quit changing parts and making guesses and do some real diagnostics... for instance.. when you pulled the plugs.. did they LOOK fouled?
do you have spark? good spark?
if so.. move on to fuel.
oftentimes a can of start fluid is a good diagnostic aid. if you have spark and she will run on start fluid.. you know you have a fuel issue.
post back after more diagnostics and less guessing and we can help you.
ps.. 6v don't turn terribly fast... BUT.. DO make sure all your bat cables and terminals and ground to chassis connection are clean, rust and paint free.. and cables are thick, not wimpy auto 4 ga.. and are not coroded under the insulation.. also sheet metal bat tray connections are usually bad...