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Good Question. 1971 Datsun 1200 was fitted with a small #135 secondary jet. That proved to be finicky, so for 1972-1973, Datsun fitted #140. That will take care of surging while cruising at higher speeds.

Or go to a #102 Main jet from the stock #97 which is what I did to cure the flat spot when cruising at 65 mph or so in my 1972 coupe.

Yes, 1972 had its own problem.

1971 - lean secondary
1972 - lean primary

Although I must say I never experienced a flat spot with all the working stock equipment. If you modify the engine (for example change exhaust, gap the plugs too narrow, or remove smog equipment) then the jetting can go over the line. 1972-1973 was factory jetted for maximum fuel economy but if anything altered that then it could run too lean. Going to a bigger size jet has a small penalty but gives the engine more leeway in the tuning department.