Webasto Thermo 90s over fuelling at start

[SV Trumpeter]

Really strange one here with my Webasto Thermo 90S. Refuses to fire unless the fan is pulled back to allow additional airflow. This indicates too much fuel... Fires up within 40s with the fan to heater joint cracked open otherwise no ignition..... It will even fire late in the start cycle (2m10s) once the fan joint is cracked open. After ignition it goes really well, cycling on and off with the HW & central heating, but will not restart if the heater gets cold, or restart after a shutdown while the unit is hot. NB the fan is bolted back down tight as soon as the unit fires. The helpful "one flash = fail start" code is shown on the controller.

• The burner is clean all holes clear,
• The heat exchanger is clean,
• There is no carbon on the evaporator gauze,
• Fuel is flowing nicely,
• Glow plug works well - tested off unit,
• Flame sensor works fine - tested off unit,
• Voltages are fine.
• Big fan O ring is in good shape.

Took the air inlet hose completely off but the unit still requires the fan joint to be cracked open to get ignition. In my head this feels like an over rich fuel mixture in the combustor but can't see how to affect that with the gauze. After a couple of fail starts, the fan trick works and then the unit smokes until the rest of fuel is burnt off. Thereafter the exhaust is nice and clean.

Tried tweaking the rotation of the glow plug and adjusting the bracket to get it properly close home.

Not sure how the air regulating screw works, but tried that in the recommended coarse setting 1 turn counter clockwise as well as 1/2 turn, and 2 turns but does not impact lack of firing when the unit is buttoned up. Perhaps it's more sensitive?

Any other ideas? Currently in Maine and opening up the lazarette and taking the heater apart every time we want heating is a bit of a pain.