<VV> suspected carb problem

[Rburger]

Gentlemen,

I have a '62 sedan, 110-hp, PG drivetrain. Engine has about 30K on a rebuild, and has run very well until the last few weeks. The symptom I have first manifested itself near the end of an 80-mile drive, when the engine began losing power and by the time I got it home, was running on three cylinders. What I discovered when I began troubleshooting was that all the plugs on the driver side were wet with gas and dead. When they were replaced with some used spares, the car ran great again, for a few days. Then, the same thing happened again. I replaced all the plugs with new ones. I started paying more careful attention when I drove the car to work each day, and I noticed that after about 15 minutes at freeway speeds, i.e. 65 mph, I could feel the engine begin to surge and labor slightly. Sometimes the symptom would just clear up, and sometimes it would get progressively worse, and the car would only do about 45-50 mph until I got off the freeway and slowed down. The last time I drove the car, it got to the point that when I pulled onto the off ramp, the engine died. It started again easily, but ran rough and there was a very rich exhaust smell. I let the car sit for a few days and when I started it again, it ran fine, with plenty of power, when I pulled it into the shop. 

I've concluded that the problem must be with the fuel metering in the carburetor on the driver side, which I removed today, expecting to find something wrong with it internally. Nothing apparent. Floats float, needle seats well and shuts off fuel, no apparent contact or binding between the float and the bowl. Since the problem seems like it could be temperature related, I looked carefully for cracks that might become problematic when the engine reaches a certain temp and the carb body expands. Again, nothing apparent. One thing I did notice was that the driver-side thermostat has failed, in the open position, as it should.

The carburetors are a matched set, but, according to the previous owner, are '69 smog carbs with the enrichment screw bottomed out. Anyone have a similar experience? I'd rather not replace the carbs until I'm fairly certain that's the problem. I just can't think of anything else that might cause the symptom I'm having.

Thanks,

Richard Burger