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Micha
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Hi. I've changed oil recently on my R51/3, and I noticed that the old oil smells like petrol. Is this normal? Is there a problem? If so, where should I search? Thank you all.

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

Darryl.Richman
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Engine oil smells from petrol

Possibly a leaky petcock.

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Micha
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Engine oil smells from petrol

Where does it leak to? How is that petrol getting inside the engines oil?

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

niall4473
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Lancashire, UK
Joined: 04/09/2007
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Engine oil smells from petrol

Surprise Hi Micha,
This is quite common with British bikes, especially the ones with downdraught carburrettors, less so with BMW twins because gravity is the culprit, and the Bing carb is designed to drop excess fuel on the floor.
For it to happen you need fuel flowing through the petrol tap (whether through leakage or neglecting to turn off), and also past the float valve. The fuel level reaches the slow-running jet, (just as when you tickle it) and gets into the main choke. From there gravity takes it down the port to the inlet valve where it either sits or passes the valve and runs into the cylinder. It then has to pass however many compression and oil rings before running down into the crankcase. As you can imagine this is far more likely on a vertical motor than a flat twin, especially as the fuel tends to overflow from the carb itself.
So, another possibility, if your mixture is too rich, (for instance a punctured float allowing fuel to flood the carb, very, very common with Bings or a clogged air filter), and nothing is done about it, the excess fuel dissolves the oil film on the bore and the rings start to scrape the barrel. The charge then starts to blow past the rings to the crankcase cavity. By this time the motor is running like a bag of nails and smoking like the Grand Fleet at Jutland, but if an unsympathetic hand keeps the throttle open the engine oil will became contaminated with petrol to the extent that you can smell it, however, by now so much oil is blowing out of the breather due to excess crankcase pressure that it isn't in long enough to pick much up.
Thirdly, when you say old oil, how old? Used engine oil does degrade if left and can smell odd, I once stripped a Norton and would have sworn that there had been rats living in the crankcase if there were enough room for them!

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Micha
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Engine oil smells from petrol

Thanks for the replies; the engine got a rebore, new liners, new pistons and of course new oil. After a couple of months of riding, I decided to change the oil. This is how old it was. The engine is running a bit rich, but the Bing's aren't dripping petrol or so. I just can't get them adjust right and I think of sending them to Mark Huggett for restoration. So where can this petrol smell come from in the oil?
Thanks again.

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

EuroIron
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Engine oil smells from petrol

1) do you turn your fuel petcock into the off position when the engine is not running?

If no, you should and you always should.

If yes

then

2) Remove fuel hose from petcock or at either fuel bowl, with the petcock turned off

Is fuel flowing out the fuel line?

If no, somebody is pouring fuel into your crank case

If yes, then you should realize you very likely may have a leaky float needle or a needle seat poorly adjusted.

If you merely have a float needle which is somewhat leaky, you can lap it into the seat if the needle and seat are not excessively worn. You can do this with some fine valve lapping compound, or very abrasive tooth paste applied to the taper seating portion of the needle and then chucking the needle into a slow drill motor, and simply lap it into the seat.

Clean all traces of lapping compound and reassemble.

probably not a terrible idea to lap even a new needle into a new seat as that whole setup is less than precise even when everything is new..... including the carb body even.

The float level can be adjusted by warming the top and drifting the brass seat downwards or upwards in the cap, I use an aluminum drift for this.

You can also convert your bowl tops to the side needle type and they can be set up with vition tipped float needles which work at least 5000 times better than the caveman skinny hardchrome? metal needles attempting to seal in brass.

Good luck, any questions and do you understand that if you turn the fuel off on a warm engine........

and if the fuel really isn't off..........

it can leak into the carb........

and the thermal cycle as the engine gets both hotter and then cooler

can generate a very effective "syphon" effect

and this is how fuel can enter into your crankcases

right into the intake manifolds

past an open intake valve

into the cylinder

and right past the rings

check your oil for fuel smelll often

enough fuel in the oil and good bye crankshaft and connecting rods!

Allan.Atherton
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Engine oil smells from petrol

"Micha" wrote:

.. the engine got a rebore, new liners, new pistons and of course new oil. After a couple of months of riding, I decided to change the oil. This is how old it was.... So where can this petrol smell come from in the oil?

After a rebuild like that, the oil should have been changed within one or two hundred miles. Since it was the first oil change after rebuild, I would not worry about it yet, as long as the bike is running OK. Change the oil in another couple of hundred miles and check its condition and smell again. And do keep that petcock closed, though if not closed, I am only familiar with it's dripping externally.

Micha
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Engine oil smells from petrol

OK. Thank you all Big Grin .

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Michael Steinmann
R51/3 1952
Engine Nr. 529466

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