8hp starts but runs only at high rpms

[Plandeck]

My 8hp 2007 Nissan outboard starts but runs only at high rpms. It dies when throttle is decreased. She ran fine the first day in the water, but after an hour of high rpm boating she would not idle without dying. Any suggestions?

 

[MattFL]

Does the choke help it idle? If yes the start by cleaning the carburetor. If no then start with a compression test.

 

[Plandeck]

Does the choke help it idle? If yes the start by cleaning the carburetor. If no then start with a compression test.

Do you know what size compression tester I would need? 14mm or 18mm? And if it is compression does this mean new rings, rebore or what? Thanks!

 

[MattFL]

I don't know what size spark plug is in that motor, but that's the size adapter you would need for the compression tester. Most of them come with a few adapters to fit more than one size hole. If the compression is found to be too low, there are multiple potential causes and it would be impossible to tell which is your problem until you look further. Test it first, then take it from there.

 

[km1125]

You likely need to clean the carburetor GOOD. Like soak it overnight. There are passages that feed all the low speed circuits and different ones that feed the high speed circuits and it's likely you have some blockage in the low speed circuits. I had a very similar problem and tried the clean the carb several times with spray and a 4 hour soak. No difference. Let it soak overnight and the problem was gone.

 

[pvanv]

You likely need to clean the carburetor GOOD. Like soak it overnight. There are passages that feed all the low speed circuits and different ones that feed the high speed circuits and it's likely you have some blockage in the low speed circuits. I had a very similar problem and tried the clean the carb several times with spray and a 4 hour soak. No difference. Let it soak overnight and the problem was gone.

Yes. Likely the case.

 

[Sea Rider]

Before ordering a pressure tester, seems the motor it's idling rpm low and shuts down, probably the low idle rpm screw is playing tricks, with time and motor vibrations the screw tends to unscrew itself. With motor fully warmed up screw it CW a tad more and check if still dies ? If doesn't that's it, if still does clean the carb as suggested..

Happy Boating​

 

[racerone]

There are very small passage ways in 4 stroke carburetors.----Often it takes a novice 3 or 4 attempts at cleaning to get the desired results.----Always store the motor with carburetor empty.----An empty carburetor does not plug up.

 

[pvanv]

You can also swap to the 9.8 carb if yours won't come clean. Then you add power as well.

 

[Plandeck]

Sorry to confound all the experts...the carburetor soaked overnight in OMC cleaning fluid still won't run unless rpms are 4,000+. As a note soaking the o-ring around the high rpm jet screw made it balloon and it had to be replaced - luckily still had the old 8hp carburetor (believe I bought the 9.8hp with electric choke from Paul a couple years back) and salvaged the o-ring from it. However, no improvement. Also swapped the old 8hp carb in and still the same problem. If the choke is held open the motor will run ok at 3,000 or less rpms. Seems there is an air leak somewhere preventing the jets from providing enough fuel at low rpms. I even swapped in the old fuel pump without effect. Adapter for the compression tester should arrive shortly and I'll run that. Any other thoughts? When I put her in the water she was purring like a kitten, then an hour at high throttle through pretty good wave heights and now she won't run at low rpms.

 

[km1125]

Not sure if this is the same kind of carb as the 9.9 or 15, but if the float bowl below looks familiar then make sure that passage in the bottom is free from obstructions.

 

[MattFL]

I'll put $1 on low compression. A 2-stroke that acts lean, won't run at low RPM and lacks power is a common symptom of a damaged piston and/or rings. It can be a symptom of other things too, but you've already ruled out the carburetor and the fact that it was run wide open for a while just before the problem started makes me suspicious that something may have failed. If it's a reed valve engine then a slightly damaged reed or reed cage can also behave similar, but most outboards I think have metal reeds and damage is not as common as it is with fiber reeds used on more high performance motors, so I don't think that's it. But that said; do check all of the bolts between the carburetor and motor to be sure nothing is loose allowing for an air leak.

 

[Sea Rider]

Tohatsu's metal reeds are near bullet proof and usually never go bad unless groped. If suspicious there's an air leak, with motor ON spray some soapy water with a small spray bottle behind the carb and the beginning of the reed valve assy and check if with bubbles. An air leak usually revs the motor way higher when at idle. Check that the cardboard carb gasket is in a flat opt state and adjusted tight against the inlet manifold.

Happy Boating​

 

[km1125]

I'm not familiar with all the models they offer.... is this a two stroke or a four stroke?

 

[Faztbullet]

with motor ON spray some soapy water with a small spray bottle behind the carb and the beginning of the reed valve assy and check if with bubbles.

LMAF!!!!

 

[pvanv]

Sorry to confound all the experts...the carburetor soaked overnight in OMC cleaning fluid still won't run unless rpms are 4,000+. As a note soaking the o-ring around the high rpm jet screw made it balloon and it had to be replaced - luckily still had the old 8hp carburetor (believe I bought the 9.8hp with electric choke from Paul a couple years back) and salvaged the o-ring from it. However, no improvement. Also swapped the old 8hp carb in and still the same problem. If the choke is held open the motor will run ok at 3,000 or less rpms. Seems there is an air leak somewhere preventing the jets from providing enough fuel at low rpms. I even swapped in the old fuel pump without effect. Adapter for the compression tester should arrive shortly and I'll run that. Any other thoughts? When I put her in the water she was purring like a kitten, then an hour at high throttle through pretty good wave heights and now she won't run at low rpms.

You must disassemble the carb before soaking. After, blow out all passages.
You may have a sticky float valve.

 

[Sea Rider]

LMAF!!!!

I know...Leak Down Test pressurizing the motor with a manual pump to at least 5 PSI for 5 minutes, the issue is that's impossible to perform without the right tools for such specialized test which the mayority doesn't have...

Happy Boating​

 

[Faztbullet]

with motor ON spray some soapy water with a small spray bottle behind the carb and the beginning of the reed valve assy and check if with bubbles.

No..the reason I am LMAO is I have never seen a vacuum bubble!!. Anything behind the carb throttle plate is vacuum...

 

[Plandeck]

You must disassemble the carb before soaking. After, blow out all passages.
You may have a sticky float valve.

Compression measured in at identical 70psi for both cylinders. Seloc manual says they should be 128psi. Oil added to each cylinder before testing increased psi in one cylinder by 5-10psi and saw no change in the other cylinder. However, introducing oil into spark plug hole was difficult since motor is still in the boat. New spark plugs have carbon coating but no oil which I think means rings are ok(?)...so what is it? Where to start?

 

[Faztbullet]

If it will run at high rpm it will run at low also, maybe not idle well but should run. Holding the choke makes it richer so my first look would be to see if a bolt/stud vibrated out of intake manifold causing vacuum leak