Does this engine surge more or uneven as you say if you are trying to hold and maintain a certain speed. But is not as noticable powering all the way to top speed?
Yes, it runs unevenly when I try to maintain a certain low speed, say at a throttle position about 5 mph. If I need more throttle because of an uphill or just because the trail is straighter then it seems to be running normal again. By running uneven I mean the rpm drop to almost idle rpm and its like it at deceleration and then the rpm pick up and it surges back to the throttle position speed.
Sounds like a governor issue or loose linkage or play in throttle linkage bewteen the governor and carb. Check the holes that the links go into for wear. Jack the rear wheels of the ground and run and locate the throttle shaft to see it it is moving back and forth quickly while you hold a steady slow rpm.
It been a long day and my spelling is off. Sorry
This engine FD620D CS26 has a direct fuel inject pump and I believe does not have a mechanical governor. There is no linkage at all that goes to a governor from the throttle body. I think the speed/rpm is controlled electronically. Actually it has a speed limiting switch that limits it to 20 mph if used. Its top speed is 30 mph and I do not know the rpm ranges at these limits. I can take a photo of the intake and send it to you if you like. It has the throttle body with controls and sensors that limit and air and injector output I believe.