I have the same problem. I hope I can find something like that on my B&S 18 Twin II.Not sure if its the same or similar throttle assembly, but I had the same issue on my GX345. There is a big nut on the side of the throttle assembly that can be tightened to adjust the amount of friction on the throttle arm. It's been a while but I thought I was able to get a box end wrench on it to tighten by just opening the hood.
That's not where we advised making the adjustment.