Mercury 15hp oil/fuel problem

paintman · 08 August 2016, 00:38

Not my boat, but friend has a Mercury 15hp 4-stroke twin cylinder Big Foot - I don't know the year.
Recently started to cut out at idle but OK at wider throttle openings. Fine before that. Oil level also around 1/2" to 3/4" higher than than it was when he last checked it before this issue. No water in oil & (to my surprise) no particular smell of petrol in the oil or obvious thinning of the oil.
My own suspicions would be a perforated fuel pump diaprhagm, sticky float valve or leaking carb float.
Any known issues with these engines in this department?

jyasaki · 09 August 2016, 21:15

A buddy of mine had his fuel line disintegrate from the inside; it was apparently some sort of plastic lined rubber hose. Any evidence of contamination in the filters or fittings? Unlikely, but something to look at.

Also check that the primer bulb firms up when pumped; if it doesn't, a small leak in the line may be the issue.

Carb cleaning and rebuild (with appropriate replacements) is always a good thing.

jky

jeffstevens763@g · 09 August 2016, 21:43

So the engine oil has increased by 50 - 75%? I would drain the oil completely before anything, oil will float on top of petrol & water unless that's what you have done but very strange

paintman · 10 August 2016, 00:07

Thanks for the replies. His mechanical ability is fairly limited so he's decided to get it looked at by one of the local outboard specialists.
Hopefully that will be in the immediate future & I'll post the result when & if I find out!

Nos4r2 · 10 August 2016, 17:52

Quote:

Originally Posted by jeffstevens763@g

oil will float on top of petrol

No it won't, it'll just thin the oil.

jeffstevens763@g · 10 August 2016, 18:00

Quote:

Originally Posted by Nos4r2

No it won't, it'll just thin the oil.

Yep got that wrong way round oil & petrol will float on water cheers nos

paintman · 21 August 2016, 09:49

Problem solved.
Diaprhagm - part no 9 in the attached - had failed & petrol was passing down a couple of the bolts - part nos 8 - and into the oil.
Diaprhagm - part 4 - was fine & is being kept as a spare as the repairer managed to track down a pump overhaul kit.
Mercury Marine 15 HP (4-Stroke) Fuel Pump Parts