The best place to start is with a full standard tuneup. You know, check the plugs, points, timing & have a listen to the tappets. If they are a bit quiet, then run the feeler gauge through them too.

Spray some of the good stuff through the carb & if it has a bad flat spot, then swap the plug leads on cylinders 2 & 3 then start it up. A few nasty backfires should dislodge any rubbish in the main jets, but do it with the air cleaner in place for best results & greatest safety.

If the carb is the problem & the backfires clear it, then drop the two brass plugs from the bottom of the fuel bowl & with any luck the dislodged particle will be washed out when the bowl drains as these plugs are at the base of a well at the bottom of the bowl.

Make sure that you refit the plug leads correctly & that all the drained fuel has dried.

Oops, just noticed, it's a non standard carb.