Erik,
My FY engined Flyer had a spitting back problem which was due to weak mixture at partial throttle openings and very noticeable when retarding the ignition slightly (to climb hills under load etc.).
I’ve managed to improve things by trying carburettor slides with different cutaways. A No. 3 cutaway completely cured the spitting back but made the engine ‘chug’ (or 4-stroke) when cruising. A No. 4 cutaway made the engine very crisp but the spitting back ocurred when retarding the ignition or when throttling off after accelerating. Turned out that a no. 3 1/2 was the best compromise to prevent spitting back and a reasonably smooth engine at cruising speeds. I am now running a 190 main jet after having a minor engine seizure on a 180 jet after a prolonged, flat out hill climb. I do use oil drippers to the main bearings and 50:1 petroil mix in the fuel much to the horror of some Scott riders (it doesn’t smoke !). My timing is set as per Ian Parson’s details above.
Good luck, these things can be sorted,
Paul
