The #1 failure reason for JD snowmobiles is dirty carbs. The plastic JD used in their tanks seems to leech something into the gas that produces a substance similar to Tapioca pudding when a machine sits unused and the gas isn't stabilized. This clogs jets and jams floats, and the result is a burned piston on the first ride of the new season. This tends to destroy both the pistons and cylinders, and the result is a $100 snowmobile that needs an engine rebuild. Every engine failure I've seen is for this reason - JD Owners KEEP YOUR CARBS CLEAN!
When the rev limiter in 80-84 Liquifires goes bad, it does not let the engine engage the clutch. A quick fix is to simply disconnect it. .
The plastic hyfax slides tend to wear rapidly near the front of the skidframe. If they wear through, the cleats will quickly wear through the aluminum slide rails causing expensive damage.
The aluminum cross shafts tend to sieze in the suspension arms. These should be removed and greased every season.
The throttle safety switchs are very fussy about proper adjustment of the throttle cable. Improperly adjusted cables will cause an ignition miss and hard starting as the switch intermittently shorts the ignition.
The ignition coils tend to go bad, replace them with OEM JD coils for maximum life. Symptom: The engine runs for 20 minutes or so before shutting off or running rough. The failure happens when engine heat is built up in the coils.
The metal gas tanks rust when the gas goes stale inside. Oven cleaner, along with a lengh of chain, or some loose nuts and bolts rattled around in the take can help dislodge the debris.