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1973 F-100 idling issues

  #1  
Old 06-08-2018, 08:52 PM
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1973 F-100 idling issues

Hello everyone; after letting my truck sit all winter I have recently encountered some problems regarding what I think is the carburetor on my truck. It will rarely idle and when it does it is a very rough, looney idle. I originally thought it was an ignition problem so I replaced points, plugs (which were very black for being ~1000 miles old) wires, and a rotary button. I then reset the idle on my carb until it ran smoothly, I assumed that it had been running rich before so I adjusted it accordingly until it smoothed out. This worked for about 30 mins until it started running rough again and now Iím back at square 1. Note: this is a stock 302 engine with the original Motorcraft 2 barrel carburetor. Also, I am seeing a lot of white smoke out the throat of the carb when I try to start the engine, and the choke is closed.
 
  #2  
Old 06-08-2018, 09:25 PM
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Ah, you might just need to adjust your choke to a Summer position.
 
  #3  
Old 06-08-2018, 09:44 PM
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Originally Posted by meangreen92 View Post
Ah, you might just need to adjust your choke to a Summer position.
Thank you for the reply; so I just open it up a few clicks?
 
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Old 06-08-2018, 10:14 PM
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In electric chokes there is typically a bimetal spring (like an old thermosat) housed in what looks like a really big black bottle cap. This spring controls where the choke plate begins at cold startup and slowly opens as it heats up via 12v current. If you loosen the retainer around the black choke cap, you can rotate it, increasing or decreasing the initial tension of the spring on the choke plate linkage. You should see the choke plate open or close as you do this, assuming all the pieces are there and nothing is stuck. The choke plate should only be cracked open slightly at first in most cases. There are also manual chokes without a spring and thermal chokes which are usually warmed by a small pipe connected to the intake manifold. You can get better info from the people here if you can provide pictures or numbers off the carburetor. Good luck!


PS. You might be flooding a bit if your choke is completely closed and you're seeing white smoke out the top. The smaller the gap, the richer it starts, the wider (more air) the leaner it starts, so closed choke will be really rich.
 
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Old 06-08-2018, 10:40 PM
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Originally Posted by XSoph View Post
In electric chokes there is typically a bimetal spring (like an old thermosat) housed in what looks like a really big black bottle cap. This spring controls where the choke plate begins at cold startup and slowly opens as it heats up via 12v current. If you loosen the retainer around the black choke cap, you can rotate it, increasing or decreasing the initial tension of the spring on the choke plate linkage. You should see the choke plate open or close as you do this, assuming all the pieces are there and nothing is stuck. The choke plate should only be cracked open slightly at first in most cases. There are also manual chokes without a spring and thermal chokes which are usually warmed by a small pipe connected to the intake manifold. You can get better info from the people here if you can provide pictures or numbers off the carburetor. Good luck!


PS. You might be flooding a bit if your choke is completely closed and you're seeing white smoke out the top. The smaller the gap, the richer it starts, the wider (more air) the leaner it starts, so closed choke will be really rich.
ah, sorry I donít have any pictures of it at the moment, but I believe itís an electric/ thermally assisted choke. I will try opening it up a tad tomorrow. Itís on the default setting now, it isnít completely closed (small gap for air. In between the choke plate and the carb) but Iíll open it more tomorrow because I think you may be right about the flooding issue
 
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Old 06-11-2018, 05:29 PM
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Tried setting the choke. I adjusted it all the way open (according to the marks on the top of the choke housing) and then back to the original setting, (the middle mark) nothing changed. Any other ideas?
 
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Old 06-11-2018, 05:50 PM
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Have you adjusted the idle mixture screws? It's that or the choke for idle. Try going clockwise until they are closed all the way, then come back open 1 1/2 turns on both screws. Then try to fine tune it from there.
Also, when was the last time your carb was taken apart for cleaning/new gaskets, etc? You could have a stuck float, or a saturated float that doesn't float anymore.... My truck had a stuck float when I got it and flooded constantly until I found the problem. It still sounds like you're running too rich, especially since you mentioned very black plugs. If you rebuild/tune the carb and it still doesn't behave, then your problem may be ignition or timing related. It doesn't cost anything to check your grounds..
 
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Old 06-13-2018, 12:08 AM
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Originally Posted by XSoph View Post
Have you adjusted the idle mixture screws? It's that or the choke for idle. Try going clockwise until they are closed all the way, then come back open 1 1/2 turns on both screws. Then try to fine tune it from there.
Also, when was the last time your carb was taken apart for cleaning/new gaskets, etc? You could have a stuck float, or a saturated float that doesn't float anymore.... My truck had a stuck float when I got it and flooded constantly until I found the problem. It still sounds like you're running too rich, especially since you mentioned very black plugs. If you rebuild/tune the carb and it still doesn't behave, then your problem may be ignition or timing related. It doesn't cost anything to check your grounds..
I guess I need to rebuild the carb. I have a hard time adjusting the idle mixture screws because it idles really rough without my foot on the gas and dies as soon as I get the screwdriver in place to adjust them. I have them set now in the only spot where the engine will run.
 
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Old 06-13-2018, 12:05 PM
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Have you checked for a vacuum leak? This is the issue I am having with my truck. My leak is at the base of the carb.
 
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Old 06-13-2018, 01:20 PM
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Originally Posted by CelticOne
Have you checked for a vacuum leak? This is the issue I am having with my truck. My leak is at the base of the carb.
Bingo. Any vacuum leaks, even very slight, whether intake manifold, carb base, throttle shaft or wherever will make tuning idle mixture impossible. Engine may ping, it leans things out.

Also make sure ignition timing is straight. Retarded ignition timing will also reduce manifold vacuum. A strong manifold vacuum signal through the carburetor idle circuit is what makes it work, there simply isn't enough air moving through a carburetor at idle otherwise. Though it's called the idle circuit, it is in play well into the transition zone, where most driving is done.
 
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Old 06-13-2018, 03:29 PM
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Originally Posted by Tedster9 View Post
Bingo. Any vacuum leaks, even very slight, whether intake manifold, carb base, throttle shaft or wherever will make tuning idle mixture impossible. Engine may ping, it leans things out.

Also make sure ignition timing is straight. Retarded ignition timing will also reduce manifold vacuum. A strong manifold vacuum signal through the carburetor idle circuit is what makes it work, there simply isn't enough air moving through a carburetor at idle otherwise. Though it's called the idle circuit, it is in play well into the transition zone, where most driving is done.
That's a good suggestion. You can spray a little wd-40 or carb cleaner around the base of the carb. If the idle changes, then you probably have a leak. You could also try closing other vacuum connections to isolate a bad hose or vacuum driven part. At least do this before you go and rebuild the carb. Did you also check the resistance across your coil and plug wires? You could have a weak coil or something not grounded well. If I were in your shoes it would probably be a little bit of all-of-the-above.
 
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Old 06-13-2018, 08:41 PM
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Yesterday before I went in I poured gas in the throat of the carburetor. Then, I noticed a small fuel stain seeping all around the base of the carb. However, when it was running, there was no sign of seepage. I assume thatís most likely the vacuum leak?
 
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Old 06-13-2018, 08:42 PM
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Also, how does one go about checking resistance? I put a new coil on last year, and everything but the cap and the actual distributor is brand new
 
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Old 06-13-2018, 09:27 PM
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Originally Posted by RileyCarroll View Post
Yesterday before I went in I poured gas in the throat of the carburetor. Then, I noticed a small fuel stain seeping all around the base of the carb. However, when it was running, there was no sign of seepage. I assume thatís most likely the vacuum leak?
Probably so. After settin' all winter with no activity you're bound to have fuel issues - the junk gas we have will turn to jell-o in a month of no usage - and the carb gasket will dry out.
Try a new gasket and check for more vacuum/fuel leaks....around the throttle shaft, carb base, etc.

Then run that puppy hard......
 
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Old 06-13-2018, 09:31 PM
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Wow. I had a guy rebuild the carb (before I learned how) a while back and havenít messed with it since. He didnít even put gaskets on there
 

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