I hate to say it, but sounds like you dropped something in the cylinder. If you did and run it, it will destroy the engine. But then again, it could be something else. See if you can find a friend or mechanic with a scope to look down in each cylinder and see if there is anything there.
Well the big question is how long had it been siting since you had it running. If it had been a long time, you could have a bearing in the pump going out. IF you just ran it and it was fine and the only thing you have changed was the plugs, Then it sounds like you dropped something in the cylinder.
The only way to tell is to scope the cylinder and see if there is anything in there or any scaring on the walls.
The easiest way to tell if it is a pump bearing is to start it on the trailer in the water. The water will quieted down a pump bearing a lot. But if that is not the problem, you may be causing problems running it if something is in the cylinder.
Matt