I posted this in my "home" forum, but haven't had much in the way of suggestions, which is odd, because those guys are usually so helpful. Anyway, I thought I'd try in the 300 forum. This is in my 83 F100.

Problems:
1) Random, intermittent misfire.

This is most evident at idle. It isn't noticeable under load on a flat road with no cargo, but I did definitely notice it a few weeks ago going up hills at 55 with about 600lbs in the back.

2)Pinging at slight throttle.

I get this in all gears when the throttle is just open a little. If I let off the gas, or give it more gas, it goes away. I have not been able to reproduce this in park however.


So far, I have replaced the plugs, wires, cap, dizzy, and coil.

I have run the engine with plug wires disconnected in turn, and it runs the same regardless of which wire is disconnected.

Ran a can of SeaFoam split between the gas tank, crank case and down the carb as directed.

Set timing, idle and mixture.

Made sure carb is tightened down.


I have 2 possibilities in mind, with what I believe is evidence to support both.

1) Air leak.

The signs of overheated plugs, and the knock at slight throttle to me indicate there is air getting in somewhere. I've checked and rechecked the vac lines, and they all seem to be fine. The carb is nice ant tight on the manifold. Possibilities I can think of would be, the gasket between the carb and intake manifold, the intake manifold gasket, cracked intake manifold, or a vac line I somehow missed. I hit all the vac lines I could see with starting fluid and noted no change in idle.

2) Valve issue.

I understand a good compression test doesn't necessarily mean the valves are OK. I have a definite ticking from the valve cover. Also despite the good compression test, I have enough blowby that the crankcase has more pressure than the PCV can handle, as i get some vapor out of the breather tube, and my dipstick is always pushed out an inch or so (though it could just be shaking loose).



So, where should I go from here? I've never been a fan of just throwing parts at a problem, and with money tight, it's not an option. What can i do to determine if these are symptoms of the same problem, or two separate problems, and how do I best identify the problem part? Please keep in mind that I have average mechanical skills and an average set of tools.

 

What is the timing set to? Might need to back it off just a bit (2-3 degrees).
Get a can of carb cleaner, and spray a little around the base of the carb, engine idling. Listen for changes in the idle. The base plate to carb main body screws are known to back off, and the gasket can be leaking even after resnugging these screws.
As for the higher than normal blowby, try this. Substitute 1 qt of ATF during an oil change, and drive it. When the oil starts getting really black (500-1000 miles) change the oil with another qt of ATF in place of 1 qt oil. Repeat until the oil stays fairly clean until time for a change.
ATF really cleans up the inside of the engine, without dumping so much crud into the pan that a pickup tube or oil filter can clog.
If the engine is really gunked up inside, this can make it run much better. ATF also seems to make the valve stem seals work better, to reduce oil consumption.

One last thought on the intermittant miss. Check for a small exhaust leak. It's really amazing what a minor exhaust leak will do for an otherwise smooth idling engine.

 

Has the EGR system been bypassed or disconnected?

 

No, it's all in place. I can't swear it's all working, but it's all there at least. I know how to test the EGR (I don't have a vac pump, but I know how it's done), but I have no idea how to test the air pump.