Six in line BMW dieseling issue

[Gerard C]

Hi there. Car is an historical BMW ex-TWR, six pot 3,5 litre running on Webers. Some issues plus "dieseling" (also called run-off I think). Any idea/advice? Thanks.

[Tel 911S]

Originally Posted by Gerard C

Hi there. Car is an historical BMW ex-TWR, six pot 3,5 litre running on Webers. Some issues plus "dieseling" (also called run-off I think). Any idea/advice? Thanks.

If it has been standing and not used for a while , then the carbs will need cleaning out as modern fuel will " Gum Up "the jets and other bits inside .
Then check fuel delivery rate to carbs , fuel levels in float chambers , and balance between all of the Butterflies so that they are all opening at the exact same time .
Then a new set of spark plugs , give it a run and see what the plugs look like .
They should all be a light brown colour .

Just another thought , if the ignition timing is retarded , that can also cause "Run On ".

[Gerard C]

Thanks Tel for your advice. The carbs are OK, mixture too rich, plugs too hot and CR is slightly above 11,2 (we believe…). Didn't think about re-adjusting the ignition timing. Another issue could be the fuel, the best we can find at any regular station is SP98 which could be better with some additive or few drops of ethanol. Will let you know, thanks again.

[Tel 911S]

Originally Posted by Gerard C

Thanks Tel for your advice. The carbs are OK, mixture too rich, plugs too hot and CR is slightly above 11,2 (we believe…). Didn't think about re-adjusting the ignition timing. Another issue could be the fuel, the best we can find at any regular station is SP98 which could be better with some additive or few drops of ethanol. Will let you know, thanks again.

Usual reason for "running on " is that something in the combustion chamber is getting too hot .So that when ignition is turned off , the fuel still being sucked in is ignited .
Could be exhaust valves or possibly spark plugs are hot enough to ignite the fuel .
Reasons for extra heat can be retarded ignition timing , or more often a weak fuel mixture .
Sometimes even the wrong grade of spark plugs .

If possible , use a laser temperature gun on exhaust manifold which might give a clue about what is happening .

[Gerard C]

Originally Posted by Tel 911S

Usual reason for "running on " is that something in the combustion chamber is getting too hot .

Thanks Tel for briging back to the origin of the troule, very useful. Did you ever one of those? https://www.motomobil.com/en/product...eb527-WEB001-1 Do you know how it works, meaning do they send fresh air to kill the engine?

[Tel 911S]

Originally Posted by Gerard C

Thanks Tel for briging back to the origin of the troule, very useful. Did you ever one of those? https://www.motomobil.com/en/product...eb527-WEB001-1 Do you know how it works, meaning do they send fresh air to kill the engine?

Have seen them before .
When fuels went lower octane ,[ unleaded ], running on became more of a problem , and I remember a lot of FORDS fitted those valves .
I believe they are closed when ignition is on , and open up when ignition is off ,so as to let air in. And the last few rotations of the engine is just sucking air and not fuel as well .

[Gerard C]

Now you mention Ford engines, dieseling was a concern when I started with the full iron 3.0 Essex engine. Never had an issue, but the heads are said "heron", menaning they are almost flat and the combustion chamber is in the pistons heads. I'll ask my friend what kind of shape he made in his Bimmer head but I thing its an aluminium one. Looking for any hot spot now!

[Tel 911S]

Originally Posted by Gerard C

Now you mention Ford engines, dieseling was a concern when I started with the full iron 3.0 Essex engine. Never had an issue, but the heads are said "heron", menaning they are almost flat and the combustion chamber is in the pistons heads. I'll ask my friend what kind of shape he made in his Bimmer head but I thing its an aluminium one. Looking for any hot spot now!

From memory , if the engine is running too hot , the exhaust valve can become actually red hot , or the protruding part of the spark plug can as well .
But another possibility is a build up of soot/ carbon in the combustion chamber can get hot enough to ignite the fuel still being pulled in as the revs drop off .

So check the fuel mixture is correct through various throttle openings and the ignition timing is good . Good quality petrol as well .
After that it does get a bit more difficult to pinpoint the actual cause
.