[Baffler]

Something else for you to consider...as you look at the front of the engine, on the inlet manifold, check the shut off throttle is reopening, ready for the next start, after you switch off the engine. If it is sticking in the closed position the engine can't/won't breath. As it's made from plastic it deforms over time so a new replacement is the best option. The reason it eventually starts is because the shaking releases it.