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

ArtVandaley

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Sep 1, 2026
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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

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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

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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

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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

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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 😁😁
 

racerone

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Dec 28, 2013
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I doubt it is a fuel issue.----Instant improvement points to a spark issue.---Could be a broken wire.----Could be the powerpack.-----Those factory pumps are very reliable.-----And so easy to install a new diaphragm !!----Run with a timing light to check for spark issue.----One cylinder at a time.
 

ArtVandaley

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Sep 1, 2026
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Thank you for the replies! In part, I'm trying to determine if I should put money into this engine or get something newer. racerone, plug wires and the power pack are two areas I have not addressed. Those components would cost me around $150, not too bad I suppose. I have a video of this engine misbehaving, let me see if I can upload it.
 

racerone

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It is the wires from the stator to the powerpack that should be inspected.-----Also look carefully in the rubber plug in connecters.----A pin / socket out of place can be the issue there.-----All simple to check out in my opinion.
 

ArtVandaley

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Sep 1, 2026
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It is the wires from the stator to the powerpack that should be inspected.-----Also look carefully in the rubber plug in connecters.----A pin / socket out of place can be the issue there.-----All simple to check out in my opinion.
Copy that! I thought you were talking about the spark plug wires :) So, I need to check the connections between the stator and the power pack first, then replace the power pack if the connections look solid, correct?
 

saltchuckmatt

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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 😁😁
Yeah, plus being able to see is a high priority!
 

saltchuckmatt

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Still think something is bouncing around in the low idle circuit....but what do I know. Here's the same motor and I fixed 2 weeks ago.
 

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airshot

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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 .😁
Not worth the gamble when I know the oem stuff always works.
 

ArtVandaley

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Hey saltchuckmatt! I unscrewed the slow speed needle (15) and shot a bunch of carb cleaner through it. I did eyeball the main jet/high speed orifice (24) but I couldn't budge it using standard power 😁 (I didn't want to booger the slots in it) but I did shoot a considerable amount of carb cleaner down through it.
 

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saltchuckmatt

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As far as the idle circuit your guessing it's ok...but, that being said...it looks more electrical. Is the timer plate wobbly under the flywheel when running? Timing light would be useful here.
 

ArtVandaley

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As far as the idle circuit your guessing it's ok...but, that being said...it looks more electrical. Is the timer plate wobbly under the flywheel when running? Timing light would be useful here.
While I didn't pay attention to the timer plate, I did drive around for the entire trip with the cowl off. So, perhaps something wobbling around would've caught my eye? I no longer own a timing light, but I could ask my friends and co-workers if they have one I could borrow. Part of the problem tinkering with this engine is, I live in a townhouse, so I cannot park it on-site. Thus, it is stored almost 40 minutes away. But luckily, it's near one of my favorite lakes.
 

saltchuckmatt

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Yeah, Racerone's advice is solid. Under the flywheel many wires come out...sometimes they rub on stuff and get chaffed. Also, there is a plastic bushing or wear ring per say and if that's worn, cracked or broken then that creates issues. Again, woodruff key. 1/4 20 bolts (3) and an inexpensive steering wheel puller does the trick.

Get rid of those ngk plugs. Champions only.
 

Mc Tool

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.

Get rid of those ngk plugs. Champions only.
Every plug in every motor I own is ngk , granted there are some crappy imitations out there ( google to see how to spot the dif) .
Cant remember the last time I saw a new champion plug . We mostly have NGK ,Bosch , Nippon Denso and autolite here now , used to be all Champions and AC Delco.🙂
 
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