A little clarification on adjusting idle air mixture on eddy carb

It doesn't matter much if you use rpm or vac for adjust for strongest pull.
Vacuum is more sensitve and will better inform you of efficiency if you compared it with another tune at the same rpm.

When you're setting the idle mix in neutral, the only load on the engine is the internal friction. This why it is often better to turn the idle mixture screws a bit richer (1/16 to 1/4 of a turn) after establishing the best idle in neutral.

I'd experiment with the part throttle and cruising mixtures too (if there is a safe place to test).
The Carter design gnerally has two idle restrictions. So off idle/ very low throttle is not user friendly adjustable.
But the higher speed cruising and part throttle mixtures (50 -70 mph) can be leaned by changing the rods. Do this in small steps. When you go too lean the engine will surge or trailer hitch. You'll have to slow down then to complete your drive back to the shop. Then change back to a little richer. Use the chart in the instruction book to make sure you haven't altered the wide open throttle AFR. If you have, then change the jets to get that back where it was using the chart for guidance. Obviously don't play games by going WOT while testing for lean in cruise. Test for WOT at the track and to be safe experiment by going richer first.