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There are several 'indicators' used for a well designed and maintained boat fuel system:

1. Calendar --- scrub out the insides of the tank about every two years regardless of fuel quality.

2. Vacuum gages on all fuel filters. Here's how to set up for YOUR system:
a. Clean the tank, put in NEW fuel and NEW filters
b. Run the engine at near/below wide open throttle - WOT, read and record what you see on the gages ---- this is the 'benchmark' pressure/vacuum for a new/clean system, you'll probably see 'nil' vacuum on the gages.
c. Have an assistant standby on the tank valve, engine running below WOT; at your signal the assistant slowly closes the tank valve while you monitor the 10µM filter's vacuum.
d. RECORD that vacuum value where the engine starts to 'stumble' due to fuel restriction - This is your fuel system MAXIMUM vacuum value read at 'any' filter.
e. Multiply that max. value by 0.80 --- this is your new 'its time to change the damn filters value'. When a filter has consumed 80%+ of its 'dirt capacity', the time to full shutdown is exponentially quick ...... meaning with respect to gallons or time remaining, you probably only have 5-10% reserve capacity remaining in the filter before clogging.
d. option - usually the 10µM filter is the one to clog first, even if preceded by a 30µM which is installed to extend the in-service life of the 10µM. Get a fuel oil compatible 12vdc operated vacuum/pressure switch with adjustable 'set point', wired to an alarm. Adjust the 'set point' on the press/vac. switch so that the switch closes and turns on your alarm at the 80% value of where your engine begins to 'stumble' at near WOT.

tank --> 30µM --> 10µM --> 15-17µM small engine 'guard' filter ---> engine.

Note: if youre plugging filters and you cant get inside and clean the tank ... ADD a 30µM filter to extend the service life of the 10µM. If that's not enough in-service time increase, then add another 30µM in parallel to the first 30µM and run them BOTH at the same time. Nutherwords, if your system is always plugging, increase the SURFACE AREA of your filters and your prefilters.
If youre plugging even occasionally, ultimately you will have to get inside and clean the tank.
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