Enter a Quadrajet four-barrel in my wife's '68 Chevy C-10. The truck gets driven about 1000 miles per year, but in winter it sits a lot. So when we took the truck out the other day on a mission to pick up some 93-octane non-ethanol fuel, the vehicle "stumbled" at high speed, surging as if the engine were being starved for fuel.
"Uh-oh," thought I. We've had the Chevy since 2003, and that particular curse had never afflicted it. In fact, it never has had any problems.
Safely home with the gas, I began to do some research. It looked as though two things might be at fault: the carb needed tuning or the gas tank had some water in the bottom. That can happen even in a nearly full tank that sits in our changeable weather. Fool that I am, I ignored Occam's Razor, the principle that the simplest explanation is probably the place to begin, and starting reading about tuning carburetors. Time to earn some gearhead points, since I'm writing a bit for Hemmings Motor News.
Spoiler alert: I got it right the first time. I must credit Super Chevy's Web site, where there are easy-to-follow instructions for the three main types of carbs. I purchased a vacuum gauge and as I do when cooking a new dish, I followed the instructions to the letter. Soon I had a truck that idled with maximum vacuum pressure at 850 RPM, the guidelines for four-barrel carbs. Then I took it on the road.
It stumbled again. I made a round-trip drive of 20 miles, and on the way home, the truck ran perfectly with better power than it had before. I began (finally) to employ Occam's Razor.
As a brother-in-law once explained, with old motors before sensors and computers, two things affect running or not: getting spark and getting fuel. Spark and fuel sound rather primordial to me. It's amazing that a vehicle as new as our 68 truck, or for that matter, my 74 Buick Apollo have more in common with my 1950 tractor than my Mini Cooper. But that's how it worked for decades and decades of internal combustion.
I'd assured myself, then, that the truck was getting ample fuel; the carb was well tuned and clean. The gas was new. The electrical system was delivering spark; the motor never missed.
Thus is was back to the bottom of the gas tank, a place I could never go with human eyes. What to do, so the motor gets the RIGHT fuel? For those using old equipment, water is always a threat, but there are products like HEET that absorb water and safely burn in the cylinders. That's my next step, and since all three of our old vehicles have high-output GM 350 motors with four-barrel carbs, HEET and a bit of tuning will be part of my old-school rituals from now on.