66 Mustang Carb/Fuel problem - Ford Mustang Forum
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post #1 of 6 (permalink) Old 08-16-2019 Thread Starter
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Question 66 Mustang Carb/Fuel problem

Hey guys, first post here and in need of some serious help.

First off, the specs: 1966 coupe with straight 6 engine and 3 speed auto transmission, both rebuilt and installed in the last 2 months and have about an hour on them. Holley 1940 single barrel carburetor with original log style exhaust manifold. Brand new transmission cooler as well.

Car didn't run for years and sat, took it to garage 1 who installed the rebuilt engine. Gave me the option of running it like it is or cleaning the whole gas tank out and everything. I thought I had drained the tank completely prior, so stupid me being cheap says just throw a gallon of 100 octane in it and see what it does. Runs rough and the fuel is junk, but she starts up and gets going, although she struggles. If she dies, she turns right back over, so drive to shop 2 where they rebuild the transmission (167K miles on original transmission)

Shop 2 finishes their work, says the same thing, she idles rough and they struggled to start her unless they played with the choke, then she ran fine. All things point to dirty carb, so I drive her home (35 minute drive) with little to no issue. She stalls out at traffic lights, but whatever.

Pull the carb, and take parts out thinking I can get away with carb cleaner since the dirty fuel ran through her for so little time, and now's where it gets weird. Mount the carb, start her up and she fires right up almost as soon as I start the car and sounds phenominal. Take her for a spin, drive her about a minute and she just dies as soon as I hit a low RPM and round a corner to head back. Will turn over and turn over but absolutely will not start. Grab my tools from my house, pull the carb, open it up, clean it up a bit and mount the carb and again, starts right up. Drive back to the house and she dies again at the bottom of the driveway. Rinse and repeat (pull carb, open her up, spray it out, put it back on and it starts right up then dies about a minute later.)

Figure all right, I'll buy a carburetor rebuild kit and do all the gaskets, seals and everything, not a big deal, the accelerator needle cap (I don't know what to call it, that plastic cap that sits on the end of the needle and seals the cup) looked in rough shape any ways. Change everything out except the main jet (It was really locked in there and the valve opened and closed like it should, so I just left it). Made sure I replaced the check ball and weight (there's only one that I saw...) resealed it and literally the exact same thing happened.

I'm pulling my hair out again and have no idea what's going on, floats are working, fuel level seems to be right where it should in the carb, new needle is working. The only thing that's different post carb rebuild is that when I turn the engine over, if I push the gas pedal in almost all the way, it tries to start, but dies off the second I lift off the gas. Fuel/Air mixture problem? Did I miss something somewhere? No leaks that I can see anywhere and clean gas is coming out of the mechanical fuel pump. Sorry for the long post, but any help is greatly appreciated.

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post #2 of 6 (permalink) Old 08-16-2019
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Varnished gas and crud will cause all kinds of issues. Have you checked the condition of your points, set dwell and timing? What about adjusting the idle air/fuel mixture? Is your cub idle speed set correctly? Are there any vacuum leaks?

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post #3 of 6 (permalink) Old 08-17-2019 Thread Starter
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I keep thinking of taking a look at vacuum for a leak, but I can't keep it running long enough to start spraying around the hoses. Didn't think of checking points since I was so set on it being a carburetor problem, and adjusting air/fuel doesn't seem to solve the problem. Going to play with it some more today, hoping for better luck than yesterday.
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post #4 of 6 (permalink) Old 08-17-2019
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Turn up your curb idle speed until it stays running if you can. Most carb problems are related to the ignition.

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post #5 of 6 (permalink) Old 08-17-2019
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The runs then dies repeatedly sounds more like a fuel supply issue. If there was any residual crud in the tank/lines, they would be picked up by the fuel pump. Did you happen to put a fuel filter between the tank and pump? If not, the check valve in the pump could clog and stop fuel flow. Or the crud could have gotten to the fuel filter at the carb. When it dies and sits for a bit, the crud could float back toward the tank until the pump starts sucking fuel again. Hopefully, none of the crud got into the passages/ports in your carb (that would entail a full strip and boil out to clean the carb). I had this issue with the carb on an old Dodge Dart (it cost me a reman carb).
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post #6 of 6 (permalink) Old 08-17-2019
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Just read a story about a guy with the same symptoms. Turned out to be a bad connection to the coil.

15/5/1964 260 Manual Coupe.
23/3/1965 289 Manual Fastback.
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