Too lean/too rich
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Mountain Pass
Joined: May 2021
Posts: 183
Likes: 21
Too lean/too rich
My 82 F350 351W, manual trans and two barrel carb starts right up in the morning. But, on cold days if I let it sit there and idle it begins to run rough. However, if I put it in gear and take off, it pulls great and runs smooth. Could I have the carb set too lean or rich? Or could this be another problem? Any suggestions?
Moderator
Joined: Jan 2001
Posts: 56,966
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From: Virginia
Sounds like the choke, and it's not easy to get set right. One thing you can check is the choke pull-off or choke qualifier.
On the Fords it's usually a round diaphragm mounted on the pass side with a short vacuum line going to it. When you set the choke in the morning, it slams completely shut. You start the engine, but the choke being completely shut starts choking the engine too much. This is where the pull-off comes into play, the vacuum line is hooked directly to manifold vacuum. As soon as the engine starts, vacuum builds and the choke pull-off pulls on the choke linkage and cracks the choke open a small amount to give the engine some air. This is adjustable, on my China knock-off I have a stop adjustment on the diaphragm and I believe some of them you have to take pliers and bend the pull-off rod to adjust it. If it opens it too much, it will cause the engine to start and then stall.
On the Fords it's usually a round diaphragm mounted on the pass side with a short vacuum line going to it. When you set the choke in the morning, it slams completely shut. You start the engine, but the choke being completely shut starts choking the engine too much. This is where the pull-off comes into play, the vacuum line is hooked directly to manifold vacuum. As soon as the engine starts, vacuum builds and the choke pull-off pulls on the choke linkage and cracks the choke open a small amount to give the engine some air. This is adjustable, on my China knock-off I have a stop adjustment on the diaphragm and I believe some of them you have to take pliers and bend the pull-off rod to adjust it. If it opens it too much, it will cause the engine to start and then stall.
Freshman User
Joined: Mar 2021
Posts: 43
Likes: 7
From: Mt Airy, NC
Choke. As above and have you watched how the choke plates move? As the engine warms the plate should open wider and wider. How long are we talking before it runs rough? Sounds like all choke to me if choke is operable.
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Mountain Pass
Joined: May 2021
Posts: 183
Likes: 21
Thanks for the comments. I set the choke as described back in the summer. Chinese knockoff. I rotated the choke to just slightly open at full choke. Maybe it’s slipped since then or because it was warmer in the summer (90 degrees) the slight opening is now completely closed when so cold outside. I always get in and push the throttle to the floor, release, then start. Always fires up easily and runs fast, so the choke does set. I will readjust with it colder outside. Interesting that it runs fine with load immediately.
Freshman User
Joined: Mar 2021
Posts: 43
Likes: 7
From: Mt Airy, NC
The choke should close completely when you press the pedal all the way down. Every time. Then the vacuum pull off opens it a crack when engine starts. As it warms, it stas on high idle but should start coming down after you use the gas pedal again - to move or even stab at it. And it may take a few minutes for that to start happening. May still be choking out when it sits too long on idle?
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