Here's the jetting I came up with for my '75 MR175, after almost a year of trial and error:
120 main (stock is either 118 or 122)
200 air (stock)
48 pilot (stock is 52)
271005 (stock) needle in #3 (middle) position
3.0 throttle valve (stock)
air screw 3.4 turns out (my air screw is hollow - not all are)
This setup has worked well with 93 octane pump gas at temps from 40 to 75 degrees F, at basically sea level, with stock carb & pipe, ported cylinder, and high-compression head, using Bel-Ray H1R at 36:1. I have a full set of jets, throttle valves, and a 271002 MT250 needle/jet set (real lean) on hand, plus I was experimenting with Sunoco MO2X and GT Plus race gas in the summer months, so I tried pretty much EVERY combination.
For what it's worth, the ported cylinder seemed to make the bike run leaner, while the compression increase had the opposite effect.I hate running the air screw that far out, but if I turn it in the bike won't idle, and if I then turn in the idle speed screw to compensate, the bike 'runs on' when I shut down the throttle at high RPM.
In the hot/humid months (80 degrees plus), I was running a 118 main and a 3.5 throttle valve, with the needle at #2.
One thing to keep in mind: older bikes were jetted for lots of oil in the gas (16:1, 20:1, etc.), and tend to run a bit rich on modern 32:1 to 50:1 premix ratios.
Ray