I have a 79 F150 with a 300 and it has really got me confused right now. The motor will not run decent at all, shakes badly at idle and has no power. Compression is between 160 and 170 on all cyls, previous owner had head milled, and it will only hold 15 Hg of vacuum no matter what I do and that is about 1200rpm idle, and higher or lower and it drops to about 10 Hg, the confusing thing to me is the gauge is rock steady no tick to the needle at all. Now for the really confusing part to me is that #1 and #3 are totally dead, pull the plug wire and there is no change in the way the engine runs. I tried a different set of plug wires that I know to be good and no change, I though mabey a cross firing issue but no. Checked the resistance through the plugs and wires, just checking to see if they were bad and all read the same and they were, pulled the dura spark unit of my other truck part number were an exact match and still no change. Now I am looking at the distributor, it seems to have a lot of end play at he rotor, is there a way to check the pick up in the dist? The truck has no EGR and no cat, was having carb issues earlier but I have that straightened out, tired the carb on another truck and it worked great, also I pulled the valve cover the gasket was leaking any way and all valves seem to open and close normally. Thanks for any help, need to find out how to check that dist.
-Johnboy

 

Vacuum leak and worn dizzy good starting place.

 

Do you have a timing light you can hook up to wires 1 and 3 while the engine runs to verify they are dead?

 

No vacuum leaks, and I tested using a plug against the block and nothing. It is also getting plenty of fresh gas, tried include every think I had checked in the original post but I missed a few.
-Johnboy

 

Is the cam stock? Because with no vacuum leaks you need a fairly radical cam to pull 15 inches.

I didn't really catch in your first post though, so you switched out dizzys in the truck for a known good one and still go those two dead cylinders?

 

Did not switch the dist, just the dura spark box up on the fender the old one leaked all that nasty goo out like they do, but no change. I am assuming the cam is stock but I guess you never know. I was thinking the dead cyls might cause the low vacuum.
-Johnboy

 

I guess the dead cylinders may drop your vacuum like that. Did you pull the cap of your dizzy? Maybe you just need a new cap and rotor?

 

Cap and rotor look to be brand new, this thing has got me kinda stumped. Right now looks like the distributor itself but I have never seen one just drop two cylinders. Seems like the either work or they don't never seen this weird stuff before.
-Johnboy

 

Yeah I haven't either. Is there continuity between the inside terminals and the outside posts on the dizzy cap?

 

All terminals read 0 ohms from inside to out side, but hey finally found some thing I hadn't checked.
-Johnboy

 

I'm sure someone tomorrow will come around that's a good brainstormer and will have the answer

 

On your two dead cylinders have you tried using new spark plugs? Tried running the engine at night and looking for errant sparks/grounding of plug wires? Are the plug seats in the head clean?

You can have plugs and wires that test good "cold" but die under load.

 

I grabbed some off a 400 I have but when I went to start it the battery was dead, I had started it and shut if off probably 20 in an hour while working on it. I checked the resistance in the wires and they were all about 4 ohms. I am thinking cross firing mabey, but am currently waiting on a new distributor. The old one just has way too much end play, and it is worn and starting to round off the oil pump drive so they are both getting replaced.
-Johnboy

 

Runs like a top now, just went for broke new plugs, wires, cap rotor, distributor, oil pump drive, and threw an accel coil I had layin around on it. Well a top with a 198,000 miles on it any way. Thanks for the help.
-Johnboy