I've got several threads going related to a carburetor / distributor conversion and getting rid of some emissions stuff after the computer is gone. That all got me to thinking about the base issue that started me down this path, and perhaps taking a closer look at that before going ahead with the conversion (Now THERE'S a thought..)

So..

The truck was running pretty good, a little rough at idle but not bad and certainly not dying. It always started right up, no issues.

Then..

I was coming home and noticed it seemed like it was losing power. I got home, parked it and then later that night went to start it and it would just crank.

I did some messing around and found that if I opened up the chock plate a bit (stuck a screwdriver to hold it open) the truck started right up. Running rougher than usual and not as much power, but started and ran.

I figured it was a choke thing, but now it will hardly stay running at all, cold or warm. Still need to 'crack open' the choke plate to get it started and the only way it will stay running is with the choke mostly closed. Open it too far and it dies.

This all started what seems like all of a sudden and the only thing I did right before that was get fuel. (I can't believe I got a tank of bad fuel)

It seems to me that it's running very lean and keeping the choke close riches it up a bit to let it run. That's my theory anyway.

I can't find any obvious vacuum leaks, but I'm thinking that may be what's going on. There IS a sort of hissing sound when it's running, but can't find any leaks (maybe it's just the carb sucking air).

In any case, I think I need to figure this out before I start the conversion. Other thing I saw was that manifold vacuum is only about 12-13 and fluctuates a bit, but I'm guessing that's because the engine is running so rough. I'm going to do a smoke test next to see if there is a leak, but anyone have any other bright ideas?

Again, this started all of a sudden and seems to be getting worse. Vapor lock?

1985 F150 4x4
Stock 4.9 Feed-Back Carburetor (at the moment)
4 speed w/granny gear


Thoughts?
Anyone?
Bueller? Beuller?

 

That sounds like a good theory. If you are correct, patting the gas pedal quickly should make it run a little better also. This will shoot little streams of fuel in the carb each time you lightly pat the gas pedal, which will be adding fuel by the accel pump. If it is running lean, then the engine should act favorably for a couple of seconds till it uses that extra fuel up.

If that seems to be the case, then the carb may be clogged inside, or you have large vacuum leak like you mentioned. You could also have a piece of carbon stuck in the EGR valve, which would cause it to run very rough at idle. I don't mind the theory and the EGR being on there, but that's why I don't like them, they fail or get dirt stuck them too often and cause trouble.

 

Disconnect & plug the vacuum hose from the brake booster and see if the hissing goes away.

 

I remember in my younger days learning to cover the throat of the carb gradually to get a feel for richness/leanness.