Check your timing if you have sheared a key on the fly wheel it might : 1.just work if the plug is out because it might just have enough friction to turn. 2.Your valve seals are letting oil threw and it is dead heading on compression stroke.
The list goes on.
Hope this helps.
5/8 inch
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.
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
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.)
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.
Clean the carburetor with special attention to the high speed jet and fuel passage.
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
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.
Drive shaft is vertical- like a lawnmower. Cylinder is horizontal.
You failed to list engine size and make. If it has a horizontal shaft 5−7 h.p. Briggs & Stratton non-Vanguard engine the total oil capacity is 20 ounces. If it is a Vanguard 6 h.p. engine the capacity is 24 ounces. Add 20 ounces and then check the oil level. If it has a Tecumseh engine it can be 20 or 21 ounces. Do not overfill.
On a vertical shaft Briggs & Stratton 12.5 h.p. engine it takes 48 ounces. If it has an oil filter and you change it add 4 ounces for the filter. So, and oil/filter change would require 52 ounces.