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Intermittent Drop in power from full throttle after running 10-15mim

Mindrunner88

New member
Experiencing an intermittent drop in power on my ‘95 Nissan 40hp 2 stroke.
Conditions
- backdrop - ran seafoam treatment the previous time out before this occurred the first time
- engine had a hard time staying started, but came around
- idled smoothly after spending 3 minutes getting started
- ran at WOT for about 10-15 min and then suddenly power dropped to about 40-60% (no rpm gauge on boat). Went from going 22mph to 9mph. Engine wasn’t running rough just not getting rpm. Idle still good.
- went back towards docking area at 9mph, WOT, took about 30 min. Checked out under the hood. Visually all seems fine (no wiring issues visible)
- pumped the bubble 20 times, took it back out and it ran fine, right back up to 22 at WOT. Over the next two hours, no issues, including running at WOT for a good 45 min straight.
- a week later, took it out again, same thing happened , except this time I was about 85% max throttle. Took about 25 minutes this time. Tried the bubble again, no immediate effect.
- went back in a fished near shore for a bit, an hour later, opened it up and was fine... 15 minutes later, happened again at 85% throttle.
- slow trolled for 45 minutes, tried it again and went back up to full RPMs, no reoccurrence on way back in 30 minutes+ at 85%)

Have read a bunch of posts, but they seem a bit different. Appreciate any help!
 
Well-----You should not have to operate the manual fuel pump 20 times to fill carburetors.---On first start does the bulb go hard after about 5 pumps , yes or no ?------Test run with another tank and hose.----Check for water in your fuel too.
 
Well-----You should not have to operate the manual fuel pump 20 times to fill carburetors.---On first start does the bulb go hard after about 5 pumps , yes or no ?------Test run with another tank and hose.----Check for water in your fuel too.

20 pumps is about what it takes each time to get bulb hard. That said, each time out we flush engine by disconnecting fuel line and letting engine run itself out of gas inside. So maybe that’s why it takes so so many pumps?

Tank is brand new. Will try a new line. Thank you!
 
OK, so you likely have an NS40D or D2. That is a 3-cylinder, with 3 carbs. If all carbs are dry, I can't imagine using the primer more than maybe 5 to 10 pumps to fill all 3 carbs all the way from completely empty. You may well have a bad o-ring in the original primer hose, allowing air to get in. That would explain 20 pumps to fill, and might also cause your issues.

FWIW, sea foam is basically mineral spirits, so it is in the category of snake oil. Some swear by it, some swear at it.

If the bulb doesn't firm up after 5 or so pumps, either there is air in there, or it isn't pumping well. Yes, a new primer hose sounds like a good start.
 
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