Assuming your ignition system is in good shape, your air filter in good condition and your engine compression is good, your likely looking at carb issues. If you never need to use the choke even on cooler days to start your bike, it is most likely running rich, you can also check the color of your plugs. remove your diaphragms and check the needles and seats as well as for proper needle settings. Enrichment valves stuck open is a common problem for this, You may also want to check your choke cable adjustment, or the float level is too high. Before pulling your carb apart to OH it and if the choke cable adjustment appears to be OK, I would check those enrichment valves they can be removed and freed up without pulling your carbs apart. Check and set your float levels, 0n the carbs (easier done with the carbs off the bike and sitting on a flat level surface). There are different ways of doing this much easier than indicated in the manual and you will find various favorite techniques in the tech section that work very well.