8hp Yamaha 4-stroke smoke and rough idle

[SalmonBonker]

Greetings. Long time lurker, first time poster.

Looking for clues before diving into this one. I have a 2004 Yamaha 8hp 4-stroke tiller drive outboard. Model F8MSHC.

This motor has been my 'go to' motor for trolling on the big boat and also the main motor on my little 12 foot jon. Runs smooth, starts easy, etc. Very reliable until this year.

From the first time out this spring, it started with a big belch of gassy smoke, then smoked for a few minutes until warmed up. However, it never seems to idle down properly. At a slow trolling pace, it will rev up on it's own, then calm down to normal speed and fluctuate like this until i kick it in neutral, rev it up a little, then go back to trolling.

And this scenario has been repeating on every trip out this year, so I need to quit waiting for it to fix itself.

It kicks and sputters like a fuel rich two-stroke at most speeds under half-throttle.

Ideas on where to start?

Thanks,

Mike

 

[SalmonBonker]

Re: 8hp Yamaha 4-stroke smoke and rough idle

Alright, so the Yamaha parts dealers seem to grasp onto "It's always the fuel pump or thermostat when it runs like that. Change them"

$100 later, it still runs like crap.

Maybe I smell gas in the oil. However, I can tell you it is extremely rich running. It starts fine, but belches light blue smoke at first, and then keeps giving me the blue haze and gassy smelling exhaust when running. Sputters consistently whether struggling at low RPM or WOT. Won't idle, but I can pull the choke or wiggle the throttle to keep it going.

Carb clean is coming up next unless someone wants to chime in.

Do the Yamaha service centers have computer diagnostic equipment? Just noticed this motor has a brain. (Or mind of it's own.)

Mike

 

[SalmonBonker]

Re: 8hp Yamaha 4-stroke smoke and rough idle

The Lord helps those who help themselves.

Cleaned the carburetor...TWICE... Second time I took the advise of a friend and really really scrubbed the tiny holes with a soft wire and cleaner.

However, still a little funky. Might just need more scrubbing. Symptoms are the same as described above, but idles and less smoke on start-up. As if the cleaning was 80% better and I need to get back in there with another 20%.

Idle is ohhh-kaaay....could be better. Listen for the revving. it revs all on its own, then runs down to almost stall and catches itself and keeps on puttering.

Yamaha 8HP Idle Demo - YouTube

 

[Toddboat]

Re: 8hp Yamaha 4-stroke smoke and rough idle

Check plugs. Change them and blow out plug boots with compressed air. Condition of old plugs may tell if mixture was too rich in a given cylinder.