A tip is not to start and stop cranking when it happens. Just keep cranking- after 20secs the M30B35 will fire.

The engine is flooding.

The check valve is the issue, and is a kit which is best sourced from bmaparts.com.

Fix the valve, else you will have to fix the starter and/or deal with flat batteries- it isn't good for any battery to crank an M30 for a long time. Nick