The piggy-back filter referenced above on Merc V6 and optimax are half canister types that are engine mounted. Same type, smaller capacity, that has been described over, and over, and..... Any additional filter can "possibly" restrict the flow. You need one that is at a feed level greater than your system requirement. Moeller, Land-N-Sea, Perko, Mercury (yep, the same people referenced above sell a "recomended" in line water-seperating fuel filter. LOL!) and BPS plain label all are filters that exceed the minimum, just to name a few. (The BPS is a Merc compatible match. The parts, i.e: filter can, will interchange.)
I've ran, blown, R&D'd and tested numerous Merc powerplants and almost everyone rigged by the support crew with factory techs included a 85GPH min. canister type water-seperating, in-line fuel filter. Call the Hi-Perf division in Fon Du Lac and you can get the word from the techs that build, blow and rebuild these things day in and day out. The rate that the canister allows is higher than your lil' motor can draw it through the picup screen in the tank. Adding one of those cheap, in-line cheese cloth type filters will restrict the flow, slightly.
Just a word of advice; if you have a technical question, ask a service tech at BPS or any other qualified Merc dealer. Not to "dis" a salesman, but if I want to know something about my prostate, I'm not gonna ask the guy selling Prep H to the Dr.'s office. :lol:
P.S. You should be able to prime your canister filter without filling with gas and trying to screw that in place. If you are trying to prime and the canister will not fill or the ball won't prime to hard, you have a leak or bypass somewhere in the system. Check all fittings and seals for leaks and/or air draw. It doesn't take much of a breach/bypass to suck air in and not push fuel. Good luck1