I've got a 74 F100 with a 302 and motorcraft 2100. I rebuilt the carb a month ago, and it ran fine for a couple of weeks. But then it started running pretty poorly. The idle seems too low, and it's a loping idle when before it was really smooth. The exhaust smells really rich and it's very sooty (it stains your hand, and left a big black splat on my motorcycle in the driveway). It runs worse when cold (it idled so low, it died at the first stoplight I came to the other day), but not much better when warm. When it's warm the choke appears fully open.

The only other clue I have to go on, is that when I rebuilt it and reset the air mixture screws (1.5 turns out) it wouldn't start. I kept backing them out until it started and ran. I think I'm like 3.5 turns out.

Any ideas?

 

Here are a couple of basic online carb tuning guides to help you dial in the screws..

1. Setting Idle Mixture The Right Way
2. How to Use and Interpret a Vacuum Gauge

Seems to a common problem these days, The forgotten art of dialing in those old fashioned carbs...

 

Thanks that helped! I agree, it is getting to be a forgotten art. I've never been very good at tuning by ear, so I usually left it to others. Can't afford to do that now. Thanks again.

 

Glad to hear you got it, armed a vacuum gauge, and a screw driver you'll never have to depend on anyone else to dial it in for ya. You now know more than they do.... I also use a cheap old dwell meter with the tach option to keep an eye on the rpms while under the hood...

 

Perhaps it's a coincidence, but my truck blew it's muffler apart yesterday. It only backfired once (that I could hear), but it was a big one.

If I were to float a hypothesis, I would think that after weeks of running very very rich, the baffles were coated with gas/soot, and the backfire was enough to light it on fire. And the muffler was rusty enough that I couldn't handle the pressure.

I'm thinking it backfired because the ground wire of the petronix was loose and making intermittent contact.

Does that make sense? Or is there something else I should be looking for.

 

Anything is possible... Make sure your timing is dialed in along with the carb and rpms.. Could be a lot of carbon built up everywhere if it was running that rich... Pull a spark plug or two to evaluate the engines running condition...

 

Ok another mystery...

After tuning it according to the instructions posted above, it ran better. Except for a rough idle and dying at stop lights. I double checked my timing (10 deg btdc with the vac advance unplugged), and double checked my idle and air screw settings. Everything was supposedly correct. Turning the screws in more than 1/2 turn, from 1 1/2 turns out made it die. However there wasn't much change at the vac gauge turning the screws out. I turned them so the most vacuum was showing on the gauge and then fiddled a little each way for the smoothest idle, which still wasn't very smooth.
Further searching on this board led me to reexamine my float height to deal with the dying at stop lights. I set it a little less than 7/16ths dry and the dying stopped. Then I drove it for a week. It ran, but idled poorly.
Then I changed all the plugs, and decided to triple check all my settings. When I pulled the top of the carb off, there was very little fuel in the bowl. The tank was full, and the pump had been changed a few hundred miles ago. It started fine, and after letting it run up to temp, rechecked the carb. Now the bowl was full. Maybe a leak? The plugs did smell gassy.
For grins, I started it up and began turning the air screws in, and now I could run them all the way in and the truck ran better. Smoother idle and less rich smelling at the pipe.

So that's how I'm running it now, air screws all the way in, and it starts ok when cold and a little reluctant when hot.

I'm at a loss to explain it. Worn carb? Air leak somewhere else?

 

The Bermuda Triangle Strikes again... lol... It very well could be a vacuum leak somewhere else... These trucks are getting old... Hoses, gaskets, brake power booster, auto trans modulator, cruise control, ect... When you opened the top of the carb to find the bowl low on fuel, had it been sitting overnight, this new fuel evaporates very quickly, or as you stated above, the float was simply off... Maybe the jet, rod size or spring is off as well... Keep at it, Once you've dialed it in... No Worries....

 

Might have figured it out: bad power valve.

A friend who works on old fords for a living told me to replace the power valve as soon as I described the symptoms. He said he's come across three bad power valves in the rebuild kits in recent months.

As soon as I replaced it the air-mixture screws started working as expected, and once adjusted correctly, everything seems to be running smoother. I'll post an update tomorrow after I get a few more hours on it.