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1974 F100 starts to chug after 15 minutes

Maintenance/Repairs
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#1

1974 F100 starts to chug after 15 minutes driving although infrequently when driving at a slower pace. Would happen around the 70mph speed and stop chugs when slowed to 65mph.

Only have experienced this after it’s been warmed up, in the past 3 sessions of driving it will do this on freeway when driving 70mph, slow down to 65mph and no issue. Although, it seemed to have chugged once this morning when driving 45mph up a hill when during time engine was partially warm.

Any thoughts would be greatly appreciated

#2

Check for a frozen heat riser valve in the exhaust manifold. If it is not opening completely the intake manifold floor below the carburetor will get too hot especially at high speeds. If you have a 6 cylinder the valve is in the ontake/exhaust manifold connection. On V-8s it is at the rear of one of the manifolds. It is a bi-metal spring operating a flapper valve and the counter weight should be free to swing back and forth about 90*. Also, on the V-8s a restriction in the exhaust will result in the same problem.

#3

If not the heat riser valve I’d also check the fuel filter. If not that I’d try a test: loosen the gas cap and see if it happens (remember to tighten it back up). A clogged vent might do this.

#4

I have a truck of similar make & vintage too, Ford w/302 V8. I have to say I’ve never had that particular problem. Most of my truck’s drivability problems have involved vacuum leaks. If allowed to venture a guess, I’d say it is either something with the EGR function, the fuel system (i.e. pump or filter), or it needs a tune-up, new points, condenser, dizzy cap, ignition rotor, wires, plugs, set ignition timing, and verify both vacuum and centrifugal advance is working.

One thing easy to do, suggest to visually check that the carb choke plate is fully open when the engine is warm.

#5

Make sure your choke is working properly…sounds like it is stuck to me…Stuck ON that is… Easy to check.

Remove your air filter and notice where the choke is positioned AFTER you pump the accelerator prior to initial startup…Is the choke ON? Then drive for a while until hot or chugging…and look at the carb again…is the choke still on? If it is…you found the problem.

Other areas that could cause this…especially if the issue is heat related? The ignition system…Cap Rotor…Wires… Very common to experience symptoms when hot in the ignition department…if its going to act up…it will act up when Hot…

Blackbird

#6

If you have electronic ignition, change the big module above the left front inner fender. Do this especially if it feels hot after the malfunction happens. If you have ignition points, don’t look for a module.