Make sure that you don't have plugged jets in your carburetors. If carbs look fine then it could be timing off. If I remeber correctly you can turn the stator plates on the 81 Jags. It could also be a partially sheared key in the flywheel.
The best way to check your timing is with a Strobotic (Xeon) Timing light. A standard timing light can not keep up with the speed of the 2 cycle ignition.
I would recomend tring to find a tech manual for this so you can be accurate with your timing specifications