I had my R1 engined car dynoed last summer and its been running nicely. Recently however, it has developed a habit of stumbling a bit if you snap open the throttle. Roll the throttle open over about a second and its fine. Its also good at any throttle opening at any revs. Take less than say half a second to fully open the throttle and it grumbles for a second before normal service is resumed.

Where in the carb should I look first?

It sounds like a slide/s have a diaphagm leak to me.

The slides act as little chokes when the throttle is opened quickly to speed the flow of air across the venturi and richen the mixture momentarily but it sounds like yours are opening too quickly - but not by much.

Id start by checking that all the hoses which interconnect the slide diaphragms are all nice and tight and havent come off and arent leaking. Then if all looks well, take the top off the carbs and check that the diaphragms arent torn or perished or unseated - and when you put the caps back on make sure the rubber beading of the diaphragm seats properly around the edge of the carb top.

I reckon thats a decent starting point. If that does nothing Id be resorting to squirting WD40 around looking for leaks and maybe suspect worn spindles or something a bit more annoying like that.

Geoff

Have you balanced your carbs? Try that first if not....