1979 Ford F-100 Engine not running right

DOITAZATEAM

MEMBER
1979 FORD F-100
4.9L
6 CYL
2WD
AUTOMATIC

Hello,
I have a 1979 Ford F100 with a 300 Inline 6 cylinder. It runs pretty bad. Here's what it does: It runs like it is running off 4 cylinders, BUT I have installed NEW spark plugs, plug wires, coil, control module, plug cap, rotor button, new intake/exhaust manifold gasket, new pcv valve, new head gasket, new intake valves on front 2 cylinders that were missing, reseated all intake valves with compound, retorqued all bolts when reinstalled everything, still runs the same?
Heres the thing though, when you pull the plug wires off while running, on front 2 cylinders there is no change in motor, BUT if you leave the wire off and drive it, it has no power at all and will barely move, but if you hook wire back up, it will move on down the road, and if you floor it with all the wires hooked up, it pops out of the carburetor(almost like backfiring but not loud, just rapid), only when you hold it down over half way. There has been no change in the front 2 cylinders, still acts like there not firing til you drive it and it still dont run right. I am stumped, Could it be timing chain jumped, something to do with the distrubuter(Pick up or something)? Motor going bad or?
I did notice while I had the head off that the spark plugs have.040 stamped on top. PLEASE someone help me with this!

2 Replies

CARADIODOC

MECHANIC
33,873 POSTS

My first suspect would be worn lobes on the camshaft. Next, I think I'd throw an inductive-pickup timing light on it and put the trigger on each of those spark plug wires, (one at a time, of course), to see what happens to the spark on those two cylinders. If it disappears at higher speeds or at lower speeds, you might consider a worn bushing in the distributor, or the gap is set wrong for the pickup assembly.