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Old 25 August 2013, 04:15   #1
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Tohatsu 9.8 won't idle, rough, cutting out...

Been on the River Dart this last week - engine has been running fine all summer until this week. After the first day's pootling on the river at 5 kts or so it started cutting out at idle/taken out of gear towards the end of the day. Got slightly worse for the next two days and was a real PITA when trying to manouver in the harbour as each time into neutral or if trying to go dead slow it died. Out to sea at other rev ranges or WOT it was fine and got full power.

Cleaned carb about 15 hours ago and it was spotless anyway, new plugs then too.

I checked plugs after issues above and they did not look too oily but cleaned them to no affect.

Pumping bulb when engine idling/dying has no affect - bulb is hard too.

Just had it in water bucket and the same again, will not happily idle below about 1400, drop it to less and then 1200 and it gradually gets rougher and rougher, more erratic, slows and dies.

Any ideas please chaps?
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Old 26 August 2013, 04:19   #2
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How old is the engine? Sounds like a blocked idle jet to me.

My Tohatsu 9.8 (two stroke) is not smooth at under 1400 rpm, never has been, but it never stalls. The idle settles at a rough 1200 and under load an uneven 8-900rpm.
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Old 26 August 2013, 04:20   #3
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2006 and low hours, very well maintained.
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Old 26 August 2013, 12:10   #4
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Almost certainly the idle jet. Also once the jet has been taken out and thoroughly cleaned (don't use a wire - use something like a nylon brush hair and wash out in clean petrol or similar solvent) blow out the orifice it screws into. In fact blow out all the holes around the jets. If you haven't access to compressed air use a small diameter plastic pipe over the hole and blow hard.
Good luck
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Old 26 August 2013, 12:55   #5
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I will check out the carb again then - surprised though as I'm really careful with fuel cleanliness, there's a filter in the pick up and in the carb of course and the carb was cleaned a short time ago. Is the fact it was trolling for a few hours a factor?

As a follow up to my OP I did try a pair of used (but clean spare) plugs and the same issues remained.
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Old 26 August 2013, 13:19   #6
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You've tried it on fresh fuel mix, of course?
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Old 26 August 2013, 15:57   #7
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You've tried it on fresh fuel mix, of course?
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Old 26 August 2013, 16:01   #8
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Hmmmm

You wouldn't be running on "double" mix - 25:1?
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Old 26 August 2013, 16:05   #9
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99.99 % sure, I always habitually fill from a 5 litre container to which I add 100ml measured at filling time.
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Old 26 August 2013, 17:44   #10
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Check gas tank is inmaculate clean along internal mesh filter, carb filter too. Probably your slow carb circuit is bit varnished. Will need to dismantle hole carb, remove all plasitic and rubber parts and soak for at least 3 hours on a proper dip carb clean fluid. Squirt all fuel passages with any good quality spray carb cleaner, assemble back carb and test. Adjust fuel/air mixture screws and slow idle screw back to specs,

Happy Boating
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