Bogging down, stumbling, hesitating from a dead stop or when accelerating at speed is indicative of a bad accelerator pump and accelerator pump check valve.

These two parts come in the carb kit, also available separately.

 

Don't forget to check the push rods. If one is bent this could also be your problem.

 

While you are warming up the engine to do the compression test adjust the carb. I'd be curious to find out where those idle mixture screws are now. I have a feeling one screw is 1/2 to 3/4 turn different than the other. Before you start it screw each screw all the way in counting and recording how far out they were. Lightly tighten. If I am right screw them back out splitting the difference and adjust both the same back and forth as you are adjusting the carb. Example, if one is 1 turn out and the other 2 full turns, set them both at 1 1/2 turns out and fire up the engine. Then watch that vacuum gauge and set the carb so you get the smoothest idle and highest vacuum reading.

 

I put a Quick Fuel 600 CFM Vacuum Secondary Carb on it. I haven't adjusted anything on it yet but I will do what you told me. There is Four Screws on it much like a Holley. I will give it a try and see if I can get it dialed in. I know there is a Measurement for the Accelerator Pump on that carb as well. Its like .o14 gap between the pump arm and the Linkage.

 

I forgot about the QFT carb. I have a sinus headache this morning that could kill a cow. Four corner idle? The off balance I was thinking is prolly front to back then instead of left to right. You can double check to make sure QFT set them the same but I still think that carb needs adjusting to your particular engine. My guess is the front two are close to what your engine wants but the back two are off. I think this is why your vacuum signal starts out high then settles down lower.
Then the wet test to figure out that low cylinder.

 

Thanks JEFFFAFA, Hope you get to feeling better. I don't think this truck was driven much and am wondering if maybe it just needs to be taken out for a long drive and see if things settle out. It seemed when I had my 74 F100 the more I drove it the better it ran. I will adjust the Carb and do the Wet test on #6 and see what I have going on. If it is a Ring issue is there anything I can do short of Pulling the Engine and doing a ring job on it to help the situation? The other thing I did last night was widen the gap on the plugs from 35 to 40 since I put the Flame thrower Coil and Electronic ignition in it. Seems to be running really strong and starts instantly.

 

A few things. Depends on what the wet test shows. And whether or not you have some blow by going on when you remove the oil cap while it's idling. Blow by indicates rings. Since you have a steady vacuum needle I don't think it's in the valves. But don't be worried yet. If it wasn't driven much or sat for a long time the rings might just be stuck in that piston. BTW, you have fresh gas in the tank not Old gas right? To free up rings you can do one of two things. 1st, slowly pour Marvel Mystery Oil down the throat of the carb while the engine is at a fast idle. This will help clean the combustion chambers and most times lubricate the rings. 2nd, put some Seafoam additive in the engine's oil pan with fresh oil and filter. A GOOD filter. Not some Wallyworld special. Like a Genuine Ford Motorcraft FL1A. But before you do this I'd take that valve cover off 1st. While you're checking for bent pushrods also look and see if the head surface and rocker assembly is all gunked up with mud or if it's clean. The Seafoam can free sticking rings and will also help clean up gunk build up. But that gunk will fill up the oil filter quick.

 

Okay, So I tuned the Carb and its Running Steady 21 for Manifold Vacuum and I took it for a Run and its still missing but its running better. The results of the Wet Test is still 120 in Cylinder 6. So Im guessing its in the Valve train. My Grandpa taught me a trick a long time ago and what I did was put the end of a screw driver on the Valve cover and put a glass bottle on the end of the Screwdriver Handle. I can hear a tapping sound directly over the #6 Valves that sounds different than the rest of the Valves. I am going to let the Engine cool off a bit before I start messing with removing the Valve cover. I will take pictures of what I find and I hope its something I can adjust or maybe replace a Push Rod to fix the problem. Also hoping its not a Dead Hydrolic Lifter. I love this kind of Troubleshooting!!! Thanks again for

 

Instead of pouring it down the intake I would recommend installing the marvelous mysterious oil in the fuel itself at 4oz. per 10 gallons of fuel as per the directions, and replace 1 quart of crankcase oil with 1 quart of the MMO. This is the proven, longstanding method of freeing stuck rings and lifters. It does work. Buy a gallon (it's cheaper that way) and run it in the fuel at the 4oz rate for a while and see what you think. Just try it, before mumbling "snake oil".

 

No Wallyworld Filters for my F100!!! LOL I put a tank of Premium in it when I bought it and it was running on Fumes when I filled it. So I have Chevron Premium in the Tank. So how long should I run the engine with the Seafoam in the Oil and should I just let it idle or take it out and run it? I will check the Valve train before I do anything to it.

 

When I did my initial Oil Change I put a quart in with the oil change but I didn't put it in the Gas tank. Thanks for the Info and I will go get some in a Gallon. Do you know how big the Stock Tanks are in these trucks? 13 Gallons?

 

You mentioned it was running on fumes and filled up, how many gallons did it take?

 

If you put MMO in the oil pan you don't have to do the Seafoam.

 

70/72 F100/750 without evap emissions and without diesels is 19.5.
71/72 F100 with evap emissions is 18.5.

 

I took the Valve cover off and I found nothing out of order under there. I did check the torque on the Valve train bolts and one of them was pretty loose and I torqued them all down to 45 Foot Lbs. Its still missing and Im at a loss. I guess if I had a bad valve it would be impossible to see without taking the head off. unless I wanted to get one of those Crazy Camera's that you can snake down a hole.