Given Rob's findings so far and no mention of a hiss when removing fuel cap I'm favouring ignition as the problem. Condenser or coil maybe, the latter being excessively hot, causing missfire then no spark would be a giveaway.
Richard
2018 Roadster 3.7 1966 Land Rover S2a 88 2024 Royal Enfield Guerrilla 450 1945 Guzzi Airone
