When I briefly ran my '74 CR engine with a modified piston (@ 6mm shorter rear skirt length) it had these same symptoms - my air filter would actually get wet with gas, it was pretty scary - so I went back to stock. The shorter skirt drastically reduced low end, while adding almost nothing on top.
If your intake port floor has been cut away and it's running the stock piston-skirt length, you might try an MT125 piston (not an MT125R piston!), as I believe most of the Honda 125 pistons use slightly different skirt specs and the MT125 should be the longest. BTW, the MT125R road-racer engine used radical intake timing & wouldn't run at all below 500 rpm, it had to be push-started (no kickstarter).
You can also try jetting the carb very lean on the pilot circuit (#48 - #52 pilot, unworn needle jet), which should at least give you some initial throttle response to get the revs up; more compression & ignition advance wouldn't hurt either. The best fix would be another '75 CR cylinder with standard intake timing.
Ray
_________________ '74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric
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