There are Walbro carbs that DO have accelerator pumps, 2 types...One is a plunger that operates from a cam on the throttle shaft, and the other is just a few holes drilled through the shaft and carb body that uncover and let more fuel in...Electronic advance is full at anything over 4000 (syncro spark, don't know about others) so mechanical advance is slower over 4000 rpm...
Syncro spark CAN be configured for full advance occurs at 6000 instead of 4000...There is no detectable difference either way....