Well start checking backwards from the starter. Could be bad solenoid, bad ground from solenoid to frame, bad battery connections, bad battery cables, bad ignition switch, bad safety switches between ignition switch and solenoid, or did you say those were bypassed? Walt Conner

I have a mower which used to behave rather similarly ... When the mower was on an incline the engine would start running very poorly ... and it was rather hard to start. It turns out I had a good amount of water in the tank and eventually the float bowl ... when going up an incline the washer would slosh into the perfectly wrong position and the float bowl would fill with water - the motor would sputter and slowly die out on me ... had to frequently pull the bowl off the little Briggs and drain it and life was good all over again... Then i picked up a riding mower which would deliver the same problems... the carb had water in it ... BUT the bottom bowl of my larger riding mowers has a spring loaded brass drain button on it ... simply pressing a the brass button drains the water and probably "some" small debris - very convenient ... What comes out is generally a little bit of water followed by fresh gas ... For some reason - I seem to get a lot of condensation on my fuel tanks - so I generally "tickle" the little brass drain buttons once or twice a month... Im a scavenger - so I get my stuff used and I typically try to get my new found junk running first - and then start to invest in the parts and stuff to keep it running well... So I STRONGLY suggest that you pick up a fuel filter (I like clear ones so I can see the dirt / water in them) Be careful if you have a vacuum assist fuel pump - some of those filters are specialized. You might want to drain your gas tank and leave the cap off for a few days so as to evaporate all gas and water from the tank. Put it all back together and at least you know that your eliminated any potential water problems. I once had so much water in a gas tank I ended up draining the liquid into a clear plastic water bottle and i could clearly see that i had a ton of water in my gas. You might want to consider this - and then as your ready to start the engine back up you know exactly what your running through your motor... While your at it you might consider fresh fuel line and a shut off valve. And finally - pull the float bowl and make sure its really clean. If possible try and pull the bowl off over a cut up water bottle and you can see if you captured any water while you were pulling the bowl off. A shut off valve can be very handy - avoiding your carb flooding the crank case with gas and risking engine seizures. good luck to you.

The seal remover looks like it will do the trick. Alternatively you can punch (or drill) a small hole in the metal face of the seal and thread a sheet metal screw into that hole. Grip the screw with locking pliers and pry out on the pliers with a heel bar or crow bar and out pops the seal. Installing seals where a long shaft is present can be a bit more troublesome than just selecting a suitable socket from the drawer. I have often used a length of PVC pipe (be sure it's schedule 40, not the thin stuff) either 1-1/4" or 1-1/2" diameter and long enough to reach the seal bore. If you can find a metal washer big enough to cover the metal OD of the seal AND fit over the crankshaft, use that washer against the seal, the PVC is sturdy enough to drive the seal in. Buy a PVC end cap to cover the end you plan to strike OR you can just "hammer" the open PVC with a short piece of 2 X 4 about 18" long. It doesn't require a sledge hammer to seat a seal. Be sure to lube the rubber lip of seal with a couple drops of motor oil before installing seal.

If the deck belt tension stays constant, maybe the key in the drive pulley has spun?