Re: 1985 mercury 150 blackmax problems!
A lot of times this is caused by low compression. It will start and run fine on the hose with no back pressure but once it's in the water it has a hard time holding fuel in the cylinders and creating adequate crankcase pressure. Typically this will happen when the engine has sat for a while after a run at moderate or wide open throttle. What's happening is that the fuel in the crankcase has evaporated leaving behind only the oil and because of the low pressure in the crankcase, you're not pulling in as much fuel as it requires to get the mix back to a point where it's combustible. Opening the throttle slightly and tilting the motor up sometimes helps to get it started.
I would recommend doing a compression test on the motor to see if all the cylinders are within 15% of the cylinder with the highest compression. If that cylinder is more than 15% off from what it was new then you should consider rebuilding the motor or replacing it. In any case, a compression test will get you started in the right direction for continued trouble shooting whether it be carbs, synch, ignition, etc.
