator pump adjustment.

exhaust sound is key also.

when you punch it listen for the exhaust if you hear something that sounds like marbles rolling around in the muffler that is the engine pinging from detonation it also indicates a lean condition in the motor. basically the engine needs more gas at that throttle position. if you climb a steep hill and hold the throttle steady around 35mph and you hear braaa ratatataataa then you know you covered the hesitation off the line but the engine still needs more gas. you want a rich rumble sound as you accelerate up to the point that black soot comes flying out the tailpipes and you look like the 1980s spyhunter smoke screen.

lean on acceleration will be hesitation then go, backfire through carburetor up through air filter. real lean will be stall out on acceleration and may blow the air cleaner off, and puncture the vacuum booster on the brakes (ask how i know, and took 6 months to have eyebrows :-D )
rich on acceleration will be hesitation, lots of black smoke, your stomping on it and you can feel the car wants to go but its making Burbling sounds. the hesitation lasts until your over 40 mph.

It may take you trial and error, the motor will be finicky when cold, and run better when it warms up.
options for that might be to go up a range on the spark plugs heat range. like if you have autolite 24 and go to autolite 25s. that increases combustion temps 75 to 100C you want to save that for last as well. a lot of cars do run too cold there is a optimal engine temperature range. there are ways to tell if your engine is running too cold from the spark plugs but another time on that.

basically look at your accelerator pump first make sure it is adjusted. a quick fix try upping timing 2-4 degrees and take for a drive. if that shows no improvement that you want to start looking at upping the accelerator pump shooter size i think most holleys start at .26 or .28 shooter size, you can toss a .32 in and see what happens.
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