Follow Cab's advice.
Lean surge at cruise means main jetting is a little small. This could be because the engine has a weaker signal.
My example, a carb that worked very well on a squarebore 4-hole intake became quite lean when mounted on a spreadbore intake with an open adapter. Open adapter cuts down the signal or pulse strength, pulls less fuel through the jets, results in lean condition.

After you get carb right start on the ignition. In my experience a low compression engine feels much livelier with a quick curve. But don't try to fix a carb problem with ignition.

R.