why are you wanting to put an electric on an 850? is the 1st question. if you have a built in tank, and not getting fuel, i would look for air leaks, and restrictions, clogging. tank pickup, vent clogged.
When doing a $20.00 Fuel pump repair, I snapped one of the screws on the check valve retainer. I ordered a new Fuel pump assembly which costs $150.00! I thought maybe a little electric pusher at the tank with 2+ PSI would do the trick. It was just a thought. I already ordered the new Mercury fuel pump for $150.00.
