I have 1980 sportfire that I am getting ready for this season and I have discovered that I have a dead cylinder. here is what I have done . I have rebuilt the carb not for this problem but because I have only owned this sled for 2 years and I have not done it yet. We did put 250 miles on this sled in Feb ( ride with the champs) and it performed great with no problems. I replaced the spark plugs(champions) fresh fuel and fuel filter. started it up and it idles at 2400 /revs good so I checked the plugs to see how they were burning MAG side golden brown PTO side white/slightly wet so I reinstalled plugs started it back up unplugged PTO side and the no change (dead hole) unplugged MAG side and the engine stalled. swapped plugs with the same result. checked compression 165psi on both cylinders. cranks seals are good I even went as far as removing the clutch to look at the PTO side crank seal and it looks ok. I tried a different coil off another one of my sleds-same result. tried another NEW spark plug -no change. Also the old plugs from last year I took out both looked the same (Golden brown and normal). Also the temperature of the PTO side plug cold(room temp) and the MAG side plug is HOT to the touch. the same goes for the exhaust manifold. I would think if you have good compression/ fuel and spark all would be good I Tried a different CDI box problem is still there. I even tried a set of NGK spark plugs-same results ? Thanks for help.
OK so after talking to JD Rob we come to the conclusion that it is a leaking seal issue or crankcase leaking so I have taken the engine out and I'm going to reseal it
I will let you know the outcome
I've had a dead side arise in the past due to a leaking diaphragm in the fuel pump. It was flooding the side the pulse is from. I don't think this is your problem because IIRC, the pulse line is on the mag side.
UPDATE, I did spray carb cleaner at both crank seals-and the idle did not change. So after a complete reseal new gaskets and new seals . I now have both cylinders back up and running . After talking to JDROB before the teardown he came up the conclusion the maybe it was sucking air through the crank halves. so during the teardown I inspected the case halves and that's what it appeared to be. the crank seals looked ok. it idles great.
so I reinstalled plugs started it back up unplugged PTO side and the no change (dead hole) unplugged MAG side and the engine stalled. swapped plugs with the same result. checked compression 165psi on both cylinders. cranks seals are good I even went as far as removing the clutch to look at the PTO side crank seal and it looks ok. I tried a different coil off another one of my sleds-same result. tried another NEW spark plug -no change. Also the old plugs from last year I took out both looked the same (Golden brown and normal). Also the temperature of the PTO side plug cold(room temp) and the MAG side plug is HOT to the touch. the same goes for the exhaust manifold. I would think if you have good compression/ fuel and spark all would be good 
I Tried a different CDI box problem is still there. I even tried a set of NGK spark plugs-same results ? Thanks for help. 
