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Old 21 August 2021, 17:36   #1
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Tohatsu 9.8

Hei everyone
Im having problems with my tohatsu 9.8 mfs.
It starts no problem but as soon as you throttle up it dies. If you open and close the choke the engine will rev up then as soon as you use the throttle it just dies. What is the fix ?
Thanks
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Old 21 August 2021, 19:56   #2
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Have you tried a squirting of carb cleaner into the air intake. Perhaps if that does not help it’s time to give the carb a strip down and check the jets for foreign bodies?

I have found the infrequent use of outboards often leads to muck and bits residing in the carburetor and good clean out often resolved issues. Only takes a few minutes to remove.
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Old 21 August 2021, 20:17   #3
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Yup. All the symptoms of a carb needing whipping off and given a clean. It doesn't take much of a bit of muck to produce the above symptoms but does this model have a carb or is it an EFI?
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Old 21 August 2021, 20:31   #4
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It’s a carburetor on a four stroke. I have a 9.8 but a two stroke so not sure if exactly the same.
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Old 21 August 2021, 21:02   #5
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I've had the Mariner version of the OP's engine.

First this model is known for needing warming up on choke/part choke for much longer than many outboards before it will pull away cleanly.

This behaviour is improved if you drill out the idle mixture plug and unscrew 1/4 to 1/2 turn.

Finally this engine will soon misbehave with the slightest dirt in the carb.
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Old 21 August 2021, 21:22   #6
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Have you opened the air vent on the fuel tank before you start pulling things to bits ?
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Old 21 August 2021, 21:58   #7
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Only a complete idiot would forget to open the tank valve. Luckily for me, only yesterday, someone on the pontoon kindly reminded me.
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Old 21 August 2021, 22:01   #8
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If it has a carb, will idle and you can trick rpm out of it by playing with the choke then it's usually a bit of crud in the carb.

Never fully understood the reason for small outboards not having air filters? I assume maybe it's a moisture thing but there must be a logical reason?
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Old 21 August 2021, 22:07   #9
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I can only think the air filters would be saturated most of the time.... as in with salty moisture laden air not direct splash.
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Old 22 August 2021, 08:51   #10
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I can only think the air filters would be saturated most of the time.... as in with salty moisture laden air not direct splash.
That's the only reason I can think of. You could make a filter from a non absorbent material though. I guess that even so the environment outboards run in really don't require a filter and that putting one in would tend to vary the air flow more between different conditions and so impact the running.

When you consider the route that the air takes around the outboard as the engine sucks it in from outside, I actually suspect the largest risk isn't from a sand particle that randomly flies in fresh, whizzes around the casing and into the carb needle but instead is far more likely to be a grit particle that is already sitting in the lower casing.

It doesn't seem uncommon to lift the cowl off an outboard and see large amounts of sand, shell and dust/grit sitting on the floor. I'm sure that when a particle does find its way into the carb that it has almost certainly originated from the pile of thousands of particles sitting inside the housing as opposed to one lucky particle arriving fresh from outside.

When I bought that old 15 I sprayed the base with carb cleaner and vast amounts of much came out. Once completely clean and dry I sprayed with lithium grease, my theory being that grit will subsequently stick to that thin layer enough to not be dislodged when the throttle opens up and then when flushing the engine after use I'll flush out the lower casing.

I'd forgotten this practice when I started using the 4 again after years of non use and that's the only time I've had an issue with that engine and found a tiny speck wedged into the carb needle and the engine deck was dirty with loose grit.
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Old 22 August 2021, 12:53   #11
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The reason outboards dont have air filters is they dont need them. Very little dust or debris to get sucked into the engine when on water. Anything getting sucked into the venturi will go straight through the engine & get spat out of the exhaust. Debris sucked in will not block the carb or cause any issues, fitting a home grown filter is pointless & asking for trouble. Any debris blocking blocking the jets will have entered via the fuel line or be congealed fuel in the float bowl. A total strip down & a few cycles in an ultrasonic bath is probably whats needed to clear the problem
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Old 22 August 2021, 13:09   #12
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Quote:
Originally Posted by Sharki View Post
Hei everyone

Im having problems with my tohatsu 9.8 mfs.

It starts no problem but as soon as you throttle up it dies. If you open and close the choke the engine will rev up then as soon as you use the throttle it just dies. What is the fix ?

Thanks
Those engines are pretty fussy when it comes to fuel cleanliness. Did a carb on one yesterday and although it looked relatively clean it still needed a dip in the ultrasonic bath before it behaved
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Old 22 August 2021, 18:57   #13
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The reason outboards dont have air filters is they dont need them. Very little dust or debris to get sucked into the engine when on water. Anything getting sucked into the venturi will go straight through the engine & get spat out of the exhaust. Debris sucked in will not block the carb or cause any issues, fitting a home grown filter is pointless & asking for trouble. Any debris blocking blocking the jets will have entered via the fuel line or be congealed fuel in the float bowl. A total strip down & a few cycles in an ultrasonic bath is probably whats needed to clear the problem
My mind seems more clear than it was this morning!! You're right. The grit in the carb on my 4 came via the fuel line which was why I subsequently put a fuel filter on it.
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Old 22 August 2021, 19:40   #14
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One of the great benefits of plastic tanks over the old steel ones. Chucked my old can when it contained more rust than fuel ⛽️
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Old 23 August 2021, 20:26   #15
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Tohatsu 9.8

I Cleaned the Carb out it still wont work with the throttle. The choke if pulled out doesnt work but push it in (the off position) it then puts the engine on full throttle. As soon as you try to rev with the throttle it just dies. Why ?
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Old 23 August 2021, 22:54   #16
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The only way the engine can go on full throttle is if the throttle butterfly in the carb opens fully... are you saying that happens when you move the choke knob to closed? If so surely something weird going on with the linkages??
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Old 23 August 2021, 23:13   #17
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The only way the engine can go on full throttle is if the throttle butterfly in the carb opens fully... are you saying that happens when you move the choke knob to closed? If so surely something weird going on with the linkages??
Yes thats exactly what happens ive never ever encoutered this before … its seriously odd
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Old 24 August 2021, 07:59   #18
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So with the choke in and the throttle at idle the engines runs at max revs but if you then turn the throttle it dies?

But the engine will run at idle if you pull the choke out but also does when you apply any throttle?

That does seem odd unless the butterfly is jammed as it sounds like the engine is stuck at full throttle?
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Old 24 August 2021, 10:38   #19
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So with the choke in and the throttle at idle the engines runs at max revs but if you then turn the throttle it dies?

But the engine will run at idle if you pull the choke out but also does when you apply any throttle?

That does seem odd unless the butterfly is jammed as it sounds like the engine is stuck at full throttle?
With the choke in the off position its putting the engine on full…if you try the throttle it will rev then die…pull the choke out and it will also die …push choke in full throttle i need help with this
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Old 24 August 2021, 11:04   #20
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This behaviour as stated is pretty well what you’d get if it were ever possible for the throttle and choke rods to be transposed. But I can’t think that could be possible however my next step would be to observe the throttle and choke butterfly positions as the controls are operated.
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