First a belated welcome to CF burleigh66, clearly you have been reading for a long time and so it is nice to see your first post.
The are only a few reasons a 2GM20
won't start easily.
Usually the air is not a problem but a blocked exhaust
can cause poor air induction.
You haven't given us much background on the engine
like age, hours, how well it runs after starting so the following is therefore generic in nature and you will have to work
out if it applies to your situation.
First assumption - you are using the starter motor
- not hand cranking!
As you are using the decompression method of starting, I am going to assume you have a weak battery
or a problem with the battery wiring
to the starter motor. Normally with everything working properly, you don't need to decompress a 2GM20 to start. In fact, it should start within a couple of revolutions i.e. less than 1 second and 2 or 3 at the very most.
Slow cranking when starting results in low compression
Will the engine spin over rapidly with the starter motor without being decompressed?
If not, fix the starting circuit. It could be a weak battery, poor battery cables
, poor terminations, problems with the wiring
harness (sort of common on older Yanmars) or less likely a dodgy starter motor.
If it does spin over well but won't start, then air in the fuel
seems likely. Replace the fuel filters and bleed the fuel system. It's not hard once you know how.
Another possibility for low compression is zero (or negative) valve lash. Easy to check the tappet clearance. It should be 0.8mm or 12 thou (IIRC).
Finally check the exhaust
manifold / mixing point for carbon build up.
There could be other reasons but most are unlikely, especially if the engine is running OK after starting. That is, it delivers power OK and doesn't blow clouds of smoke!