U/K: The carb is a Walbro WT 74A E3. I have converted one of the engines and it runs pretty strong---20 X 8 MA wood semitar @ 6950
BME :Funny but I find they dont want more than about 25-26 degrees timing. I am going to try a bigger carb and see if it gains more......I put the carb from a g-23 on it and gained some power. Its now turning a 16x8 APC at 9300.
av8tor::In doing some research, most of the 25cc and under engines seem to use the 7.94 mm carb. That would be a 15% increase from what I have, and would probably work well. The exception is the G-23 carb at 11 mm that a lot of people use for more power
zagnut: all my little 21.2 echos came with 8.5mm zama carbs and i haven't tried anything else yet.
volfly:Each venturi size corresponds to an ideal range of flow rate, where it develops the optimum volumetric efficiency. Too small and the engine run out of breath on top; too big and the venturi doesn't develop good velocity to draw and meter fuel well at the low end.
When the weedwackers run out or break the string, they can over-rev with no load on them, so they put an undersize carb on to restrict the rpm. so don't hesitate to play with bigger carbs