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Have you tried putting the '74 carb back on? Maybe something else got knocked around while you were replacing it. I'd put the old one back on and see if the problem goes away. That would at least verify the problem is in the carb and not something else.

Assuming it is the fault of the new carb, see the picture in the link. IIRC, it has a piston (#23) that lifts (via a spring #25) two tapered needles (#24) out of the jets (#49) to richen the mix At lighth load, high vacuum pulls it back down to lean it out (it's been a long time since I was into one of those...). When you load the engine, vacuum decreases, the spring pushes the piston up, and the tapered needles allow more fuel to pass.

You might pop off the cover (#11) and see if it's free to move up and down in the bore. With the engine shut off, the whole assembly should be up, but I think you can still push it down against the spring if it's free to move. If the spring is wrong or missing, or if it was stuck, it would be at the bottom of the bore and couldn't rise to allow more fuel through the jets when the engine vacuum signal decreases (as in under load). There is also some linkage to the throttle for that piston (items 20, 21, 22), but I sure can't remember how that was set up.

I'm doing this all from memory and I'm sure someone will correct me if I recall things improperly. But it sounds to me like your motor is going lean when you try to put a load on it.

How's your ignition? I know you have electronic ignition but I still hear my father and grandfather preaching to me (about breaker point systems), "Son, 90% of all your fuel problems are ignition-related."
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