i have the mikuni 40phh (got em from a friend for cheap) and was worried about my l28 running out of breath in the upper rev range. does the size of the venturi move the power band upwards and if so where may i buy replacement 34mm venturi's instead of my puny 32's

While the 34mm venturis will help, it will still run out of breath vs. using 44PHH Mikunis. My buddy did the same thing you want to try and ultimately went to a 44PHH and the high end immediately got better than what he was able to do with his 40PHH. Try Rebello Racing http://www.rebelloracing.com for Mikuni parts. Although for the street the 40PHH is sufficient.

Or this could be corrected by sending your 32's to Dave at Rebello Racing and having them re-bored to 36's...............just another angle.

Carbs need a venturi effect to work properly. As you increase the choke size you decrease the venturi effect and the carbs are less accurate/responsive at smaller throttle openings. If its a race car then 40's with 36mm chokes will work fine. But on a street car they tend to stumble and run poorly when most of the time the carb is running at part throttle. 32mm chokes are about the limit for a nice running 40 on the street.