I drive a 1981 Ford F-100 with the 4.9L inline 300. I have been having a problem with the carburetor when I start it when it is cold about 40 or so degrees and take off down the road, after a while it starts flooding out and the idle goes down and it puts out a lot of smoke. it only happens when it is cold and if i stop and turn it off and let it sit for a few minutes it runs fine. I asked a mechanic and he told me that the float valve needle had something in it. i took the carburetor apart and everything inside was clean. It started only about a year and a half ago and on chilly mornings it will do this it is very annoying.
The float valve needle will stop the fuel pump from overfilling the carb float bowl. I find that diagnosis to be flawed, since if there was something blocking it, it would constantly overfill, and never run right, whether it was hot or cold.
As Cougar said, the choke is a good place (probably the best place) to start.
that is another thing, i needed a new carb for it and instead of paying 200 or more for a new one i bought one from a guy that came off of a newer 300 and the choke did not want to stay open i think the automatic choke is the problem there, so i have the choke zip tied open. It has worked fine like that for years, the only thing about that is i have to pump it more to start it during winter
Have you checked the PCV valve? That might be failing, and pushing oil into the system. That would account for the smoke…why it would start to work once warm, I don’t have a guess.
Obviously, you checked to make sure your zip-tie fix is still in place…??
Maybe the diaphragm in the power valve is ruptured. A cold engine may be able to use the additional fuel being dumped because of a problem like this but a warm or hot engine won’t.
If the power valve is the problem then I would overhaul the carburetor (properly) and fix the choke at the same time. Inspecting and replacing the choke pull-off pod should also be part of a proper overhaul.
In a few very rare cases some Ford carburetors have been known to have a porous casting which will leak fuel and create the same symptom as a bad power valve.
The only way of verifying this is to remove the carburetor, fill the carburetor float bowl with gas, wipe the bottom of the carb off, and play the waiting game. That means watching the bottom of it to see if it becomes wet with gasoline.
when i had taken the carb apart the first time everything looked brand new, and it has only been about 2 years (give or take a little time) since i put a new kit in it
I’ll throw out a couple more possibilities: It sounds like fuel is puddling in the intake manifold until the engine heats enough to boil it off. This would explain the black smoke and why it goes away after sitting for a few minutes.
Two possible causes:
Rare, the floats have saturated with fuel making them heavier, raising the float level.
The air-bleed/anti: siphon/percolator port (located at the top of the throat) is partially or fully blocked. This port stops fuel from siphoning from the fuel bowl into the engine.
At any rate, it seems like another carburetor rebuild is in order.
Best of luck.
1981…A VERY bad year. Is this one of those feedback carburetors that have electronic mixture control and use an oxygen sensor to monitor fuel mixture?? It’s going to take more than a few new zip-ties…
Does the carb have a choke pull off, either internal (piston) or external (vacuum diaphragm). The pull off opens the throttle plate slightly after startup. I had an 83 Mustang GT with a Holley 4 barrel. The piston for the pull off would stick, the throttle plate would not open and the engine would flood and stall.
I got around it by adjusting the choke so it would start opening immediately.
If the pull off is working, the throttle plate should open slightly after the engine starts.
thanks to every one who put in there two cents and ideas, when i have some time this week i will go out and tinker with it some more and try these ideas.
to caddyman, it does not have any kind of electronics or any thing like that for me to look into, thanks, the only thing with this engine is all of the vacuum lines they put on them and the smog crap that gets in the way, which was removed before i got the truck.
"I asked a mechanic and he told me that the float valve needle had something in it."
Chassois, "The float valve needle will stop the fuel pump from overfilling the carb float bowl. I find that diagnosis to be flawed"
I find the diagnosis possible.
I would assume he meant there was a piece of grit or something meaning the needle valve does not shut off the overflow of gasoline. In that situation it is a probable.