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351C Carburetor Coughing
#1
Wrench 
Hello everyone,

So my dad and I have been repairing his 1971 mach 1 mustang 351C 2V that he had in high school and college until it broke down. It's just sat for 30+ years until this past summer when we sent it out to a shop in town to get it to start, run, and stop. While they did manage to do that, they didn't do the best job in the world so for the past couple months, whenever I'm home from college, he and I always work on it together and I've gotten almost everything running beautifully now except for one big problem we have. The carburetor coughs. So when it's idleing in park it's just fine and in drive it's perfect too, but when driving it, when you step on the gas too quick the engine will "cough" out of the carburetor. I can't seem to find anywhere where someone's got the problem figured out. Some people say the fuel mixture is too lean, but when you accelerate in park it's fine so I didn't think that was the problem. Others say the Carb is just old and needs to be replaced or rebuilt, and others say it's the fuel pump itself. Honestly we're both lost on this one so if anyone has a list of things to try to identify the problem and try to fix it that would be a lifesaver. Thank you so much.
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#2
(04-16-2017, 11:09 PM)patrickob17 Wrote: Hello everyone,

So my dad and I have been repairing his 1971 mach 1 mustang 351C 2V that he had in high school and college until it broke down. It's just sat for 30+ years until this past summer when we sent it out to a shop in town to get it to start, run, and stop. While they did manage to do that, they didn't do the best job in the world so for the past couple months, whenever I'm home from college, he and I always work on it together and I've gotten almost everything running beautifully now except for one big problem we have. The carburetor coughs. So when it's idleing in park it's just fine and in drive it's perfect too, but when driving it, when you step on the gas too quick the engine will "cough" out of the carburetor. I can't seem to find anywhere where someone's got the problem figured out. Some people say the fuel mixture is too lean, but when you accelerate in park it's fine so I didn't think that was the problem. Others say the Carb is just old and needs to be replaced or rebuilt, and others say it's the fuel pump itself. Honestly we're both lost on this one so if anyone has a list of things to try to identify the problem and try to fix it that would be a lifesaver. Thank you so much.


Mate mine does the same when it's not quite up to temp, under hard acceleration it likes to cough at the carb.

Once it's warm it's much better.

Does engine temp change yours at all?


Sent from my iPhone using Tapatalk
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#3
patrickob17,

Accelerating in Park does not replicate the engine under load. It could be a bad vacuum diaphragm on the distributor, it could be the old carburetor does need to be rebuilt. You said it has been sitting for 30 + years. Time sitting is an enemy of automobiles.

All of the rubber components in the carburetor are probably rotted along with the vacuum diaphragm in the distributor. The coughing could also be timing related issues. The accelerator pump is suppose to elevate that issue when the carburetor and engine are cold.

Keep us posted on your findings.

mustang7173
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#4
The accelerator pump diaphragm is probably stiff from age and not allowing the pump to work properly.
http://www.carburetor-parts.com/Carb-Type_ep_665.html
Chuck
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#5
I would suspect the accel pump as well. A simple test. Start the car and let it run for about 30 seconds then turn it off. With the car off remove the air cleaner. Look down into the carb, hold the choke open if needed. Either have someone step on the gas, all the way to the floor or open the throttle quickly by hand. You should see a Spray of gas on both of the ventri's. Similar to what you might see from a squirt bottle when you pull the trigger. If nothing comes out or it just dribbles your accel pump is probably bad.

Good deal getting the old mustang on the road again!

'Mike'
73 Convertible - 351C/4V CC heads/4bolt/forged flat tops/comp 270/rhodes/mallory unilite/tri-power/hookers/glasspacks/c6/3.50 limited slip/Gear Vendors/Global West sub frames, strut rods and shelby style traction bars/ Rear sway bar/tilt steering (not original)

Pics of modifications included in:
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#6
I agree, likely the accelerator pump, possibly power valve.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#7
Okay so I tested what Will C. said to do and it did spray just like you said it should from both ventris. Also the problem is actually worse as you drive. Like when we first take it out she'll run pretty well, then as we continue it starts to cough more and more. We tried to seafoam the engine like this guy did here http://diysrc.com/how-to-seafoam-your-ca...n-buildup/ and that definitely helped out a ton, but after about a half hour of driving the cough came back. I don't know I'm starting to think that Mustang7173 is right in that it may just be that even though the place that did the repair either didn't do a full carb rebuild or just did a poor job of it because it doesn't look nearly as good as what I've seen from pictures when people say they do a carb rebuild. Any suggestions on what to do or what kit to use best works for this carb? Also I know this sounds crazy probably and it's something we probably wouldn't want to do simply because it's great to have almost all original parts, but someone said something about switching from a 2V to a 4V. What're y'all's thoughts on that?
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#8
(04-17-2017, 11:36 AM)patrickob17 Wrote: Okay so I tested what Will C. said to do and it did spray just like you said it should from both ventris. Also the problem is actually worse as you drive. Like when we first take it out she'll run pretty well, then as we continue it starts to cough more and more. We tried to seafoam the engine like this guy did here http://diysrc.com/how-to-seafoam-your-ca...n-buildup/ and that definitely helped out a ton, but after about a half hour of driving the cough came back. I don't know I'm starting to think that Mustang7173 is right in that it may just be that even though the place that did the repair either didn't do a full carb rebuild or just did a poor job of it because it doesn't look nearly as good as what I've seen from pictures when people say they do a carb rebuild. Any suggestions on what to do or what kit to use best works for this carb? Also I know this sounds crazy probably and it's something we probably wouldn't want to do simply because it's great to have almost all original parts, but someone said something about switching from a 2V to a 4V. What're y'all's thoughts on that?

Were they suggesting switching the heads from 2v to 4v or just putting an aftermarket intake on the 2v heads that will accept a 4bbl carb? The head swap is going to be a considerable amount of engine work and the cost of the heads as well as new intake and exhaust manifolds. The intake swap with a 4bbl carb is pretty straight forward and is a common upgrade.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason


[Image: 082-hot-rod-power-tour-2017-1970-1970s.jpg]
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#9
(04-17-2017, 11:44 AM)73pony Wrote:
(04-17-2017, 11:36 AM)patrickob17 Wrote: Okay so I tested what Will C. said to do and it did spray just like you said it should from both ventris. Also the problem is actually worse as you drive. Like when we first take it out she'll run pretty well, then as we continue it starts to cough more and more. We tried to seafoam the engine like this guy did here http://diysrc.com/how-to-seafoam-your-ca...n-buildup/ and that definitely helped out a ton, but after about a half hour of driving the cough came back. I don't know I'm starting to think that Mustang7173 is right in that it may just be that even though the place that did the repair either didn't do a full carb rebuild or just did a poor job of it because it doesn't look nearly as good as what I've seen from pictures when people say they do a carb rebuild. Any suggestions on what to do or what kit to use best works for this carb? Also I know this sounds crazy probably and it's something we probably wouldn't want to do simply because it's great to have almost all original parts, but someone said something about switching from a 2V to a 4V. What're y'all's thoughts on that?

Were they suggesting switching the heads from 2v to 4v or just putting an aftermarket intake on the 2v heads that will accept a 4bbl carb? The head swap is going to be a considerable amount of engine work and the cost of the heads as well as new intake and exhaust manifolds. The intake swap with a 4bbl carb is pretty straight forward and is a common upgrade.
They were suggesting the intake swap with a 4bbl carb I believe
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#10
Honestly though, I just want it to drive without coughing and take off when I stomp on the gas because no new car has that same sound and powerful feel to it that this car does. It's intoxicating just to listen to.
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