Maybe the previous owners had a buildup of crap in the
gas tank and you're using gas that is helping to disolve
it. I was once warned about using gas with alcohol in it
because the alcohol would disolve things gas alone would
not. I don't know how much truth there was in it.

I had a motorcycle that fouled the carbs or plugs because
there was some serious corrosion in the gas tank.

Maybe a tank inspection is in order.

This one was above the crankshaft nose VIN, right?

Leon

Techron - the stuff in the bottle, is recommended by the mechanics I
deal with.  Its supposed to clean up the injectors and help clean up
the crud in the intake manifold and the valves (remember, you are
receirculating exhaust gas and sucking crankcase fumes throught he
intake manifold and after a few thousan miles, and it gets nasty in
there.)

Did you inspect the old fuel filter?  You may have sediment, water, or
something else, in the tank.  If you run less than a quater tank you
may be picking up wha or water at the bottom of the tank, or you may
also have had a bad load or gas.

Draw a sample of gas from the bottom of the tank and inspect it, or
have the tank scoped.

Having gone through a water and dirt in the gas tank (silt in
carburator bowls) experience, this is what I try to do:

Put in a bottle of Heet in the gas tank every six months or so  to
scavenge any water.

Put a bottle of Techron once a year (usually before emissions test)

Stick to a single quality gas station whenvern possible, and get a
receipt, it helps track mileage and gives you proof in case of
trouble.  

Like good wine, you want gas that has had a chance to rest and you
don't want old stuff.   Water and solids  are at the bottom of the
tank at the gas station.  Avoid filling up from a near empty tank.
and avoid filing up when the station is receiving a delivery, pumping
the gas is stirrs up the muck in the tank

How about Lanny's suggestion to me about cleaning the grounds?

Leon