GT380, Looks like a rebuild is needed.

[hahahaurugly]

I've been hunting down my running issues and have determined the left cylinder firing 50% of the time is not an ignition, carb or oiling problem. I have an even 125psi compression on 3 kicks and 145psi by 5 kicks. It leads me to believe there has to be a major tear in the seal or something in the bottom end. Any advise on what to look for when tearing down the motor. Are the crank seals replaceable with out major specialty engine shop tools? The Clymer book glosses over the entire engines lower end stating "send it out for repair". Any advise on using aftermarket seals, new from dealer, or new old stock?

[Alan H]

Take the left carb off and kick the engine over with your hand over the inlet stub.
Does it 'suck'?
In fact if you take all the carbs off and try each one in turn, do they suck the same amount?
It's really a specialist job to do crank seals, and if one HAS gone, the others will be tired at best.
Not sure who you can use in your neck of the woods.

[hahahaurugly]

Could I put a vacuume gauge in a rubber plug to measure the suction at each cylinder as a diagnostic step?

[Alan H]

If there's somewhere to put them yes. Not sure on the 380.

[hahahaurugly]

Used an automotive evap smoke tester to find I had a minor base gasket leak on the left cylinder and sris check valves passing both ways or plugged intermittently. Got a set of top end gaskets and a line on a local shop who knows old bikes to rebuild the crank if it doesn't run right afterwards.