Recently, I replaced the heater core in my 1981 f100 I6. I also found a ducting issue for floor/defrost flap, which I temporarily fixed by moving the flap closed and leaving the selector in the heat to defrost range. The ducting issue often caused a "hiss" under the dash that I figured was due to the vacuum switch being stuck in a wierd place; that hiss is now gone. Unfortunately, since the repair, my pickup is idling really rough and low rpm. I runs better when I'm moving. I would think vacuum leak possibly, but when I pull vacuum hoses to create an artificial leak, it runs better (idles higher, almost too high). Any thoughts or suggestions here?

 

Hey, krazy, do me a favor and fill out the rest of your profile so we can see where you're at,
ok? Click the User CP in the upper LH corner, you can edit stuff in the LH column of the
following page.

About your idle, I think you just *closed* a previous vacuum leak (the hiss under your dash)
and I'm guessing your carb had been adjusted in the past based on that leak being there.

I don't know anything about adjusting the idle mixture on the 6-banger YF or YFA (or
whatever) carb but others should be able to help ya. You can also search the archives or
look/ask in the 6-banger forum:

Ford Inline Six, 200, 250, 4.9L / 300 - Ford Truck Enthusiasts Forums

 

If you are letting air in at idle and it runs better and faster, it's running too rich.

 

Okay, I added the personal info as requested. I wondered about the "fixing a leak" creating a problem. It reminds me of my other vehicle ('93 pontiac). I had a mechanic friend look at. He saw a plugged vacuum hose on it and hooked it up properly, causing the car to run rough. He told me it was probably timed and set to run with the vacuum hose unhooked and plugged. It sounds like I might have a similar problem here. I do notice more smoke now than before, and when idling really low and rough, lots of black smoke. So---any ideas? You all think I need to adjust the timing and/or the fuel mixture, and any tips on how to do that?

 

Black smoke == unburnt fuel. Investigate setting the mixture on your carb, leave the
timing alone for now.

 

That will be my first investigative step. I will research what I need to do to adjust the fuel mixture on my old 1bbl stock carb; if anyone has any suggestions, I'm all ears. 8-)