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
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