Fox Carburetor Question

[84foxyconv]

Hey everyone, new to the site and to the world of carburetion. I recently purchased an 84 mustang gt, stock internals and basic bolt ons. The car came with a Quick fuel 650 carburetor. Is this carb to big for my set up, should I replace with a smaller carb like a 570 as I plan on leaving stockish as far as internals or am I good? Thanks everyone.

 

[CarMichael Angelo]

Hey everyone, new to the site and to the world of carburetion. I recently purchased an 84 mustang gt, stock internals and basic bolt ons. The car came with a Quick fuel 650 carburetor. Is this carb to big for my set up, should I replace with a smaller carb like a 570 as I plan on leaving stockish as far as internals or am I good? Thanks everyone.

That depends on whether the carb has vacuum, or mechanical secondaries.
Both will work, but one tends to be a little more economical right out of the box, and works better w/ an automatic transmission. As with camshafts, a lot of people are notorious for throwing the biggest carb they can get on a car regardless as to whether or not it needs it.
A carb w/ vacuum secondaries will only open the back two throttle blades and commensurately add more fuel as engine load requires it. So despite the fact that you may have your foot in the throttle a little harder than required to get the car up to speed, if the primary front two throttle bores are all that is needed, the back two holes stay shut. That equates to a carb that is more efficient, and therefore gets better milage because of it. The smoother opneing rate of the secondaries makes this type of carb better for an automatic transmission. also because of the above, this type of carb allows a little more carburetion fudge factor CFM-wise than a mechanical "double pumper" type of carb.

A mechanical secondary carb ( double pumper) opens the secondaries up as you press down on the accelerator pedal regardless of whether or not the engine needs more fuel.
This means that excessive fuel that the engine didn't need, or require is typically wasted straight out the tail pipe. The obvious down side is poor fuel economy, and a engine that doesn't make as much power as it could have. A carburetor like this is typically installed on a more modded engine and used w/ a manual transmission, as the acceleration curve w/ a manual transmission is helped w/ multiple gear changes versus just having 3 gears like an auto has. Because of the tendency of this type of carb to get poorer economy, and use more fuel to typically make less power, a "double pumper" really has no business on a street only engine.

In 99.9% of instances, this is the only type of carb you'll find on a race engine.

A stock internal-ed engine, kept to stock RPM redlines will not need alot of carburation if you do some research. The average guideline will be between 550, and 650 CFM. Given that you are gonna be close to the CFM-O-Meter recommendation, the primary determining factor that goes towards deciding if that carb is right for your engine is done w/ your eyes and nose.

#1 How does the engine run with the carb that's on there now?
( no reason to fix something that ain't broke)

If the answer to #1 "is not that good" then:

#2 Is the tail pipe black? Have you pulled a few plugs and looked at them, and are they black or brown? (specifically look at 1,4,5,8) If those plugs are black, then in most cases the other 4 are even more so. The reason being is, typically on a carbureted intake, the 4 outside corners run the leanest, because the fuel has to travel farther to get there. So for example, if #'s 5, or 8 are brownish colored, then the carb "tune" is close to being right, and probably adequate for the engine.

** If the plugs are black, then:

#3 When it's idling, and you stand behind it, does it smell like its too rich?
#4 If a friend follows behind you and you stand on the throttle, does black smoke come out the tail pipes?
#5 Is the car an auto, or 5 speed?
#6 Does the throttle curve come in smoothly w/o bogs, stumbles or coughs?

A carburetor can be tuned to be made less rich, and someone that knows what they're doing can make that happen. Jets, power valves, squirters, accelerator cams, and diaphram springs can all be changed to affect how the carburetor flows fuel and air. Unless you're gonna pay somebody to do it for you, and despite the implied complexity, a "Holley designed" carb like you have is really quite easy to master, that same basic carb type has been made for 50 years.

Like all things that come with it when you mess w/ a car, it just requires some time spent working on it to truly learn how to fix the thing if it's not right.

 

[84foxyconv]

That depends on whether the carb has vacuum, or mechanical secondaries.
Both will work, but one tends to be a little more economical right out of the box, and works better w/ an automatic transmission. As with camshafts, a lot of people are notorious for throwing the biggest carb they can get on a car regardless as to whether or not it needs it.
A carb w/ vacuum secondaries will only open the back two throttle blades and commensurately add more fuel as engine load requires it. So despite the fact that you may have your foot in the throttle a little harder than required to get the car up to speed, if the primary front two throttle bores are all that is needed, the back two holes stay shut. That equates to a carb that is more efficient, and therefore gets better milage because of it. The smoother opneing rate of the secondaries makes this type of carb better for an automatic transmission. also because of the above, this type of carb allows a little more carburetion fudge factor CFM-wise than a mechanical "double pumper" type of carb.

A mechanical secondary carb ( double pumper) opens the secondaries up as you press down on the accelerator pedal regardless of whether or not the engine needs more fuel.
This means that excessive fuel that the engine didn't need, or require is typically wasted straight out the tail pipe. The obvious down side is poor fuel economy, and a engine that doesn't make as much power as it could have. A carburetor like this is typically installed on a more modded engine and used w/ a manual transmission, as the acceleration curve w/ a manual transmission is helped w/ multiple gear changes versus just having 3 gears like an auto has. Because of the tendency of this type of carb to get poorer economy, and use more fuel to typically make less power, a "double pumper" really has no business on a street only engine.

In 99.9% of instances, this is the only type of carb you'll find on a race engine.

A stock internal-ed engine, kept to stock RPM redlines will not need alot of carburation if you do some research. The average guideline will be between 550, and 650 CFM. Given that you are gonna be close to the CFM-O-Meter recommendation, the primary determining factor that goes towards deciding if that carb is right for your engine is done w/ your eyes and nose.

#1 How does the engine run with the carb that's on there now?
( no reason to fix something that ain't broke)

If the answer to #1 "is not that good" then:

#2 Is the tail pipe black? Have you pulled a few plugs and looked at them, and are they black or brown? (specifically look at 1,4,5,8) If those plugs are black, then in most cases the other 4 are even more so. The reason being is, typically on a carbureted intake, the 4 outside corners run the leanest, because the fuel has to travel farther to get there. So for example, if #'s 5, or 8 are brownish colored, then the carb "tune" is close to being right, and probably adequate for the engine.

** If the plugs are black, then:

#3 When it's idling, and you stand behind it, does it smell like its too rich?
#4 If a friend follows behind you and you stand on the throttle, does black smoke come out the tail pipes?
#5 Is the car an auto, or 5 speed?
#6 Does the throttle curve come in smoothly w/o bogs, stumbles or coughs?

A carburetor can be tuned to be made less rich, and someone that knows what they're doing can make that happen. Jets, power valves, squirters, accelerator cams, and diaphram springs can all be changed to affect how the carburetor flows fuel and air. Unless you're gonna pay somebody to do it for you, and despite the implied complexity, a "Holley designed" carb like you have is really quite easy to master, that same basic carb type has been made for 50 years.

Like all things that come with it when you mess w/ a car, it just requires some time spent working on it to truly learn how to fix the thing if it's not right.

Hey Mad Mike,

Just looked again and the carb is a 600 not 650. It is a mechanical secondary carb. To answer your questions.. It runs OK. Idle is alright and normal cruising is good however when I punch it it kinda falls on its face, there's power being held back. I do have black soot in the pipes and just changed the plugs due to them being black. And yes I can get high as a kit if I stand behind the car running. It is a 5 sped man and black smoke does come out. I had a shop dyno tune it and they put smaller jets in.. I believe 64 but it still runs the same.

 

[A5literMan]

Sounds like they didn't adjust it enough. What brand of carb? Any other mods to the engine? It's been so long ago since I messed with a carb. But a 600cfm shouldn't be to big in general. Even a mech secondary. Sorry for my "no help" reply.

 

[84foxyconv]

No problem I ap

Sounds like they didn't adjust it enough. What brand of carb? Any other mods to the engine? It's been so long ago since I messed with a carb. But a 600cfm shouldn't be to big in general. Even a mech secondary. Sorry for my "no help" reply.

No problem I appreciate the reply. It's a quick fuel 600. Only mods are a edelbrock intake mani, hei ignition, exhaust and shorty headers.

 

[CarMichael Angelo]

It sounds like the jets are still too big . That carb has 4 ( 2 in front, 2 in back)
The off idle stumble is remedied by enlarging the squirter nozzle size on the primary side. There is a number stamped on it, ( i.e. 37) up that by 2 steps or so as a start. You'll have to keep doing that until the " squirt" is adequate to compensate for the big gulp of air standing on the throttle causes. Tuned properly, you should be able to tap the throttle lightly with your foot, and instantly get a crisp rev.

Once you get the primary side to do that, you may have to bump the secondary squirter nozzle size to assist in fueling the off idle transition.

As I've always maintained, more damage to performance comes from too big a carb, as opposed to too little

The typical rule of thumb for determining if the carb is too big w/ regard to jet size is that if you have to go smaller than 10 jet sizes from how it came factory equipped, it's too big.

But that said, if it starts to clean up and run w/o black smoke pouring out back, or burning your eyeballs outta your head at idle after 10 steps down, I'd go a couple more until I got it right.

 

[84foxyconv]

It sounds like the jets are still too big . That carb has 4 ( 2 in front, 2 in back)
The off idle stumble is remedied by enlarging the squirter nozzle size on the primary side. There is a number stamped on it, ( i.e. 37) up that by 2 steps or so as a start. You'll have to keep doing that until the " squirt" is adequate to compensate for the big gulp of air standing on the throttle causes. Tuned properly, you should be able to tap the throttle lightly with your foot, and instantly get a crisp rev.

Once you get the primary side to do that, you may have to bump the secondary squirter nozzle size to assist in fueling the off idle transition.

As I've always maintained, more damage to performance comes from too big a carb, as opposed to too little

The typical rule of thumb for determining if the carb is too big w/ regard to jet size is that if you have to go smaller than 10 jet sizes from how it came factory equipped, it's too big.

But that said, if it starts to clean up and run w/o black smoke pouring out back, or burning your eyeballs outta your head at idle after 10 steps down, I'd go a couple more until I got it right.

Thanks Mike. Also I'm coming from efi so carbed cars are still a learning process for me but I don't need a dyno to properly tune these cars right?

 

[CarMichael Angelo]

A wide band O2 is all the additional stuff you'd ever need. ( and you don't have to have one of those either)

But it helps.