Welcome! Need support, you got it. Or share you ideas and experiences.
I am trying to troubleshoot my dad's 25hp Mariner. It idle's great, goes at a slow pace great, and when you "hit it" and go fast, it goes great for about 10 seconds, then dogs down to an idle.
Playing with the pull choke helps, but does not cure it.
It is an electric start with steering and remote controls (if that helps).
You didn't mention the size of the boat, but I'm assuming that this motor IS big enough for it and this wasn't always a problem for you.
Sounds like a typical fuel starvation issue/or a too lean setting on the carb/or timing.
First place I would look is at your gas tank - is it vented, and if so is the vent open/clear? Next look at the tank fuel line pickup (in the gas tank) - is it clear?
Next, take a look at your fuel line coming out of your gas tank - is it cracked, leaking gas, or letting air in. As well, before running the engine, when pumping your primer bulb - does it get remain firm? If not you may have a bad check valve in the primer hose or a leak somewhere along that line.
Now, before getting into the engine do yourself a favour and perform a compression test on all of the cylinders. Make sure that there isn't more than a 10% descrepancy/difference between any of them, and you'll be looking at about 95 - 110 psi on this motor. If you find low compression, or a difference in compression as stated above, you'll need to check the cylinders out before proceeding.
Once you've ruled out compression and external equipment, take a look at the carbs. When priming the fuel line does gas run out of the carb throat? If so, you most likely have a gummed up carb and need a rebuild - easy to do yourself. But the carb rebuild is usually the last step. Lets do another test before then. While running the motor in idle, pull a spark plug boot off of one spark plugs (with insulated plyers) and see if the idle rpm/(sound) drops or if there is a stall. If not, there is no spark in that cylinder and the ignition system must be tested - you will need a tech book on your motor for that since there are many types of ignition systems out there - Clymer or Seloc are both good! Do this test on all spark plugs obviously replacing the boot before proceeding to remove the nxt one.
If ignition and compression and all outside factors prove to be in working order, clean out the carbs and check the settings on the carbs and timing on the engine with one of the books mentioned above. Hope this novel helps!
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