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Old 09-12-2021, 22:23   #1
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Tohatsu M9.8B power loss

Hi everybody,

I have a problem with my tohatsu M9.8B it is not even 2 years old but, it is loosing power after about 10m a full throtle, with 2 person in the dinghy, it does not do it when I am alone.

I already checked and cleaned the carburator and the jets, adjust the mixture screw, checked and cleanned the sparkplugs I get a good spark on both...tried with less oil, cooling circuit seems to be working, as water is flowing correctly and temperature seems to be ok, I'd say around 60c.

Any idea?

I am in french polynesia and everything costs a fortune

Maururu, thanks.
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Old 09-12-2021, 23:19   #2
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Re: Tohatsu M9.8B power loss

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Originally Posted by olivioloyer View Post
Hi everybody,

I have a problem with my tohatsu M9.8B it is not even 2 years old but, it is loosing power after about 10m a full throtle, with 2 person in the dinghy, it does not do it when I am alone.

I already checked and cleaned the carburator and the jets, adjust the mixture screw, checked and cleanned the sparkplugs I get a good spark on both...tried with less oil, cooling circuit seems to be working, as water is flowing correctly and temperature seems to be ok, I'd say around 60c.

Any idea?

I am in french polynesia and everything costs a fortune

Maururu, thanks.
Had exactly the same problem with my M8B, which is the same motor different carburettor.
It is the plugs. Cleaning them and adjusting the gap did nothing. Replaced them with new and all was good.
No idea why, yet when the same problem reoccurred around a year later changing the plugs for new was the cure again.
Took days of messing around in a remote anchorage to work this out.
Hope it works for you
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Old 09-12-2021, 23:59   #3
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Re: Tohatsu M9.8B power loss

Thanks a lot seaslug, well that is an easy fix, I have changed them about 4months ago, although I am a fulltime liveaboard so yes I use the dink quite a lot and french polynesia have far anchorages
I'll try that tomorrow then and I will let you know.

Thanks again
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Old 10-12-2021, 02:01   #4
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Re: Tohatsu M9.8B power loss

Are you sure you haven't picked up some duff fuel?
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Old 10-12-2021, 02:20   #5
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Re: Tohatsu M9.8B power loss

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Are you sure you haven't picked up some duff fuel?
I am not, but that would be strange as it has been like that for at least three full tanks now.
I guess we'll see tomorrow with the new spark plugs

Thanks for your input
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Old 10-12-2021, 04:57   #6
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Re: Tohatsu M9.8B power loss

After cleaning a spark plug*, I always checked/reset the gap.
9.8B Manual ➥ https://www.tohatsu.com/tech_info/ow...11114-3AG1.pdf
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Old 10-12-2021, 07:43   #7
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Re: Tohatsu M9.8B power loss

If the plugs don't fix it, possibly check your fuel line coming from the remote tank. If your fuel lines are the grey plastic (not the black rubber), then they will have a liner in them that can come off the wall sides and cause restrictions in the line.

We see this more with ethanol gas, but they make things so cheaply these days. It is possible that baking in the sun may cause it also.

In general, changing the tank fuel lines is good preventive maintenance especially if they sit out in the sun for long periods. It doesn't take too much air to ruin the motors performance.
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Old 10-12-2021, 07:47   #8
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Re: Tohatsu M9.8B power loss

Ive had this problem with a different outboard. It turned out to be the vent on the tank wasnt working. Try leaving the fill cap loose and see if it will run longer.
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Old 10-12-2021, 09:30   #9
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Re: Tohatsu M9.8B power loss

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Originally Posted by Bill O View Post
If the plugs don't fix it, possibly check your fuel line coming from the remote tank. If your fuel lines are the grey plastic (not the black rubber), then they will have a liner in them that can come off the wall sides and cause restrictions in the line.

We see this more with ethanol gas, but they make things so cheaply these days. It is possible that baking in the sun may cause it also.

In general, changing the tank fuel lines is good preventive maintenance especially if they sit out in the sun for long periods. It doesn't take too much air to ruin the motors performance.
Thankfully I have the black rubber one, but I'll keep an eye on it, thank you Bill O
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Old 10-12-2021, 09:34   #10
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Re: Tohatsu M9.8B power loss

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Originally Posted by olivioloyer View Post
Hi everybody,
...tried with less oil, cooling circuit seems to be working, as water is flowing correctly and temperature seems to be ok, I'd say around 60c.

Any idea?

I am in french polynesia and everything costs a fortune

Maururu, thanks.
What do you mean by "...tried with less oil" Its a four stroke engine. There should be NO OIL in the gas.
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Old 10-12-2021, 09:36   #11
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Re: Tohatsu M9.8B power loss

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What do you mean by "...tried with less oil" Its a four stroke engine. There should be NO OIL in the gas.
Hi pcmm, it is a 2 stroke engine.
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Old 10-12-2021, 19:24   #12
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Re: Tohatsu M9.8B power loss

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Thanks a lot seaslug, well that is an easy fix, I have changed them about 4months ago, although I am a fulltime liveaboard so yes I use the dink quite a lot and french polynesia have far anchorages
I'll try that tomorrow then and I will let you know.

Thanks again
Actually I checked and that was 6months ago , and the manual state to replace them every 6months or 100h, engine works much better now, although I did only one test run, so I'll keep the thread open for now, and keep you guys posted

Thanks a lot.
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Old 11-12-2021, 18:13   #13
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Re: Tohatsu M9.8B power loss

So I replaced the sparkplugs with new ones, and the engine was working like a charm, today we went for a 1h30 ride in the lagoon and everything was perfect until we decided to come back, and the engine started to loose power again...
We were not full throttle all the time, but maybe 40m total.
Sad
Checked the sparkplugs at the end and they look like that, sorry my phone camera as fog from passing some time at 13m deep...
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