I’m having trouble with my Johnson.

Plus the boat engine ain’t workin’ right! [cue rimshot]

Bwa-ha-ha-ha! Oh man…I kill me sometimes. Anyhoo, here’s the deal: My 2005 Johnson 90 (4-stroke, EFI, Suzuki-manufactured) outboard starts fine, runs fine, and then stalls when I slow down to an idle after running several minutes.

So I’ve changed the fuel/water separator filter element, as well as the low-pressure fuel filter in the engine. See my earlier item on exploring nooks and crannies with a digital camera for some pictures.

Didn’t fix it.

Then I was reading in my service manual about the CTP (”closed throttle position”) switch, and it said “If CTP switch has failed, engine will tend to stall when decelerating”. Bingo!

Yesterday I took my electrical tester to the boat to check the function of the CTP switch, and it seems to work fine: ON when the throttle is closed and OFF when it’s open, and 12 volts to its cable lead. Since I only experience the stalling problem when the engine is hot, I decided to run the boat. With the cover off the engine, I let it run at 2500 rpms at the dock. Still no stalling, and the CTP switch was still functioning properly.

So I decided to take it out and run it at higher rpms for five or ten minutes, duplicating the conditions that have been consistently producing stalls. I left the cover off to facilitate access to the CTP switch and its lead. Went out, ran around outside the marina. No stalling! So the problem just fixed itself, then? Weird.

OK, this is where it gets good:

I put the cover back on the engine, which had been off this whole time, and ran around a bit more, throttled back to idle, and at idle the engine ran rougher and rougher until it stalled!

THE COVER! To verify, I tried it twice more, and sure enough, when hot, it won’t idle with the cover on…and if you pull the cover off before it stalls, it will recover and idle just fine.

MY THEORY: exhaust that is supposed to exit the engine completely is, instead, ending up inside the engine enclosure, thus oxygen-starving the engine. When running faster, enough air is stirred around, perhaps due to the motion of the boat through the air, that the engine runs OK.

So my problem isn’t solved, but at least I’ve got something to work with. In case it isn’t clear, any advice on this will be appreciated!