Why Won’t My Outboard Re-start When Hot?

Frayed Knot

Frayed Knot

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A year or two back I swapped my dinghy outboard for a lighter two stroke, a Tohatsu 3.5
It starts very easily, runs well but is an absolute bugger to re-start if hot - even after stopping for only a few moments (which obviously I don’t do now) & it seems to be getting worse.
Everything I can think of seems fine - fuel fresh & correct mix, carb & jet clean, float & needle valve normal, plug clean & correctly gapped.
Any (sensible) suggestions gratefully received - I don’t need a mooring sinker.
 
Chiara’s slave said:
You may find a ‘hotter’ spark plug sorts it. One with a higher number. Based on 2 stroke motorcycle experience, I must point out.
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harvey38 said:
Maybe the coil breaking down.
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When refusing, did you check for sparks?

Look for a "Sparkplug ignition tester pen". Doesn't require dismantling, fast and easy checking on the fly
 
Frayed Knot said:
A year or two back I swapped my dinghy outboard for a lighter two stroke, a Tohatsu 3.5
It starts very easily, runs well but is an absolute bugger to re-start if hot - even after stopping for only a few moments (which obviously I don’t do now) & it seems to be getting worse.
Everything I can think of seems fine - fuel fresh & correct mix, carb & jet clean, float & needle valve normal, plug clean & correctly gapped.
Any (sensible) suggestions gratefully received - I don’t need a mooring sinker.
Click to expand...
We have exactly the same! Same engine. Still haven’t resolved it. But one thing we do NOT do is use any choke.
 
Three things stop engines

Rule out mechanical as it runs

Electrical-change spark plug, ht lead coil, electronic ignition/points

Fuel. Fuel type/contaminatiin
Blockage at tank filter gauze
Does it only happen when shaken up a bit? Clean tank
Carb issue
Fuel pipe degradation
Air intake issues blocked filter, choke not opening/closing fully

Work methodically through electrical and fuel/air issues
 
Chiara’s slave said:
You may find a ‘hotter’ spark plug sorts it. One with a higher number. Based on 2 stroke motorcycle experience, I must point out.
Click to expand...
That depends on the make of plug. Champion plugs are hotter the higher the number but the Tohatsu should have a NGK BP6HS-10. With NGK plugs the lower numbers are hotter, higher numbers cooler.

BP5HS-10 is one grade hotter but in outboard use a grade 6 is already a fairly hot plug .
 
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Thank you all for the replies.
It was new a couple of years back but for various reasons I can’t expect a warranty service.
I have to admit I didn’t check for spark when it was being difficult but I think I can rule out most of the other basic things.
It’s probably only run for about twenty hours & I’ve looked after it carefully so unlikely to be age-related.
No fuel flow issue, tank is clean as a whistle & fuel is fresh. Choke is operating normally.
I’ll certainly try a different spark plug & check the air intake but I don’t know how I would tell if the coil was failing without replacing it.
sfellows: That’s depressing! I only use choke when cold - it then fires up instantly but it won’t start without.
 
I’m not familiar with the newer 2 stroke outboards but sometimes the carbureteur design has the tiniest pin hole size orifices in and along the main Venturi tube
You almost need a dentists mirror, a set of micro carb brushes to see em and get at them.

Fires up on choke, but then stubbornly won’t restart . But once you do get it going again, hot, it will run happily on and on -Could be ?

Particularly vulnerable to ethanol jelly, those teensy holes.
 
Frayed Knot said:
I don’t know how I would tell if the coil was failing without replacing it.
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Yeah, I think you have to buy a used one, swap it out and see if anything changes, if it doesn't help just be grateful you have a spare. It seems very new to have this kind of problem.
 
The OP reports checking the plug is clean and correctly gapped, but I would think (also based on past motorcycle experience!) that it would still be worth trying a new plug. The worst that can happen is he has a spare plug.
VicS said:
That depends on the make of plug. Champion plugs are hotter the higher the number but the Tohatsu should have a NGK BP6HS-10. With NGK plugs the lower numbers are hotter, higher numbers cooler.

BP5HS-10 is one grade hotter but in outboard use a grade 6 is already a fairly hot plug .
Click to expand...

But perhaps the fact it is already a hot plug is a clue that this model engine tends to need them, and that any 'variation' from the norm in a particular engine might need a yet hotter one?

Frayed Knot said:
. . . I don’t know how I would tell if the coil was failing without replacing it. . .
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Mark-1 said:
Yeah, I think you have to buy a used one, swap it out and see if anything changes, if it doesn't help just be grateful you have a spare. It seems very new to have this kind of problem.
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If the coil were breaking down with heat, wouldn't it cause the engine to misfire or stop altogether when running, rather than run OK but refuse to restart? (Not saying it's impossible, but seems unlikely.)
 
VicS said:
That depends on the make of plug. Champion plugs are hotter the higher the number but the Tohatsu should have a NGK BP6HS-10. With NGK plugs the lower numbers are hotter, higher numbers cooler.

BP5HS-10 is one grade hotter but in outboard use a grade 6 is already a fairly hot plug .
Click to expand...
I mean the plug needs to carry away more heat. NGK higher number. That’s how it is in motorcycle terms. Plug for hotter engine.
 
I once had a Ford Corsair (V4) which would actually stop in very hot weather and refuse to start until cooled.
When broken down at the side of the A12 one day someone stopped to help (he’d owned the same car & had the same problem)
The remedy was to blow hard down the filler for as long as possible and then it would start like magic.
I must have looked a proper burk, though - especially as the filler was at the back of the car, behind the number plate!
SvenH said:
I'm no petrolhead but I had the same with my moped, decades ago. It would sometimes require a cooldown after running fast for a longer time.
Could this have something to do with how fuel enters a carburator when the carburator is hot and not being cooled by continuous fuel flow?
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SvenH said:
I'm no petrolhead but I had the same with my moped, decades ago. It would sometimes require a cooldown after running fast for a longer time.
Could this have something to do with how fuel enters a carburator when the carburator is hot and not being cooled by continuous fuel flow?
Click to expand...
That was my thought - heat soak causing vapour lock. Easy enough to check, just try starting it hot with a dab of easy-start or the like
 
Frayed Knot said:
I once had a Ford Corsair (V4) which would actually stop in very hot weather and refuse to start until cooled.
When broken down at the side of the A12 one day someone stopped to help (he’d owned the same car & had the same problem)
The remedy was to blow hard down the filler for as long as possible and then it would start like magic.
I must have looked a proper burk, though - especially as the filler was at the back of the car, behind the number plate!
Click to expand...
Sorry Sven, I meant to add: thanks, that sounds like something to investigate - especially as the problem has been worse as the weather has gotten warmer.
 
It's worth just trying a new plug. Even if the existing one looks OK. Modern fuels are buggers for allowing deposits to build up and allowing tracking.
 
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