Outboard fuel ‘starvation’
QLike many, my boat Isola was left unused for a considerable time during the Covid pandemic with around half a 200lt tank of fuel. When I did eventually get to use her again I found the Mercury outboard was not performing well. Starting was much more problematic than in the past, and under way the motor was ‘missing’, coughing and choking almost as if being starved of fuel until it warmed up. Everything was fine when used regularly before lockdown. Could there be a blockage of the carburettor starving the engine, or does the problem lie elsewhere? Gary Slack
STU REPLIES:
Your symptoms are typical of using ‘old’ fuel. Unfortunately E5 and now E10 fuels become ‘stale’ much more quickly than petrol in the days before biofuel additives.
Petrol is made up of various petroleum fractions and now ethanol (the E5 and E10 you’ll see on the pumps refers to the percentage of ethanol alcohol mandated to be added to our petrol).
Petrol is a handy liquid which is easily converted into ‘gas’ by carburetors and fuel injectors, hence the American use of the term gasoline to describe what we know as petrol.
Converting, simplistically speaking, petrol into a gas, is easier in a hot engine. Old or stale petrol loses ‘light ends’, the most easily converted part of the fuel including alcohol, and so a cold engine will stumble or misfire until it is warm enough to deal with this.
There is a slight chance that the E5 alcohol is attracting water and this could also cause issues but my money is on stale fuel. I’d buy five gallons of fresh premium petrol, add it to the tank and see how you get on.
GARY SLACK RESPONDS: Thanks for the excellent advice. Interestingly, more recently with better weather I have used Isola more and added about 50-60lt to the tank and during the last two trips out it has run far better and sounded smoother (not missing). I will try premium fuel in future and try to avoid the more common E10 which seems to be taking over.