[Cedarghost]

This has been a learning process for me and will be a lengthy post, so bear with me as I explain what I have done and what is happening. :)
Bought a mower that was smoking really bad. Turns out the sump gasket was leaking onto the exhaust.
The motor is a 25 hp Briggs V Twin Intek model number 445777-0168-e1.
Other than the smoke, it started well and ran well, but bogged down a little bit under load (cutting).
Pulled the motor and replaced the sump gasket with Form-a-Gasket. I was very careful to clean the surfaces with emory cloth, spread the form-a-gasket thin and let it "skin up" good before I put it back together. I did not remove anything from the motor when pulling it, except the governor assembly and the shroud. I left the heads, carb and everything attached to the motor.
I didn't do anything inside the motor besides put the sump drive shaft back in when it fell out. It only goes in one way, so I'm not concerned about it.
After replacing the gasket, I adjusted the valves, or I should say "mis-adjusted the valves" and the motor wouldn't turn past the compression stroke without help. When it did start it ran at low rpms, no matter where the throttle was set.
I got some help and adjusted the valves correctly (.005 at 1/4" past TDC). Then the motor turned over no problem, but still ran at low rpms. So I statically set the governor (loosened the lock nut, held governor assembly in the wide-open position, and turned the governor arm coming out of the motor in the same direction and locked it down).
Now when I started the motor, it has a lot more power, but still not like it should. It's probably at about 60%
So...I checked to see that both plugs were firing (they were), I held a finger over the plug holes and turned the motor over and it had so much compression you couldn't keep your finger down. I haven't done an actual compression test, but that is on my list. I held a finger over the holes to collect gas and light a grill lighter next to the spark plug hole to see if there was gas getting into the combustion chamber. There wasn't unless you turned it over a good while (like 15 seconds).
So now I decide I better check the fuel system (even though I didn't do anything to it). I cleaned the fuel filter, put fresh gas in it, checked the fuel shutoff solenoid on the bottom of the carb, took the carb apart and cleaned the bowl and throttle with carb cleaner. No improvement.
Next I put a teaspoon of gas in each cylinder and put the plugs back in and it still took it about 10 seconds to fire. I thought "well that's too long so I checked the plugs again, and one wasn't firing.
Swapped the other plug to that side and it didn't fire. Coil problem right? Wrong, I swapped the coils to opposite sides and they both fired. Put both plugs back in and THEY both fired. So I started the motor and it still ran at less than full power but I pulled one plug wire while it was running. Ran a little crappier, but it ran. Did the same thing to the other side, same result. So it is running on both cylinders, but not as strong as it should. The governor is absolutely set correctly, as are the valves.
The two things I am going to check tomorrow are: look for a sheared flywheel key, remove the belts (so there is ZERO strain on the motor) and check the reed valve. Other than that, I don't know what I can do besides a leakdown or compression test, but again, it ran fine before I pulled the motor.
Could the flywheel key have sheared as a result of too much compression when I had the valves adjusted wrong?
Why would the reed valve have went bad JUST because I pulled the motor?
Is it possible I got some dirt or something in the carb that somehow fell into one of the combustion chambers?
And last, but not least, is is possible if some of the form-a-gasket somehow fell into the crankcase or valve covers, it could mess or gum something up? I was very careful to make sure there was none on the iside edges when I used the form-a-gasket and when I reassembled, there was very little that squished out of the side of the crankcase and block.

[DG]

First of all everything you did sounds good , except one thing,does the motor sound like its under a load.
Briggs and stratton engines have to have a gasket between the sump and crankcase , this gives proper clearance between the crankshaft and sump , other wise the crankshaft will rub .
Luck