Outboard spark plug fouled with soot. I know that this is caused by an over rich mixture in my 5hp, 4stroke, Mariner.

[Yellow Rachel]

I know it’s either too much fuel or insufficient air. I don’t think there is an air filter on the outboard so that would leave excess fuel. I’m not sure how to proceed in fault finding.

History
This engine was difficult to start from new, when it did eventually start on the choke I had to be very careful and ease the choke in over a period of minutes. If the choke was closed too quickly and the engine stopped it could not be restarted for ten to fifteen minutes. Once started and the choke closed all was well plenty of power available.

FYI book provided with the engine was for a Mercury Outboard. I phoned the Mariner U.K. distributor who told me that the only difference between Mariner and Mercury is the paint job and decals, the Mercury book was fine.

 

[KevinV]

I don't know outboard carbs, but on anything else I'd be starting by looking at the float chamber - to me it sounds like it's starving, then flooding.

Next favourite would be the idle jet being blocked, with the main jet adjusted to keep the thing running at idle, making it run rich on the throttle.

I don't understand why you accepted this from new, but that's no help now.

Get the carb cleaned,then take it from there.

 

[RivalRedwing]

try a new plug and fresh fuel before going further

 

[Yellow Rachel]

I don't know outboard carbs, but on anything else I'd be starting by looking at the float chamber - to me it sounds like it's starving, then flooding.

Next favourite would be the idle jet being blocked, with the main jet adjusted to keep the thing running at idle, making it run rich on the throttle.

I don't understand why you accepted this from new, but that's no help now.

Get the carb cleaned,then take it from there.

KevinV. Thanks for your thoughts. I wanted an outboard capable of battery charging. There was nothing local so I bought the engine online and it lay in the garage for three months before I had it in the water. That the reason I accepted it.

 

[Refueler]

For me - it would be off to the Service and get that carb / fuel run sorted ...

Otherwise one day you will find yourself stranded out on the water wishing you had got it sorted.

 

[lexi]

You have the Saildrive type with lighting coil?
These are Tohatsu engines. Very good really. These should start up on choke and then choke in within a minute. Carb clean first (Kehin type used on many Honda) May have been swarf from the beginning (unusual)

 

[Alex_Blackwood]

KevinV. Thanks for your thoughts. I wanted an outboard capable of battery charging. There was nothing local so I bought the engine online and it lay in the garage for three months before I had it in the water. That the reason I accepted it.

Clue???

 

[Steve65]

Mercury/Tohatsu carbs are notoriously bad from new...sorry bad is not quite correct. To achieve the ridiculous emission standards carbs have to be tuned very lean.
This sounds very similar to your symptoms. I think the sooted plug is from over use of the choke, in order to keep the engine running.
There is a simple cure that your main dealers are not permitted to do.
Firstly give the carb a good clean focusing on pilot and main jets.
Once clean you will notice a brass plug on the carb. This needs to be carefully drilled out. Only use a 2mm ish bit then lever the plug out.
You will see once the plug is removed you will see a slow running screw.
This needs to be unscrewed a turn, restart the engine and adjust with choke open until you get optimised tick over and throttle response