20 replies [Last post]
Micha
Micha's picture
 Offline
Joined: 10/12/2007
Posts: 350

Hi friends,
I once came up with this. My engine oil smells like petrol. I have for 2 days closed the petrol tap on the tank and guided the rubber hose to a can, which reminded dry and empty. So I don't know what to check next. Any idea will be appreciated.
Thanks and a nice week, Michael.

__________________

Michael Steinmann
R51/3 1952
Engine Nr. 529466

Allan.Atherton
Allan.Atherton's picture
 Offline
VBMWMO #2709
Joined: 10/27/2006
Posts: 507
Re: Engine oil smells like petrol

Perhaps you left the petcock open and the gas went through the carbs and into the cylinders. Change the oil and be sure to keep the petcock closed.

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

yeah keep the petcock closed and also check to make sure that closed or off really means fuel isn't still flowing

many of those petcocks need recorking and it can be done even if somewhat tedious and painful

anyhow, make sure the valve/petcock is turned off while sitting and make sure when it's off it really is not flowing fuel

not the worlds greatest fuel taps and the float needles and seats in those carbs are pretty third world crude if you ask me

if severe fuel contamination of the oil indeed happened while the engine was running so very rich in operation

it would wash the oil off the walls and wreck your rings and cylinder walls at the least

and it would not be running nice at all

as in you know something would be causing very lumpy/rough running and copious amounts of black exhaust smoke

and that is possible too but you give no indication of any symptoms which would point to rich running conditions, fouling, or running worse hot than cold,. or difficult starting once at temp......

schrader7032
schrader7032's picture
 Offline
VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6898
Engine oil smells like petrol

Michael said he ran the petcock outlet into a can and there was no leaking or drainage past the closed petcock. Sounds like that's not the problem.

If the petcock is closed at all times when the bike is not running, and it's open (obviously) all times that the bike is running, then the only way for excess gas to get into the oil is from a very poor combustion process. Coupled with bad rings, allowing the combustion gases to pass into the crankcase, contaminating the oil.

Kurt in S.A.

__________________

Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

Allan.Atherton
Allan.Atherton's picture
 Offline
VBMWMO #2709
Joined: 10/27/2006
Posts: 507
Engine oil smells like petrol

"EuroIron" wrote:

... check to make sure that closed or off really means fuel isn't still flowing.....

His first post says he closed the petcock and piped it into a can for 2 days, and no gas came through the petcock.

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

I know but unless the bike has previously set with the petcock not turned off.....

and if it is not clearly exhibiting very noticeable drive-ability symptoms

we have an inexplicable anomaly or perhaps there has been some level of sabotage

Micha
Micha's picture
 Offline
Joined: 10/12/2007
Posts: 350
Engine oil smells like petrol

Thanks for the help, guys.
Well, I'm starting to suspect the carbs, as they are old and maybe not gas tight. I've changed oil now, and hope to exchange to new carbs the moment I can afford it. I think that after closing the tap, the fuel that still is inside the rubber hose is getting through the filter and down to the carbs, passing the carbs needle and from there into the heads and down the liners. What do you think?
Cheers, Michael.

__________________

Michael Steinmann
R51/3 1952
Engine Nr. 529466

Allan.Atherton
Allan.Atherton's picture
 Offline
VBMWMO #2709
Joined: 10/27/2006
Posts: 507
Engine oil smells like petrol

"Micha" wrote:

... suspect the carbs, as they are old and maybe not gas tight.... hope to exchange to new carbs the moment I can afford it. I think that after closing the tap, the fuel that still is inside the rubber hose is getting through the filter and down to the carbs, passing the carbs needle and from there into the heads and down the liners....

I think new carbs will be extremely expensive and an unnecessary waste of money.
Gas that enters the carb must flow through the float chamber. If the float rises and the float needle does not shut off the flow, the gas overflows out of the float chamber onto the ground. I don't think the gas can also get into the intake of the cylinder. Many people have leaky carbs, but nobody finds gas in their oil. I think the gas must pass through a jet to get into the cylinder and into the oil. I just don't think it happens unless perhaps the bike is constantly kick-started without firing, and even then the amount would be insignificant.
There is many a leaky petcock, and many a leaky carb, all putting gas on the ground, but not into the oil sump.
You have tested your petcock, and it is gas-tight.
You have never reported gas overflowing from your carb float chamber, so your float needle also appears gas-tight. But if you want to test it, leave the petcock open overnight and see if gas leaks from the carb. If the float needle leaks, it is only an inconvenience and can usually be fixed with a kit from Bing.
But when Bing sends you anything, there is always included a yellow warning sheet saying the the carburetor itself (i.e. the float needle) is NOT a fuel shut off device, so always turn of the petcock.
Sometimes on a hot engine the gas in the float chamber will expand when stopped, and leak out of the carb. To avoid that, turn the petcock off as you are riding into your driveway and then let the engine idle for a half minute to draw down the level of gas in the float chamber.

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

why not simply convert to the later (R69S?) float bowl tops with the seat on the side which also uses viton tipped needles?

should cure what is ailing your ride

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

and I think they merely screw in place with no modifications required

Allan.Atherton
Allan.Atherton's picture
 Offline
VBMWMO #2709
Joined: 10/27/2006
Posts: 507
Engine oil smells like petrol

"EuroIron" wrote:

why not simply convert to the later (R69S?) float bowl tops with the seat on the side which also uses viton tipped needles? should cure what is ailing your ride

That is called the "lever top conversion", and it appeared on all the late-60s bikes. It is available from the Bing Agency (google that). But it only shuts off the gas from overflowing the float chamber. It prevents dripping carbs.

Micha has not reported dripping carbs. And even if he were to report dripping carbs, that would not put gas in the oil.

I must say that I have not heard of gas in the oil of a BMW, and wonder if Micha is perhaps incorrect about that.

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

umm if fuel is dripping from thos bings it is quite possible and very likely it can make it's way into the chamber just as easily as flowing out the upwardly tilting carb bellmouth

the viton needle tops are light years ahead of the retarded toothpick sized metal needle

everyone knows those do not work and that they never did

that's why Bing finally got a clue and then changed them

what puzzles me is that when they went to the CV's they went back to a bigger but still as stupid all metal needle

but the viton tipped needle for an Amal concentric will drop right in and allow the float height to be dailed right in

I guess I have several of those carb tops

Bruce Frey
Bruce Frey's picture
 Offline
VBMWMO #6316
Texas Hill Country, USA
Joined: 10/27/2006
Posts: 574
Engine oil smells like petrol

I suggest Micha monitor the situation and see if it happens again. I certainly would not be in a rush to buy new carbs for this reason.

I cannot think of a likely scenario for gas getting into the crankcase unless it is running WAY too rich, which would certainly cause fouled plugs and driveability issues.

Perhaps it MAY be possible for fuel to percolate through the idle circuit on a parked, hot bike, run down the carb throat, into the head and find its way into the crankcase. Any fuel in the line between the petcock and the carb could continue to feed the situation, especially with a sidestand.

Micha, do you use a sidestand?

How many miles/km does it take for your oil to darken.

Bruce

Allan.Atherton
Allan.Atherton's picture
 Offline
VBMWMO #2709
Joined: 10/27/2006
Posts: 507
Engine oil smells like petrol

"EuroIron" wrote:

umm if fuel is dripping from thos bings it is quite possible and very likely it can make it's way into the chamber just as easily as flowing out the upwardly tilting carb bellmouth...

I really think we need to settle this question. My understanding is that an overflowing float bowl can only put gas on the ground, not into the (combustion) chamber.

Thousands of Bing slide carb owners have experienced and lived with leaky carbs. The gas drips on the ground; I have never heard of it going into the engine.

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

I beg to differ

if you were indeed correct then a leaky petcock would only drip fuel on the ground

don't you agree?

if not please explain why not to me as though I were six

now the amount from that could possibly be syphoned from only the float bowl should easily be vaporized by a machine that sees regular use

at least I'd think so

Bruce Frey
Bruce Frey's picture
 Offline
VBMWMO #6316
Texas Hill Country, USA
Joined: 10/27/2006
Posts: 574
Engine oil smells like petrol

"Allan.Atherton" wrote:
"EuroIron" wrote:

umm if fuel is dripping from thos bings it is quite possible and very likely it can make it's way into the chamber just as easily as flowing out the upwardly tilting carb bellmouth...

I really think we need to settle this question. My understanding is that an overflowing float bowl can only put gas on the ground, not into the (combustion) chamber.

Thousands of Bing slide carb owners have experienced and lived with leaky carbs. The gas drips on the ground; I have never heard of it going into the engine.

I am guilty of overlooking the obvious. DUH! When we tickle our carbs as part of the starting drill, we are really simulating a sunk float or a leaking float valve. A 3 or 4 second tickle floods the carbs enough to put fuel into the engine for starting. A 6 second (at least on my bike) or longer tickle also starts to put fuel on the ground. Accordingly, leaking carbs put fuel into the engine as well as on the ground. I know from experience it IS possible to flood a bike by over-tickling.

I do not know what the intake passage is like in the postwar heads, but if there is a downward slope and an open valve, it should be possible for fuel to flow from the carb into the cylinder, seep through ring joints and into the crankcase. Because the orientation of the valves and ring gaps continuously varies it may not happen every time, but I think it CAN happen.

It would be an interesting experiment to unbolt a carb from the head, hold down the tickler and capture and measure the amounts that run down the carb throat and drip onto the ground. I wonder what the ratio between the two is?

It may be my imagination, but the odor of today's fuel seems to be stronger and linger longer than in the past. I'll bet a very small (1cc?) amount of fuel in the oil will make it smell.

Does all that make sense?

To me, this reinforces the need to run the last minute or so with the fuel valve turned off to use the fuel in the line between the tank and carb and to make sure the fuel valve is sealing tightly.

Bruce

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

if the fuel level in the float bowl is incrementally increased

it will flow into the engine before it ever leaks a drop onto the ground

as far as siphoning, during the cool down part of the heat soak, only a small amount is available from lines with shut off non-leaking fuel taps

and I am 99% certain it is not physically possible for the float bowls to be siphoned dry during

regardless

viton tipped float needles are hugely useful upgrades just as regularly bringing such a machine up to operating temp and using it with at least medium anger (just like BMW intended) seems to keep away the gremlins

amazing the problems sedentary bikes have as opposed to daily beaters

Allan.Atherton
Allan.Atherton's picture
 Offline
VBMWMO #2709
Joined: 10/27/2006
Posts: 507
Engine oil smells like petrol

"Bruce Frey" wrote:

... When we tickle our carbs as part of the starting drill, we are really simulating a sunk float or a leaking float valve. A 3 or 4 second tickle floods the carbs enough to put fuel into the engine for starting. A 6 second (at least on my bike) or longer tickle also starts to put fuel on the ground. Accordingly, leaking carbs put fuel into the engine as well as on the ground. I know from experience it IS possible to flood a bike by over-tickling....

The Bing Manual does say the tickler floods the carburetor throat with raw fuel. It does that by forcing the float and its needle down, opening the needle valve and allowing the head of fuel in the tank to fill the float bowl and push the liquid fuel past the main jet, up the metering tube and into the carburetor throat.

The purpose of tickling is to fill the metering tube to the top, so that the fuel is immediately available to air sucked through the venturi by the starting kick. Over-tickling would cause fuel to continue to flow up the metering tube and down the throat of the carb into the engine.

And I can see now that a bad float needle valve could do the same thing. Even though the float bowl is below the carburetor throat, the head of fuel pressure from the tank above fills the float bowl and pressurizes it.

At first, the fuel finds the passage to the idle jet and leaks out of the idle air screw onto the ground. If the leak out of the idle screw keeps up with the leak in from the float valve, no fuel will get into the engine. This is what most people see.

However, if more fuel is coming into the float chamber past the float valve than is leaking out of the idle screw onto the ground, then the fuel will be pushed up out of the float bowl, past the main jet and up the metering tube, to flow into the throat of the carb. From there it will run down the air tube into the engine.

This may be why many people have carbs that leak fuel on the ground, but not many report fuel in their oil. Their float valve leak is not fast enough to force fuel up and over to engine.

Bruce Frey
Bruce Frey's picture
 Offline
VBMWMO #6316
Texas Hill Country, USA
Joined: 10/27/2006
Posts: 574
Engine oil smells like petrol

"EuroIron" wrote:

if the fuel level in the float bowl is incrementally increased it will flow into the engine before it ever leaks a drop onto the ground......and I am 99% certain it is not physically possible for the float bowls to be siphoned dry

You are right, if it is a very slow leak, it may never get to the ground. I cannot imagine how it could siphon, either, but can imagine percolation if really hot.

Touching wood, I have Bings from an R50 on my sidevalve R6 and have never had a drip because of the float and it has the old style needles. The flange is vertical so the bowl is sligtly inclined, but it works OK.

Bruce

EuroIron
EuroIron's picture
 Offline
VBMWMO #7266
Joined: 04/23/2007
Posts: 414
Engine oil smells like petrol

ok I'll do a fly explanation of siphoning and how that results in a slightly leaky petcock and needle/seat facilitating the entire contents of a fuel tank being emptied into an engine crankcase........

once an engine is at operating temp and then turned off

it will indeed become hotter (called a heat soak) before it starts becoming cooler

if the fuel is still flowing during some point in this window

there is a siphoning effect which takes place and of course it is assisted by the head pressure in the fuel tank

from what I've seen of the toothpick float needles and their bizarrely small seats

I'd say plenty (a tablespoon is plenty) leaks past them and into the crankcase

but let's say it is only the contents of the fuel lines and part of what is in the float bowls.......

next start up and running to operating temp

such small amounts will tend to vaporize but not without doing at least a small amount of compromising to the engine oil

I'll name one bike which has likely lost more main bearings, cranks, and rods to this siphioning

the Yamaha XS750....... named the bike of the year when introduced in 1976 but it featured vacuum shut off fuel valves that sometimes would fail in the always on position

a few were even noted to fail in the off position and when they did

you were walking if you didn't have a portable fuel tank IV style

I have to agree with Bruce regarding those who choose to suffer those Bing needles... and there really isn't any good reason to do so

turn the gas off, make sure your petcock doesn't leak, and let the engine die from fuel starvation as it idles to finally stop

and that is one easy carb to seal up the mixture screws and misc banjo or gland type nuts which can also leak

and viton tipped needles are just plain hard to beat even though I've been very successful at lapping those toothpicks into their seats

they still won't seal like viton nor will they suffer this horrible pump swill most of us must suffer

I don't see that situation getting any better either........

Allan.Atherton
Allan.Atherton's picture
 Offline
VBMWMO #2709
Joined: 10/27/2006
Posts: 507
Engine oil smells like petrol

"Bruce Frey" wrote:

... You are right, if it is a very slow leak, it may never get to the ground...

Not what I said.
A slow leak will get to the ground but not up into the carb throat.
To get up into the carb throat, the leaking float valve has to be so bad that the fuel cannot all leak out onto the ground from the idle air screw, but rises up the metering tube into the carb throat.

Comment viewing options

Select your preferred way to display the comments and click "Save settings" to activate your changes.