I just posted a message where I stated that the engine on my 1979 F-150 Lariat missfired.

To clarify, it actually is more of a stumbling at about 1700 rpm. The speed is irrelevant when this happens. Also, a tuneup was done - new spark plug wires, distributor cap etc.

Mechanics haven't been able to find the problem. Since it has 93,000 miles, I wonder if the fuel pumps are worn - since I believe there is one for each of the two tanks. However, since this happens regardless of which tank I am running on, it would really be a coincedence if both are giving out at the same time.

Thanks for any input.

 

Does this happen only at 1700 rpm or at all higher rpms?

Try changing your ignition coil if that wasn't changed. It could be on the verge of burning out. It's cheap and probably due to be changed anyway if you haven't done it yet. When they get old, they don't put out enough voltage to produce enough spark at higher rpms.

If it's a fuel pump problem, it's highly unlikely that it's the pumps in the tanks. They are very dependable since they're constantly cooled and lubricated by the gasoline. Of course if you run out of gas often, then they'll poop out much sooner, but to have both go at the same time is quite improbable.

It could be the main engine fuel pump giving out. I think your engine still has the "old fashioned" engine driven mechanical fuel pump. If the diaphram is going bad, then you'd most likely have gasoline leaking into the oil pan which will raise your oil level, so check to see if your oil level is rising.

Try changing the fuel filters. I don't remember where the main fuel filter is on the older trucks. There might also be a filter at the fuel transfer valve that could be clogged. There should also be a filter where the fuel line goes into the carburetor. It's probably a bronze filter and if it's clogged, you can easily remove it and clean it out. This one only comes into play when the main filter media fails.

 

Thanks Dennis - I think it may be the primary fuel pump, since it hasn't been burning any oil - haven't had to add any for the last 10,000 miles or so. I will check it out. Thanks again.