1976 mercury 850 thunderbolt running on top 2 cylinders

[Bushgopher]

Hi, I'm fairly new to the forum and to outboard engines. I got this boat for free and it would start and run fine with the muffs on it but as soon as I got it in the water it had no power at all and would just die when engaging the prop. My problem is it only runs on the top 2 cylinders. Cleaned the carbs twice to make sure they are good. There's a strong blue spark at the plugs. 140 psi on all cylinders. Only thing i can think of is the reed valves for the lower cylinders because I can turn the idle screw all the way in on the lower carb and it makes no difference. Turn in the screw on the upper carb and it quits. I can stuff a rag in the lower carb and it doesn't affect it at all. Also when I block the lower carb it pulls fuel from the bowl. Any advice would be awesome. Going to make sure that all of the cylinders are firing in the correct order at the right times.

And one more question. Does anyone know where to find a service manual for these engines? I've googled a lot and can't find anything.

Thanks

 

[Chris1956]

Probably not the reed valves. Check the spark plugs for water. If they have rust, droplets or are real clean, water is likely present.

A bad lower crankshaft seal allows water into the crankcase, putting out the fire in the lower cylinders.

Of course, with the age of the motor, the exhaust baffle and or inner exhaust cover could be leaking.

 

[racerone]

Each cylinder has its own reed valves.----I suspect there is a blockage in the lower carburetor.----Sure the plug wires are on the correct cylinders ?