IMO, the best carb size for a stock '74 four-transfer cylinder is 29mm, the same size carb Honda supplied with their optional '74 'GP' 5-transfer cylinder. I'm assuming you're using the OEM piston-port design.
I've run the numbers through my simulation software (port window chordal width / down angle / side flare etc.), and the stock '74 bridged intake port flows the same as a 29.8mm diameter tube. This is nowhere near big enough for a 32mm carb, let alone 34. To maintain a decent WOT vacuum at the venturi you need at least .5mm extra equivalent diameter everywhere downstream of the carb, and 1mm would be better.
The 28hp MT125R road racer only used a 32mm Mikuni, even with the HRC liquid-cooled hop-up kit. It can get away with this because it has a much bigger intake port that stays open longer, but it also has zero low end and cannot be kickstarted. Obviously if you put a 32-34mm on your bike it will still have some bottom end and will kickstart just fine, but it will still only flow 29.8mm worth of air/fuel mixture.
I'm well aware that everyone and their uncle was throwing bigger carbs on stock motors back in the day. Mine was a 36mm - I knew way more than any of those Honda engineers
- which ran like crap despite months of jetting and rejetting, and caused me to finish way back in the pack in the only race I ever led (1st-turn pile-up). I then ported it and removed the intake bridge, giving an equivalent area of about 31.8mm (which I didn't know at the time) - it still wasn't enough. The huge drop in low end & midrange made me think I'd gained a lot on top, and the throttle response sucked unless the motor was screaming.Just my @.02.
Ray