8hp Johnson idles well, but stumbles under load, sometimes

ArtVandaley

Recruit
Joined
Sep 1, 2026
Messages
1
Hello all!

I have an '85 8hp Johnson that will start, idle, and run fine at a slow speed, but will cough and sputter (badly) once the throttle is opened. If I continue to operate it in this condition for 5 - 10 minutes, the engine will magically and instantly begin to run perfectly, as if someone had just turned a switch on, it's that abrupt. Eventually, however, the engine will return to it's coughing and sputtering state later on in my trip. This began at the end of last season, but I've been able to deal with it because I can still get around with it, plus I alternate outings between gas allowed lakes and electric only lakes (In other words, I don't need to rely on it all the time) On my last outing, I discovered if I very gradually open the throttle, the engine will accelerate nicely. This was towards the end of my last trip, so I don't know if this is a repeatable workaround or not. Aside from all that, I rebuilt the carb just before this trip (With no noticeable improvement), I also replaced the coils and plugs at the start of this season (OEM/NGK but no improvement there either), and I replaced the fuel pump two or three years ago, why I can't recall. I do recall, however, it was not a factory pump, but a cheap one I got off of Amazon. I was browsing Facebook marketplace today for another outboard, because I'm starting to lose my patience with this one, but I then realized that perhaps I should drop $100 for an oem fuel pump to see if that does the trick, but I wanted some opinions before doing so.

Thanks for reading!

Mark
 

saltchuckmatt

Captain
Joined
Jul 19, 2019
Messages
3,340
Amazon stuff does not work...OEM or Sierra but OEM parts are readily available for that motor. Use non-ethanol fuel and keep it clean. Pull the carb, new OEM parts...deep clean. Idle circuit is suspect here.
 

Mc Tool

Lieutenant Commander
Joined
Aug 7, 2024
Messages
1,859
Amazon stuff does not work...
Dumb thing to say 😁 I dunno just how many millions of dollars worth of stuff they sell but the fact that they continue to sell stuff strongly suggests that it does work . Some of you guys are real bone headed when it comes to alternate parts .😁
 

saltchuckmatt

Captain
Joined
Jul 19, 2019
Messages
3,340
Dumb thing to say 😁 I dunno just how many millions of dollars worth of stuff they sell but the fact that they continue to sell stuff strongly suggests that it does work . Some of you guys are real bone headed when it comes to alternate parts .😁
I use them for lawn trimmers, blowers, mowers and other things but for JohnnyRude outboards, there not even close for me. Sierra is acceptable too.
 

Mc Tool

Lieutenant Commander
Joined
Aug 7, 2024
Messages
1,859
I use them for lawn trimmers, blowers, mowers and other things but for JohnnyRude outboards, there not even close for me. Sierra is acceptable too.
Yeah , I get that your lawn trimmer crapping out isn't the same as swimming home . I have bought chinese carbs for both my quads and a kit for my Tohatsu and they look real good , clean castings ,decent looking rubber bits bla bla bla .
A fuel pump , specially a vacuum diaphragm one is pretty simple tho .
Gotta admit tho 😁 I just been lookin for a new welding helmet and I immediately veto'd the cheapest one just coz Im still suss about the cheap crap 😁😁
 
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