Hi everyone,
I inherited a 72 F-100 several years ago. It has a 390 with automatic transmission. It has about 75,000 miles on it. It is all stock except for a Pertronics electronic ignition. It has been running good. I was pulling a drag out back a few weeks ago, when it started missing and smoking a little. It still fires right up. I checked the plugs. They were only about 6 months old and they all looked good except cylinder #5. It was wet with oil. I checked the spark plug wires and all were good. I replaced the #5 spark plug wire and plug anyway. No change in symptoms.
Any suggestions as to problem or what to check next.

Thanks for any help.

 

Ya may have burned a valve or worse. Run a compression check and see how far of #5 is compared to the rest.

 

If plug is wet and oil fouled, compression should be right up with the others. sounds like a bad or broken oil or wiper ring. My same problem but mind is #3 unless you've lost the valve seal completely. But wouldn't think so with only 75,000 on it if rebuild..If so then you should find little pieces of it in the oil return holes at the valve-train ends where it drains back to the pan.. If you were to remove the intake manifold and look into the head intake port runners and see carbon build up on the intake stem then your seals and maybe guides to our in need of replacing.. any way that my 2cents
orich

 

I had to redo the heads, they added new seats, 390 were designed for leaded fuel so if you haven't done any work on the heads, unleaded fuel will eventually burn your valves at high speeds, until you add new seats. Mine back fired loud, It would scare the crap out of people at stop lights. I got my kicks at watching their reactions.

 

Thanks for the advice. I'll try the compression test and go from there.

 

Was that 75,000 miles on the engine or 75,000 miles on a rebuild? If 75,000 on the engine, the OEM seals maybe hard and braking up.

 

Although "pulling a drag out back" is not something I'm familiar with I'll chance my closest logical guess and say the timing chain may have slipped.

All the best.

 

OK pulled mind down. My #3cyl and # 3 was starting was sucking oil for intake gasket. An old common problem when the heads get shaved to many time.And not shaving intake manifold..Fixed mine with some virgin heads had that I came across then had hard seats ss valves put in them..my 2cents
orich