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Sometimes the most mysterious problems have the simplest explanations. Following running the tank low on gas on Track Day, our 1800S developed intermittent surging. It would run well for days, then unexpectedly reject large throttle openings and/or high rpm, just as it had at the track. The length of time between occurrences was unpredictable.
Because I'd also had a distributor problem at the track and because I knew the tank was full of gas, I gravitated towards ignition as the cause. Removing the distributor revealed nothing wrong, and the car ran well on a new points and condenser set. Perhaps that was it but no; the problem soon returned. A little work with a meter showed the plug wires to be in marginal shape; good running resumed with replacement of the wires, cap and rotor. I was sure I'd fixed it, but ten days later, it was up to its old tricks once again.
I was actually grateful when the car finally acted up in a way I could recognize: climbing a hill on an errand-running trip into town, it suddenly started firing on only two cylinders. I pulled over and smacked the float bowls on the carbs with a suitable blunt object several times (the time-tested cure for a variety of SU ills). We continued smoothly on our way.
Coming down the backside of the hill, though, the motor quit completely. I coasted onto a side street and opened the hood yet again, now with fuel problems on the brain. The odds of both fuel valves sticking simultaneously is vanishingly small, so I knew I was onto something when I saw no fuel at all in either bowl.
The top came off the fuel pump and I saw the cause: the pump's filter screen was absolutely packed with fine debris of some sort. Aha! A few minutes of work with a toothbrush (one of those "you never know when this might come in handy" toolbox items) and some paper towel cleaned everything up nicely.
Was this the result of long-dormant sediment in the tank that had gotten stirred up at the track? Had I simply gotten some dirty gas during a recent fill-up? Or was I victim of the notorious 1800S rusty fuel tank syndrome? (Failure to keep open the drain in the filler housing causes the tank to flood with water most pre-'70 1800s have had this happen at some point.)
I checked the screen frequently during the next several weeks. A very small amount of debris continued to accumulate and was easily removed. The car ran perfectly I'd just have to check the pump every month or six weeks to monitor the situation.
Everything was fine until our first long road trip into the mountains when, while using a fair amount of throttle to maintain (slightly illegal) speed on a freeway grade, the problem returned exactly as before. At our next gas stop, I inspected the filter screen and found no debris at all now what?
The next 300 miles to our destination and back were not much fun. I could pull the top off the pump, find nothing plugging it, and thereby return the car to good running for one mile or forty, no telling. This made no sense at all, but it got us home (if not in a good mood).
Give me enough time and the obvious eventually dawns on me. The paper filter between the pump and the carbs was also plugged up, of course. When I'd inspect the screen, loss of pressure in the pump body would cause the paper filter to backwash a bit, temporarily easing the plugging. It didn't look all that dirty, but when I removed it from the car, the stuff that ran out of it was more like mud than gas.
As for the source of the debris, the jury's still out. I'm still checking the screen from time to time and sometimes finding traces of accumulation, sometimes not. There's a fresh paper filter in the toolbox at all times. I imagine we're in for a thorough cleaning of the gas tank in the spring but for now, I'm finally confident about having understood the problem and having a reliable "band aid" solution for it.
It doesn't take an expert to understand that a car runs better with consistent fuel delivery than without. Nevertheless, this simple problem caused me to pull off to the side of the road dozens of times over a period of months. So, when was the last time your checked or replaced your car's fuel filters?