There is always the three steps to troubleshooting

1) spark
2)compression
3)Fuel delivery

the slide needle should not need adjustment in order to start the bike, it only controls from aprox. 1/4 to 3/4 throttle position, the fuel or air screw controls aprox. 0 to 1/4 throttle and the main jet monitors 3/4 to W.F.O..you should probably run a compression test because you said that it had been low on power and you could have tight valves causing low compression, which will cause low power and hard start you need atleast 60 to 70 psi. to make combustion happen. have you verified spark it should be a hot blue spark, pull a plug and ground it to the head away from the hole and turn it over make sure you dont have a bunch of gas in the cylinder or you can use an inline spark tester, really cheap at a local auto parts store. Oh and btw the floats should be set in the carb so that the float bowl is aprox. 3/4 to 2/3 full of gas when it closes the needle valve,you can check that with a piece of clear hose attached to the nipple on the bottom of the carb, open the drain screw and hold it up to the side of the carb. then once you get it running you can fine tune all the adjustments if required.