This happens to be on a 318 powered truck, but I believe the problem could relate to a slant as well.

I just finished installing a Vintage Air AC system in my '73 D-100 and it now has a surge that is very noticable under low speed cruise conditions and less noticable at higher speeds. The cycle is roughly 1 to 3 seconds, so it's not the compressor clutch engaging.

I once installed a ThermoQuad on a performance engine and it had a similar surge until I increased the size of the jets, but it was a 360 carb going on a 400 engine, so a lean condition was not unexpected.

Not being a carb expert, I don't understand how adding AC could cause a carb to be running too lean simply by turning it on. When the AC is off, there is no hint of surging.

FWIW, I installed a similar AC on my slant powered Valiant and it had no problems either with the 1-bbl it first had nor with the 2-bbl I installed recently. It runs as smooth as silk with the AC on or off.

Am I wrong to be suspecting the jetting on the Performer carb?

Remember, the compressor adds a load to the engine that wasn't there before. It sounds like your jetting was just barely on the acceptable side of lean before the A/C went in, and now that the extra load is there, it's a little too lean. However, before you change the jetting, you may want to take a very careful look at your ignition timing (initial advance and vacuum advance). A tetch more spark advance may take away the surging without the fuel mileage penalty that will come with larger jetting. Try advancing the base timing a couple degrees. If that makes the surge go away, look at a more targeted fix by installing a longer-throw vacuum advance can. Keep your ears opened for pinging.

Also, does this '73 truck of yours have EGR? If so, is it hooked up?

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

FWIW, I thought I'd let everyone know what resolved this problem.

What I first thought was lean surge turned out to be a combination of a bad distributor and an overcharged A/C compressor. The vacuum advance was adding over 30 degrees at speeds above 2500 rpms. I couldn't tell the total advance until I installed a timing tape and then it was obvious where the problem was.

A reman distributor from Rock Auto resolved fixed it.