I have a 1999 John Deere LT 155 mower. I have not used it about 2 months, but after jump starting with fresh gas, the engine skips/clogs down very bad on a slight incline.

Going downhill it's almost too good with the engine reving very fast (more than normal) without a skip.

I'm thinking there is junk in the fuel filter, because I've been so lazy and not using it. But then again, I think a bad fuel filter would cause problems no matter of the direction, no?

There could also be a little water in the tank since the last start.

I think tomorrow I'll start it and let it run on a flat surface. Either the piston will come out of the side or it will clear up.

 

If you suspect there is moisture in the tank, drain to remove. Drop the bowl on the carb and remove any gas/water that may be present. Check the float to make sure that it is not stuck/sticking. Invest in a can of Seafoam Deep Creep spray. If the float is sticking, spray the area where the needle valve lives to remove varnish/water, gently moving the float up and down while spraying.
Remove old fuel filter, check for water, remove if possible. Replace with new is the best. Refuel with fresh gas, spray a decent amount of Deep Creep into the tank with the new fuel. The can states that it is a 'moisture dispersant' and it will help clean any varnish buildup within the intake system and remove carbon from valves etc.
Take out the sparkplug and check it. If it has not been replaced in awhile then it may be something to think about, even if the motor is starting.
If possible, drain/run the fuel from the tank and carb if one anticipates not using for extended period of time or use some kind of fuel stabalizer for storing offseason.
Normal preventative maintenance will keep the hassle threshold low. If the above is beyond your scope in accomplishing, stimulate the small business economy by enlisting the help of a lawn mower shop in the yellow pages.

 

Seems like the float is sticking.