Yes! The motor says its 12.5 hp but that's b.s it's the exact same thing as an 11hp Briggs and Stratton, cept they have a different style carb
The torque sequence for a single cylinder Briggs and Stratton engine horizontal shaft head is increments of 2's for hp and 1's for head bolts. That makes 11 head bolts for an 18 hp.
There are many parts at are used on multiple models of engines, so its like a 50/50 chance. The 1/2 horsepower difference may use a completely different one though... Post back the model and code numbers of each of the engines, and I will look up if they are compatible or not.
5/8 inch
On most of those engines it is 1in" But it could also be 7/8"
I am afraid not Depending on the size of the shaft on the thing you are putting on it is possible to get a bushing made to increase the size of the shaft. Go to your local machine shop and ask if they can make one.)
Clean the carburetor with special attention to the high speed jet and fuel passage.
You loosen the nut own the governor arm turn the arm clockwise and then turn the governor shaft clock wise. Tighten nut That should do it.
On a horizontal crankshaft engine it is done by loosening the pinch bolt and rotating the shaft in a clockwise direction with the throttle in the high speed position and retightening
If it back fires or it gets jammed up it may have shear the key on the shaft under the fly wheel that would put it out of time.
Most likely 3/4" diameter, though it's possible it's 5/8". Although, I have never even once heard of a 3.5 hp mower with a horizontal shaft.
Unless the type # is off the wall strange, it has a 1" diameter shaft, 1/4" keyway, the end is tapped to thread size 7/16 - 20, and length from mount to tip is 3 and 5/32".
There will be a mark on both the crank gear and cam gear, these must point directly at each other (the marked teeth will mesh). On plastic cam gears, there are several marks; care must be taken to distinguish the correct timing mark from stray plastic casting marks.