How To Unclog An Outboard Motor

Q: Six years ago I purchased a new 25 hp 4-stroke carbureted Mercury outboard motor. The carb is now suffering the effects of ethanol—a clogged fuel port stalls the engine at mid-throttle. I understand there are additives that will prevent or minimize this in the future, but is there anything on the market that will clean the carb out without having to dismantle it?



A: The only one I know of that claims to use enzymes to dissolve the organics left behind by ethanol is Starbrite’s Startron.



I’ve tried some in a generator that was left to bake for two years without fuel stabilizer. Just adding Startron wouldn’t get it started, but disassembling the carb showed why—the main jet was totally plugged, and had to be reamed out.



Because your outboard is still running, the additive might make is way in and dissolve the gunk.



On the other hand, I’m reluctant to tell you to do this and then have you stranded miles off shore when the carb plugs up completely. Got a second outboard motor on board?