On my HRX217, I've noticed at the "Fast" throttle, it's not at full engine throttle. I can push the lever just a tab bit more and hold it there for full throttle and the engine revs up a tad bit more. But if I let go, it goes back to the original "max" engine rev.
Is this normal in these mowers?
Sort of.
On the fast position there is a detent to hold the throttle in that position.
The governor actually controls the engine speed.
You can loosen the clamp near the carb that holds the throttle cable and ove the cable so the throttle is fully open but not touching the choke then tighten the cable clamp again.
