Spark + fuel but won't start.

First of all, excuse my bad english!

I have a Mercury 200 (20 hp) 2-stroke, 1975 model, Belgium made engine and it won't start.

When I bought the engine it started easily and I run it with water for approx 15 minutes with no problems. Me and my son hooked it on the boat and it started as easy as the first time. After 5 minutes it just died. After that we haven't been able to get it running again.

The carburetor is overhauled, 2 new sparkplugs - no result. We put some fuel straight in to the cylinders and have tryed with start spray but there is no sign of life. The spark on the plugs are very good (we tryed with a plug with 7mm gap and the spark is very powerful). We did a compression test and both cylinders show 8 kg. When you put some fuel straight in to the cylinders it should give a bang but it's not. We have also checked the timing and it's correct.
Is there anyone who can guide me what else I can try.

Please help
Anders
(Sweden)

What do you mean by ---8 KG --- compression ??------Perhaps 8 bar ?-----Did you replace water pump impeller ?-----Mixing gas and oil at 50:1 on this motor ?-----Perhaps check the flywheel key.-----Key does control the timing.-----And I do see that you checked timing.----I say check the flywheel key.