yeah i had the same prob.when starting up only the front fired to start with but after a while the top would start up.my prob was worn cylinder so low comp.once it had warmed up and a bit of oil had built up the cylinder made some compression and it fired.solution strip down and replace cylinder and piston.try a comp test to start with clean and check your carbs check the spark plug cap and run the bike with a spare plug out of the cylinder and earthed to check the spark.