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1984 F150 with 4.9L engine [feedback system still operative]. This is my son's truck and he is out of the country, so I have had time to work on this as time permits.
The engine was idling @ 950 - 1050 RPMs and was not smooth. At this idle speed, which remained the same even with the idle screw all the way out to the point that it was not touching the throttle body, the vacuum was 18 1/2" Hg. My 1986 F150 with 4.9L engine and Duraspark conversion pulls 21 1/2". The idle air fuel mixture screw had to be turned out ~5 turns for best idle.
Much time has been spent searching for the cause. All vacuum hoses, intake manifold junction, carburetor base and EGR adapter have been sprayed with copious amounts of carburetor cleaner without revealing the leak. Sometimes, when spraying near the #4/5 cylinder intake manifold junction, I thought I could detect a slight variation in engine speed and also when spraying near the carburetor base on the firewall side, but when repeated, the change would not repeat? All vacuum hoses have been pinched close with no change to engine speed. The EGR valve has been temporarily blocked and this did not solve the issue. I have done the "jiggle" test to the wiring harness and nothing showed up.
Not only did the engine not idle well, it missed @ ~1250 RPMs when under a very light load and bucked when decelerating.
I have posted and asked questions of forum members, respected local technicians as well as retired engineers and had not found the answer, until this past weekend. After an Internet search, I found on Youtube where a professional automotive technician asked the question, "do you have a high rough idle?" and proceeded to tell that he had a smoke generating machine which he used in searching for vacuum leaks. He is quite good and showed how a Do-It-Yourselfer could duplicate this quickly and inexpensively, so I did and was absolutely amazed how quickly and easily done.
I purchased a $0.69 cigar!
With the engine off, I taped the mouth of the carburetor with duct tape. I then took the vacuum hose which normally supplied vacuum to the temperature sensor on the air cleaner, which was available since the air cleaner was off. I lit that "bad boy" and after taking a deep draw, blew into the vacuum hose. I placed my thumb over the vacuum hose and took another deep draw and blew into the vacuum line. With the 2nd attempt [very quick], smoke was evident in a place I did not know existed. On the intake manifold is what I call a vacuum tree [distributor?] which Ford used on all 4.9L engines. Depending on the trim packages, options, etc. some, but not all ports are used. The remaining ports are capped with rubber boots. Also, keep in mind that since this vacuum distributor [tree?] is threaded in, the ports may be oriented differently. WELL, there is one unused port nearest the engine block, which in this case, is pointed downward, nearer the exhaust manifold. As a result of its location, the 34 year old rubber cap has been exposed to much heat and had dried out. The end of the cap was missing and the hole is about 1/2 - 2/3 the size of a pencil eraser. Do you think that could cause the previously described symptoms???
Due to the location, which cannot be seen due to the myriad of vacuum hoses, etc., the typical "hiss" could not be heard since the port was facing downward and in the vicinity of the exhaust manifold. Nor could it be "hit" with the copious amounts of spray. Occasionally, as I sprayed the intake manifold joint near the #3-4 cylinders, I thought I could detect a difference as well as when I sprayed the carburetor base and EGR adapter area, but could not get a repeat when I tried again. In hindsight, what was happening was at some times, the angle was just right and some vapor must have reached the open port.
I carefully used a screwdriver and removed the hardened residue and replaced the cap with one "I just happened to have" [thank you, Lord!]. I then repeated the smoke test and no other leaks were detected. After removing the duct tape from the carburetor opening, the engine leapt to life at a rather steady 450 RPMs [compared to rough 950 - 1050 RPMs]. The idle screw, which had been loosened to a point that the screw did not even touch the body, had to be turned many times just to raise the idle to ~650 RPMs. I then adjusted the idle air fuel screw which had to be turned in about 2 turns, and left as is.
The idle vacuum, which had been ~18 1/2" now is 21 1/2".
This morning, I drove it to work [~13 miles] and it is one happy engine with no missing or bucking!
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