There's not one mechanic in 10,000 that can make those carbs work correctly nowdays.
One of the most overlooked problems was a worn secondary air valve cam that controls the metering needles when the secondary opens. If the cam is worn or destroyed (many were plastic, and the first backfire would ruin them) they won't pull the needles up out of the metering jets far enough, and you get a severe stumble, backfire and loss of power.
Your best bet will be a race shop that is still using carburetors rather than fuel injection. Have them rebuild the carb. In many cases that will cure the lack of power. The other suggestion earlier about the sinking float is also a good idea. It's old enough it should be replaced.
Good luck.
