Fuel mixing with oil?

[Tim Good]

Ok this isn’t on my boat engine... it’s actually a lawnmower... but... this morning I check the fuel which was low. Strange as I topped it up yesterday. I went to check the oil and out pours loads of fuel! I had also accidentally left the fuel tap on overnight.

Basically all the fuel has drained into the oil chamber!

Over the last few days it was increasingly hard to start and noticed the spark plug was getting sooted up and black very quickly. Also white smoke from exhaust. This was clearly a sign.

Now that I know fuel is mixing with the oil, what would cause this? Could a faulty carb float and me leaving the fuel tap on been enough to cause this?

Sorry it’s not a boat engine but I think the topic will be useful for others.

 

[pvb]

Could a faulty carb float and me leaving the fuel tap on been enough to cause this?

Yes, or more likely a worn float needle valve.

 

[Old Harry]

Ok this isn’t on my boat engine... it’s actually a lawnmower... but... this morning I check the fuel which was low. Strange as I topped it up yesterday. I went to check the oil and out pours loads of fuel! I had also accidentally left the fuel tap on overnight.

Basically all the fuel has drained into the oil chamber!

Over the last few days it was increasingly hard to start and noticed the spark plug was getting sooted up and black very quickly. Also white smoke from exhaust. This was clearly a sign.

Now that I know fuel is mixing with the oil, what would cause this? Could a faulty carb float and me leaving the fuel tap on been enough to cause this?

Sorry it’s not a boat engine but I think the topic will be useful for others.

Is it a B&S engine, if so quite common, hence the fuel cock

 

[Tim Good]

Is it a B&S engine, if so quite common, hence the fuel cock

It's a Honda hp250. Actually not a lawmower but I didn't want to confuse things. It's a 4 stroke. Like this:

Yes, or more likely a worn float needle valve.

But how would that mean the fuel could eventually end up in the oil? I mean that wouldn't happen on an outboard if the float need was worn would it?

 

[VicS]

It's a Honda hp250. Actually not a lawmower but I didn't want to confuse things. It's a 4 stroke. Like this:

But how would that mean the fuel could eventually end up in the oil? I mean that wouldn't happen on an outboard if the float need was worn would it?

Well it has ...
Via the carb as a result of the float valve allowing fuel to pass and the design of the carb being such that the excess fuel overflows into the intake manifold then past the inlet valve(s) into the combustion chamber(s) and finally trickling past the piston rings into the oil sump.

It could happen with an outboard with a gravity feed fuel syastem

 

[jamie N]

In much the same way as sea water does if one doesn't repair the anti-syphon device on the cooling water inlet, allowing water into the exhaust manifold, filling the barrel and letting the water drain past the piston rings into the sump, on a wooden Folkboat RC8D Renault engine, whilst moored in Pierowall and preferring to have a drink rather than sort it out OR even turn the inlet valve off instead.
Apparently....., I've heard.....

 

[davidej]

In my youth I had a Vespa. If I forget to close the fuel cock when I left it, on return I would find most of the tank had emptied through the cylinder into the silencer. Would make kicking it over very interesting. Usually resulted in a six foot long plume of fuel out of the exhaust

 

[pvb]

It's a Honda hp250. Actually not a lawmower but I didn't want to confuse things. It's a 4 stroke. Like this:

But how would that mean the fuel could eventually end up in the oil? I mean that wouldn't happen on an outboard if the float need was worn would it?

You've been fibbing! That's not a lawnmower!

However, fuel can leak from the carb into the combustion chamber, and then into the sump. It happened to my oldest mower (a Kawasaki-engined Westwood, which I now only use as a tractor to tow a trailer). I was able to buy a new needle valve (despite the engine being over 30 years old) which fixed the problem.

 

[Refueler]

In my youth I had a Vespa. If I forget to close the fuel cock when I left it, on return I would find most of the tank had emptied through the cylinder into the silencer. Would make kicking it over very interesting. Usually resulted in a six foot long plume of fuel out of the exhaust

mmmmm think you'll find that Vespas and Lambrettas shared the same system of fuel passage ... from carb it gets drawn into crankcase by piston travelling upward then pushed up into cylinder for ignition by piston descending downward - ignition ocurrig when piston is nearing TDC on next upward stroke.... its a 2str.

Reason fuel was in silencer was it drained there from carb to crankcase to cylinder to exhaust.

Not trying to be rude honest ...

 

[RichardS]

None of my four-stroke lawnmowers have ever had a fuel tap but have always, I assume, had vacuum operation.

I clearly need to invest in a better class of lawnmower/flat-bed-truck-thingy.

Richard

 

[Tim Good]

Interesting. Ok thanks all.

So as for the white smoke it’s kicking out... would this suggest excess fuel being burned as the float valve isn’t t preventing excess fuel from entering?