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Skagit 06-30-2013 11:23 AM

Fuel in oil - where to start?
Hi all-

New to me boat. Gotta love 'em.

Boat has an old Volvo MB10A gas engine. I think four of them were installed in boats:rolleyes:. After a recent test run on the hard (with cooling water) I discovered fuel in the oil. It's hard to start when cold (lots of cranking), but once it starts it runs/idles/accelerates well. Starts right up when warm.

Besides a bad ring, which I'm going to check for soon, what else could lead to fuel in the oil?

I've heard that carb problems can do this. PO(S) stated that "the carb needed to be rebuilt"...could it be as easy as that? At $300 for a rebuild and $750 for a new one, I'd like to know if there is anything else I should eliminate before I go down that route.

Fuel pump is also non-standard...the standard mechanical was replaced with a NAPA electric with a dial regulator. Possible problem here?

Any other suggestions on what to check? I'm game for a thorough list.


--Skagit out :cool:

TQA 06-30-2013 12:47 PM

Re: Fuel in oil - where to start?
Are you running the fuel line thru the old mechanical pump?

I have seen this done and if the pump diaphragm leaks AND the drain is blocked [ painted over ] then fuel finishes up in the sump.

Skagit 06-30-2013 03:03 PM

Re: Fuel in oil - where to start? fuel line is direct...tank -> pump -> regulator -> carb.

Skagit 06-30-2013 06:54 PM

Re: Fuel in oil - where to start?
Compression checked out good...95/105. Praise the Lord!

So now I'm thinking way too much fuel is being delivered. The spark plug threads were soaked with fuel, and it hasn't run in two weeks. Is it possible that the aftermarket fuel pressure regulator is set too high and it's overpowering the needle valve, flooding the cylinders with fuel? That may explain the hard start's flooded so I have to crank it over over a bunch to get it aired out?

--Skagit out :cool:

imasaluki 06-30-2013 07:09 PM

Yes, if the compression is good, it seems like you're running way rich and the oil is scavenging the fuel. You may need to put a gauge on the fuel line to check pressure and have the carb checked by a competent mechanic. Fuel dilution is definitely not a good thing and will accelerate wear a great margin.

CalebD 06-30-2013 09:23 PM

Re: Fuel in oil - where to start?
Get the right fuel pump for your engine. Find the proper psi range for fuel delivery to your engine.
I prefer the mechanical fuel pump for my gasoline Atomic 4 but they sell an electric pump that replaces the mechanical one: Moyer Marine Atomic 4 Engine Rebuilding and Parts
I just like the simplicity of the mechanical pump: it only pumps when the engine is turning. The electrical pump needs an oil pressure sensor switch which is just something else that can fail.

Mechsmith 07-01-2013 09:19 AM

Re: Fuel in oil - where to start?
I'd bet on a leaky float needle, or in an updraft carb a possibility is a leaky "O" ring or gasket around the emulsion tube. That's the pipe with holes in it that sticks up into the airstream.

If yours (which I doubt because of it's age) has an internal fuel pump the diaphram in it may be punctured. A leaky mechanical pump wouldn't wet the cylinders while sitting.

I would put a kit in the carb. Be clean and it's no biggy.

CorvetteGuy 07-01-2013 10:04 AM

Re: Fuel in oil - where to start?
I agree with smith,,i rebuilt my carb on an a4 and the little rubber end was completely gone major flooding occurred due to this, grab a carb kit and see what happens. Another thought is hotter burning plugs.

Skagit 07-01-2013 03:06 PM

Re: Fuel in oil - where to start?
Thanks guys...I'm going to mess with that fuel pump and probably send the carb off for rebuild if that doesn't work.

--Skagit out :cool:

hellosailor 07-01-2013 03:26 PM

Re: Fuel in oil - where to start?
"send the carb off for rebuild"
That would be like expecting your local stock broker to outperform Warren Buffet. Ain't gonna happen. In my limited experience, there are two kinds of carb rebuild shops. The ones that charge a bloody *ing fortune, and the ones that do the job cheap and fast, and neither one does it well.
This is one of those "if you want it done right, do it yourself" jobs. Requires a gallon of gumout (for soaking), a patient afternoon, and a handle of common tools, drill bits or wire gauges, and not any special or expensive things.
Look into buying a "major rebuild kit" for your carb, and check out what's involved in the overhaul. Any library should have reference books that will show something similar, if you cna't find it online. With a digicam to take quick cheap pix to remind you how things get stacked back up during reassembly, you can probably save $200-300 over the cost of a competent rebuild. Which some of us would call a fair rate for an afternoon of quiet work at a table.

Carbs generally should be rebuilt every 5 years or so anyway, since the rubber bits don't like the gasoline and the gasoline would rather turn into varnish and plug up all the passages by then anyhow.

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