Yamaha 20hp 4-stroke outboard problems...

Nov 1, 2019
Hi all! I don't often ask for advice here but now I'm stuck. I'm trying to fix a friend's Yam o/b.

The problem is at full throttle opening. The motor will run for about 10 secs then dies. Idles ok and part throttle is ok. Starts again ok.

To me it sounds like fuel starvation at full power.

I've changed the line from the tank to the o/b. Cleaned the coarse filter in the tank. Changed the in-line filter. Checked the diaphragm in the motor's mechanical fuel pump. Even spliced in an electric fuel pump (rated at 170l/min) and by-passed the mechanical pump but the problem is still there.

Now I'm starting to pull my hair out! A new pump costs 190€ (diaphragm costs 170€ on it's own so no point just replacing that) so I don't want to spend that and still have the problem.

So, what have I missed? Or is there some other clever system in the motor which needs replacing? Any ideas?


Well-known member
Nov 26, 2007
Is the gas tank breathing OK? It might be breathing enough to allow low flow fuel situations...but not top-end. Certainly sounds like a go/no-go situation...and I've seen just this with a tank that was not breathing. You can check this by tying the boat to the dock and running at full throttle with the gas tank cap loose to be certain it has plenty of air to breathe the tank. The biggest pump in the world will do nothing if can't backfill the volume consumed with air.


Well-known member
Lifetime Membership
Premium Member
Mar 11, 2018
Sounds like a carb problem. Remove the carb, soak for a couple days in some thing like seafoam or an injector cleaner. Get a carb kit for it and a carb to manifold gasket. Gas sitting in carbs is super destructive. I think your fuel restriction is in the carb, either the needle seat plugged and not maintaining enough fuel for more than idle or plugged man jet not allowing enough fuel for more than idle. Or, you have an air leak between the carb and engine that starves it for fuel once you throttle up. Good Luck!
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