Speaking of fuel, is it fresh? You should also be aware that a lower nitro content fuel runs cooler than a higher nitro fuel. We learned that with a real live demo with .049 engines as kids. They would not start or run well on the regular 10%-15% fuels yet were real stompers and easy to start with Missile Mist, which was 25% nitro.
Just curious on this point. If you rotate the engine so that the bottom cylinders are not at the bottom but the top, with no other changes, what happens? Do they still fail to fire?