CONTINUOUSWAVE

Q1: what value of microns in the filter element of a fuel-water separating filter is suitable for use on a MONTAUK 17 and JOHNSON 75-HP engine?

A popular fuel-water separating filter brand is RACOR.

The filter is sized for the maximum gallons per hour. For a 75-HP engine the maximum flow would be 7.5-GPH.

To separate gasoline molecules from water, the filter element should be 10-microns or smaller.

More details found from GROK.

Buy from DEFENDER:

https://defender.com/en_us/racor-spin-o ... 20r-rac-01

The length of the boat and the brand of the outboard engine are not particularly influential on choice of a fuel-water separating filter.

Gasoline stored in small-capacity on-deck fuel tanks (such as the very typical 6-gallon plastic tank) generally won’t be susceptible to developing water in the tank from condensation of moisture in the air above the fuel inside the tank. Unless water is allowed to enter the tank due to poor fuel cap sealing, or fuel is purchased from retail vendors with water already in the fuel, water in gasoline is generally not a problem in small volume tanks.

You may find the following article posted in the reference section informative. I certainly did.

https://continuouswave.com/whaler/refer ... Filter.pdf

[Moderator’s Note: The above article was published 22-years ago in 2004. The price information is very out of date.]

For a small outboard engine I don't see much value in using a fuel-water separating filter with a detachable collection or drain bowl. Every connection point or fitting in fuel system is a potential failure point and should be avoided if possible. Unless you regularly experience fuel contamination, the benefits of a detachable-bowl-style filter may not be worth the additional complexity or expense.

I have used simple spin-on fuel-water separating filters (without a detachable collection bowl) for many years and have never experienced any fuel contamination problems.

The Racor PFF5510 is one example:

https://www.fisheriessupply.com/racor-p ... nt/pff5510

[Moderator’s note: the threads on the canister filter mounting brackets from OMC are usually NOT the same as on Mercury (and other brand) engines, and the suggested filter may not fit an OMC engine.]