GE Mk IV Combustion/light-off problem

Smokey light-off, incomplete combustion. Trips on firing timer due to one flame scanner out of four showing. Checked atomizing air booster working & delivering air; checked nozzles clear; checked fuel present, and FSR at 14.7% (usual for machine at light-off); Machine ran fine days ago; need ideas what to look for next.

We are perplexed!!

Is this a dual fuel unit, or distillate only?

Have you checked the fuel pressures during firing with the selector valve at the Liquid Fuel Flow Divider? Many problems can be found with the selector valve: failed Liquid Fuel check valves, fuel nozzle problems, plugged check valves or fuel nozzles. The pressures should all be roughly equal; any pressure that is higher or lower than the majority means there's some issue with that combustor's fuel system, check valve, fuel nozzle, etc.

If the machine is dual fuel unit, have you checked the tell-tale leakoff to see if any of the Atomizing Air Purge check valves are leaking?

Has some kind of maintenance outage been completed recently on the unit? If so, what was done?

Does the fuel need to be heated before it's burned if it's below a certain temperature? If so, is the fuel heater working properly? What is the fuel temperature in the storage tank?

What type of turbine is this?
What type of alarms are recieved during startup?
What type of liquid fuel is used?
Have you checked the check valves after the fuel divider?
Is it very cold outside and turbine compartment heat is not functioning?
What temperature is the fuel?
Is there a buildup of material on the flame scanners?
What is your atomizing air pressure versus your fuel pressure at the fuel divider?
What is atomizing air pressure in regard to compressor pressure?
What do the exhaust temperatures look like, what is the spread?
At what rpm does trip occur?
What is the lube oil pressure at 63QT?
When was the last time the crossover tubes were inspected?