OK i also received troubleshooting from Lycoming: The Slick 4372 would be an impulse coupled magneto with a 15 degree lag angle, the Slick 4370 would be plain non impulse coupled 0 degree lag magneto. In this configuration the left should be used for starting and both after the engine is running. If you are having trouble starting with left only this may be an indication of incorrectly set timing possibly due to timing drift, a faulty impulse coupling, fouled plugs, or other ignition related issue. I would perform a mag drop check to see if the left side is indicating excessive drops, verify that the timing is set correctly to 20 degrees BTDC, visually inspect the sparkplugs and test them or rotate them so they are firing off the other magneto to see if this makes a change, perform a high tension lead check, verify that the impulse coupling is snapping at the correct time by turning the prop over by hand and checking at how many degrees indicated the impulse coupling snaps at, should be within 5 degrees of 5 BTDC any more may indicate a faulty impulse coupling or excessive magneto drift.
looks like i have some more investigating to perform...
thanks everyone.