Pre ignition

My engine has recently started to run on when I switch off. Since fitting a new webber it runs like a dream and tickover is great but it has just started this running on. Will this be timing or something else?
I remember seeing an article on timing not long ago but can't find it now. Any advice would be greatfully received.

It can be caused by carbon build up in the combustion chambers which carries excess heat igniting the fuel, I'll look up my notes in a minute. It's not pre-ignition (pinking or knock) as your title says though, ti's almost the opposite and is often called dieseling, as it carries on running in the same way a compression ignition engine does, ie. using heat and pressure instead of a spark.

Yeah here we go, I'm assuming that when you turn the ign off the engine runs roughly for a bit then dies, other than running as if you hadn't turned off the key (which would mean an electrical fault!)

causes can be carbon build up in the chambers like I said above, this will be due to overfueling.. your new carb isnt adjusted correctly?

If the carb is not closing properly then it will let in more fuel and air than normal which could then ignite from residual heat in the chamber. (Visual / finger check should be ok)

Your timing could be off, I'm not hot on timing so I won't give any advice on that front haha.

Engine could be running too hot or lean.

And if your idle speed is too high then it can run on.

So it all comes down to your new carb really, I'm guessing you haven't changed the timing belt.

Is the carb set up properly? Is it sticking open and not closing properly on idle?

Thanks for that TVE.
You are right I did give the wrong title.
When I fitted the new carb (about 500 miles ago) I did set it up pretty carefully. I actually bought a Gunson gas analyser and set it up bang on.
I think that the tickover may have speeded up very slightly since I first set it up, I don't know if that is a clue to anything.
I have never touched the ignition timing.
The engine always starts first touch of the button and runs great apart from this run on, which has only started happening for the last 150 miles.
I wonder if the new carb has a settling in time and needs re-setting?

"timing belt"

?

book knowledge!,that reminds me,i'd better check the timing belt on my v8.

bigchap said:

book knowledge!,that reminds me,i'd better check the timing belt on my v8.

Click to expand...

5000 word assignment on carburation I thought motorsport college would be fun!

cao said:

Thanks for that TVE.
You are right I did give the wrong title.
When I fitted the new carb (about 500 miles ago) I did set it up pretty carefully. I actually bought a Gunson gas analyser and set it up bang on.
I think that the tickover may have speeded up very slightly since I first set it up, I don't know if that is a clue to anything.
I have never touched the ignition timing.
The engine always starts first touch of the button and runs great apart from this run on, which has only started happening for the last 150 miles.
I wonder if the new carb has a settling in time and needs re-setting?

Click to expand...

It could be that you're right, and the carb has begun to settle down, it sounds like you did a good job setting it up, definately worth giving it a quick check.

TheVengefulElephant said:

It could be that you're right, and the carb has begun to settle down, it sounds like you did a good job setting it up, definately worth giving it a quick check.

Click to expand...

All the things above are right, it's probably the spark plugs glowing with the heat when you turn it off. Sometimes a change of spark plug is required to a cooler one, however you need to check the points mentioned. The gunson gadget is not much use, best to get it on the meter at an mot station for a few quid.

Make sure the fookir is not drawing air somewhere, vacuum pipe off or burst, or your manafold gasket is leaking.