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QI own a 2.5hp 4-stroke Mercury outboard that drives my Laser Stratos keelboat when the wind dies, and because there’s rarely little wind the engine doesn’t get used much.
Annoyingly, when I do need it the engine has a habit of failing, and always for the same reason: issues of fuel starvation.
The 2009 engine looks like new. I keep it clean, flush with clean water, and empty the fuel tank and run the engine dry if I have used it – in fact its only burned one gallon of fuel since new! On top of that it’s regularly serviced by a Mercury agent.
My engineer tells me my problem is a common issue because of the additives now in petrol, which will rot fuel lines and coat the carburettor with a brown vanish-like substance that blocks fuel jets.
I’m told that by running the engine once a week you can reduce the chances of it blocking as quickly – but it will still block eventually.
I’ve spoken to many other owners of small engines and they’re all experiencing the same issue – though people with older 2-stroke engines don’t seem to suffer the fuel blockage problem so badly.
The agent’s cure is to soak the carburettor in a commercial solvent – not something that’s practical when you are at sea.
Given that hundreds (perhaps thousands) of PBO readers own small 2.5-10hp outboards has anyone come up with an answer?
It’s not only a hassle to keep taking the motor to an outboard repair agent but a safety issue as the motor is my back-up. Although I carry a radio, anchor and paddle it’s still a risk if the wind dies on a rocky coastline. John Shears, By email STU DAVIES REPLIES: A bit of background first. Two strokes are effectively banned for leisure use in the UK on environmental grounds – they’re considered too smokey. So the big manufacturers were forced down the route of 4-strokes which are easier to tune for lower emissions.
Unfortunately one of the side effects is that they have to ‘lean’ the mixture right down at tick over and that’s where the problem starts. They fit tiny pilot jets and hide or do away with the tickover idle mixture adjustment to stop us richening the mixture.
Modern fuels have up to 5% alcohol/biofuel added and this is highly reactive to some of the plastics and rubbers used in our carbs.
Also alcohol absorbs water from the atmosphere, so we have a perfect medium for creating havoc with the tiny pilot jets.
I’ve stripped down a lot of these due to issues you talk about. Inevitably there’s a jellylike substance in the float bowl, but more importantly the jets are blocked or coated with a non-soluble metallic salt.
There’s a theory gaining ground that the alcohol additive is absorbing water from the atmosphere which is reacting with the brass jet screwed in to the alloy body of the carb. Electrolysis comes in to play, which then produces a hard metallic salt.
Carb cleaner was developed for dissolving ‘varnish’ left by evaporated petrol, but I’ve found is that it doesn’t dissolve the hard salts deposited in the jets. Only one thing gets rid of that, a hard object pushed through the jet.
Since being an apprentice 50 years ago we were taught never to push steel wire through jets, that it would widen the orifice causing all sorts of problems!
In actual fact people like me found out from practical experience that pulling a wire out of a wire brush gives the perfect implement to gently push through the jets and clean the hard deposits out. It rarely does any damage.
This is the only way to clean a carb properly – strip it and clean all the orifices manually.
I suspect that what is happening in your case is that the jets have been coated in hard salts and that soluble deposits are also being deposited at the same time. The carb cleaner is cleaning the soluble deposits but not cleaning all the crud out. You then have a situation where the engine sort of runs OK till the build-up gets too much… and the cycle continues.