Intermittent surging and stalling 351C 4V

Cougar Community Classic Cougar Maintenance Restoration and Repair
1

My 351C/FMX has this intermittent problem: it was running fine, then 5 miles into a local trip with engine at normal temp, it started surging a lot at mid-speeds with foot off accelerator; but it would accelerate normally, even hard, with no surging. When this surging happens, idle speed is too low, so it stalls at stops.

So I turned up the idle screw to prevent stalling. Was able to drive 3 miles like this home, even though it still surged. But by the time I got home, surging had stopped and rpm’s at idle were now too high, so I turned idle screw back to where it started. Then on a short drive same day the surging/stalling started again, so went thru same routine, idle screw up and down. Same thing since on neighborhood runs only, but the periods of surging/stalling are getting longer than the normal running.

So far have checked float (proper height and not sticking), fuel filter, gas looks/smells normal, and for vacuum leak in hoses or manifold, with no luck.

Background: this is a one owner (me) with original 351C 4V, that was professionally rebuilt with usual overbore at 92.5k miles 4 years ago. I rebuilt carburetor (also original) at same time, plus all new fuel lines, fuel pump, Pertronix, condenser, 3 core radiator, etc. etc. Not driven on the road in winter (fill tank and add Stabil), but engine started about once per month then. Driven locally in summer up to 75mph with some hard pulls, with no engine issues until now in the 1,200 miles since rebuild.

Any and all ideas appreciated. Thanks.

2

Lean surge? Could be crud in carb or vacuum leak.

3

Bill - thanks! If lean surge, wouldn’t it be constant, not on and off? Seems weird.

Carb looked clean when top off to check float. For vac leaks, blocked off main port at back, checked advance hoses, and sprayed fluid around manifold, but will do again.

4

Nice car, and very cool that you are the original owner! I first thought of carburetor crud too, but doesn’t make sense that it would clear itself that quickly. Does your car have the original dual advance distributor? The retard side of the vacuum advance should be connected to manifold vacuum which determines where the ignition timing sets at idle. If vacuum retard were working intermittently, it could act like you describe. Might be worth checking ignition timing during both conditions to see if it changes.

5

Calicat: thank you, and good suggestion.

I do have the original dual advance distributor (with Pertronix now) and it has not been checked for sticking as you suggest, or for leaking diaphragms. Will check this week.

6

Reporting back to close out this thread: it was an intermittent sticking advance plate in the distributor, as Calicat suggested (thanks!)

I believe the plate pivot shaft had never been lubricated since it left the factory, 51 years ago.

Back to running well.