My sister is an aircraft technician, so I know all too well the level of understanding someone in that field has. However a good engineer knows when they are out of their depth, especially when you don't have the one thing crucial to setting up a fuel burning appliance, the flue gas analyser. Based on the above I'm pretty certain you are recirculating POC (products of combustion). I appreciate you are struggling to find a decent engineer but playing with parts and without the proper equipment there could be all sorts of gremlins going on inside. I suggest going on the OFTEC website and searching for technicians in your area, oil is very common in your area, you should have no problem finding someone. With regards to your boiler, they don't make them like they used to, parts are still readily available for your burner, however significant improvements in fuel efficiency would happen when you decide to get your boiler replaced.