I did an experiment today. Inspired by Bert. I don't have enough time to tear into anything today. I did this. I let the mower warm to operating temperature and start pulsing. As all the shields are in place I can't get my finger to the governor lever. So. I took a long thin screwdriver and put enough pressure on the governor lever to stop the pulsing. I then released the pressure. It took a couple minutes for the pulsing to start again. I did it again, but this time the pulsing didn't resume. I had to throttle it back to low idle and back up to wide open to get it pulsing again.
Here's my theory. I think the governor lever has slipped on the shaft and is not turning the shaft far enough clockwise. I'm probably wrong, but I thought I would throw that out there.
Ed
Excellent.
Not quite what the doctor ordered but as it ran smoothly when you were holding the shaft that confirms a governor problem.
They can be tricky little buggers to adjust but yes, start with resetting it and remember to do the clamp nut up tight.
Then if it still surges, get the Dremel tool out and modify an old Allan wrench.
Never met a real mechanic who did not have a 1/2 dozen incomplete sets of them but do take the effort to find one that will go in and have enough clearance to twist the anchor point.
I have a couple with the slot at different orientations to the short end cause no matter which one you grab, the handle will foul on something a natt's nacker before the exact right spot.