I have tuned up the truck and have found it is running much too rough at a idle. Does great when given gas. New plugs, wires, cap, and rotor from NAPA. I have a 1975 f-100 with a 390 4 barrell, but the motor is a 1972 390. In the Haynes manuel (I cant find a chiltons that goes back to 75, and the haynes only goes to 76), states that the 73 (did not go to the 72 motor which I have) 390 motor had a different spark plug gag (34), and everthing from there up was gapped at 44. The plugs were a motorcraft supposed to be a 42 plug, but I have autolight 45 plugs in now. I was told by napa that they were basically the same plug and that it did not matter. Is the fact that I have the plugs set a gap of 44 with instead of 34 gap make this much of a difference? Also is the fact that the plugs are number 45 instead of 42 making the truck run this rough?

So does anyone know what should the plug gap be for a 72 390 4 barrell be? And which spark plug should I have for this motor? The ones I pulled out of it when I changed them were number 42, NOT 45. Unfortunately I did not check the gaps on the ones I pulled out, as the truck ran fine at that point.

Where could I get a Chiltons manual for a 1972-75 f-100?

 

just a quick thought,did you check to see if you accidentally knocked a vacuum line off?

 

Are you running points or electronic ignition?

 

Its electronic ignition.

As to the vacuum lines, I would have no idea how that could have happened, but I did pull the carb off recently, but made dang sure every line was hooked back up. I checked repeatedly. What lines could have I knocked off by changing the plugs cap and wires?

 

A .044" or .045" gap is what you want.

 

Thank you for that. Does it matter which plugs I have between the 42 or 45, or is that just the same plug that burns hotter?

 

I've got a tee on my manifold that lines drop off of by looking at it wrong,just a thought.Had to put hose clamp on them. Did you do a vacuum check?

 

Go to the parts house and buy a vacuum gauge. It will be the best 20 bucks you ever spend for a diagnostic tool, plus it adds another tool to the collection. New tools are always good, right? Read the instructions and get on google and look up how to use your new vacuum gauge. You can diagnose just about anything if you know how to read a vacuum gauge.

I will add to what has already been said, I think you have a vacuum leak.

 

sound like a vacuum leak to me also.

 

Get a can of oreilly brake cleaner. The flamable kind. Gently spray it around the base of the carb and intake and around all the vacume lines. You have a vacume leak. 42 or 45 plug gaps will not make a difference. Since you have had the carb off I would check the base gasket first .when you find the leak the engine will raise rpms and smooth out. Also make shoru you don't have a vacume line crossed

 

Found the problems. 1, there was a cracked spark plug that wasnt firing, and then a, you guessed it, a small vacuum leak that was easily fixed. Truck is running like a brand new engine. Thank you all for all the help. By the time I am done with the truck, I am going to be a pretty good mechanic.

 

No problem glad to be some help