Always check out ebay before buying new parts for older sleds. If you know the size/brand carb, you can buy the same carb even if its from another sled. Just make sure its jetted the same as your current carb.
I'm guessing you already tried the idle setting on the carb itself and cleaned all the jets.
This may sound like a lot to check just because of an idle problem but, hard starting can be the start of later problems.
Did you do a compression test on the motor? I don't know the stock specs but anything around or above 120 is good. If one cylinder is a lot lower than the other it could cause your motor to run poor until it warms up. (also makes it start hard). It would also feel like there is a heavy bog then the power kicks in. If you don't have a compression gauge, check your plugs and see if one plug is a lot darker than the other or if one is just wet and looks like it never fired. If your using a single carb, both plugs should be the same colors if the pistons/rings are in good shape. A nice compression guage is only $49 at Sears and it will save you time on every motor you work on.
Primer test
If both cylinders have good compression and spark, get yourself a squirt bottle of some sort and, with the throttle wide open, put a few good sprays of premix gas into the intake of the carb. See if it starts right up. If the motor starts up but then stalls, your carb could just have a problem with its fuel pump or you might just need to add a primer.
This may seem like a lot to check but, its checking all the basic things needed for the motor to run. I have seen people clean their carbs 10 times only to find out their motor was running 120psi on cly and 60psi on the other.
If you got spark, correct timing, and compression, it will start on the first or second pull when primed. This may not fix your carb problem, but it will tell you that the motor will respond to a fixed or new carb.
