Self ignition

[lotusbzz]

Problem problem problem!!!!
My elan Twincam engine , just restored, no big valves, runs great but .... he doesn?t want to shut down!
I switch the key off but he goes on to run for awhile sometimes with awful noise and a bit of knocking too.
Local engine guru ( who doesn?t know at least one guru in this matter) said that it seems too much high compression rate, but the guy that machines the block and head tested the parts with the final head gasket and the ratio is as per specs .
Are the spark plug to hot?
Could be the distributor not in synch?
Cams? They are brand new, Kent as per original spec , standard stage, nothing special or racing
An electric dispersion that feeds the coil circuit even with the key off?
Anybody experienced trouble like this?
Ciao a tutti
Paolo

This a my Elan S4 1968 ready for the Italian registration

[HCA]

Not experienced it with an Elan because I am still waiting for my first one to arrive!

But running on, dieselling, whatever you want to call it, yes, a big issue with older engines before they addressed things with an anti-knock valve.

First and foremost, starting now - DO NOT LET THIS CONTINUE! It can lead to engine damage in the form of a broken crankshaft. Until you get it sorted, before you switch off, put the car into gear with foot hard on brake then switch off - but immediately you switch off, lift your foot off the clutch and let the gear stop the engine.

Please start using this procedure from tomorrow!

As for causes there are many, but my pound to your penny, it will be poor ignition timing and poor mixture in the carbs and possibly incorrect spark plugs. Have the car tuned IMMEDIATELY (you are damaging the engine just by running it as is) by someone who knows these things. I suggest that if your local guru did not know this, you ditch him as a guru. Yes, high compression and poor octane is a factor, but ONLY after the tuning is properly done.

Once it is properly tuned, and here I am assuming that your cam timing and other basic settings are good, you will see it all go away!

[rgh0]

Lots more specific information needed in order to try to advise. I would check all the items below to try to find the source of the problem

Does the engine have Strombergs or Webers ?
What spark plugs are you using,
What fuel being used
What was actual compression ratio calculated
What is cylinder head thickness and have the combustion chambers be worked on in any way
What is the ignition timing static setting and measured advance curve

regards
Rohan

[StressCraxx]

What is your idle (tickover) speed? Make sure you are set correctly with a good tach.
Dieseling is caused by unburned fuel in the inlet tracts and high tickover speeds. It has nothing to do with ignition timing, but may have to do with spark plugs with too high a heat range leaving glowing carbon deposits when ignition is turned off.

[Andy8421]

What is your idle (tickover) speed? Make sure you are set correctly with a good tach.
Dieseling is caused by unburned fuel in the inlet tracts and high tickover speeds. It has nothing to do with ignition timing, but may have to do with spark plugs with too high a heat range leaving glowing carbon deposits when ignition is turned off.

I think all Elans have unburnt fuel in their inlet tracts, that's how they work.

For 'dieseling' or ''running on' to occur, there needs to be a source of ignition in the cylinder other than the normal ignition spark. As pointed out above, carbon deposits that have been heated by the combustion process before the engine is switched off can be a cause - but the engine has just been rebuilt so this is unlikely. A sharp edge in the combustion chamber, or badly fitted head gasket that protrudes could also be heated during normal running and provide an ignition source after the engine is switched off - both also unlikely.

Most likely is the plugs are running too hot - this could be caused by using plugs with the wrong heat range, too lean a mixture, the ignition timing too retarded, or 'pinking' impacting the combustion process (caused by too high compression / wrong fuel).

I would make sure the plugs are the correct type and that the timing and mixture are set correctly before I would look any further.

[el-saturn]

had the same issue: just make sure all of your ignition pins "get" the proper cable - i did it via trial and error: 15min!! sandy

[Esprit2]

Paolo,
Rohan's questions are on the right track to a solution, but you haven't answered him yet. I was going to post my own reply to you, but then realized it just would have been a duplicate of Rohan's. So instead, I encourage you to find the answers to his questions.

Regards,
Tim Engel

[lotusbzz]

The Carburatore are stromberg
The sparks plugs are standard maybe a bit hot
The fuel is not 98
The compression ratio is 9,6 after machining the head valve seat
All the other info must be measured but as far I know there are no strange figures

[pharriso]

The sparks plugs are standard maybe a bit hot
The fuel is not 98

Hmm, not very specific

I run NGK BP6ES in my Sprint...

[denicholls2]

In my Walter Matthau voice, "Carbon on the valves!"

You have to be pretty old to smile at that, but many of us are.

Stromberg/SU style carbs seem very good at feeding this behavior, in my experience in Jaguars and Triumphs. In my '69 Spitfire, it was an indication of high coolant temperature. The thermostats in English cars of the period failed closed, and in my experience failed a lot. So if it ran on, the first thing I did was boil the thermostat in a pot to make sure it opened.

Not sure how timing affects it (because there's no spark involved) other than a highly advanced spark usually means a hotter engine. Lean mixture means a hotter engine ...

The Jaguar 3.8 my father had briefly would run on forever if you didn't intervene. Still, I wish I'd had $600 to buy it back in the day. I'd pay even more for it today. Took him forever to find a buyer (he was selling it for a friend that repatriated.)

[HCA]

The Carburatore are stromberg
The sparks plugs are standard maybe a bit hot
The fuel is not 98
The compression ratio is 9,6 after machining the head valve seat
All the other info must be measured but as far I know there are no strange figures

This just says what you have bolted together. It says nothing about how they all interact with each other. You say the fuel is not 98 - a typing error, or are you saying it is less than 98? If so, what?

Have you done anything about getting the engine properly tuned by someone who knows what they are doing? The longer you put this off, the more the engine is suffering.

Only when you know that your mixture is right, the timing is right, the spark is right, the idle speed is right, the temp is right, and you still have dieseling will you be able to say you have a real problem. As it is, it sounds like the basics are not right.

[HCA]

Not sure how timing affects it (because there's no spark involved) other than a highly advanced spark usually means a hotter engine.

The timing has no effect on the actual procedure of dieseling - as you say there is no spark.

The effect it has is on the cause - too advanced leads to a hotter engine, too retarded and the throttle idle speed needs to be increased to keep it all running thus allowing more fuel to enter when switched off.

If the 3.8 was bad for running on, then the 2.8 as fitted to the early XJ6 was a nightmare - and being automatic, one could not induce a stall to stop it! Many broken crankshafts and they moved onto the better 4.2.

[el-saturn]

my solution is ev. too practical or simple ------------------------ too many of you are indoctrinated!!!! sandy

[HCA]

my solution is ev. too practical or simple ------------------------ too many of you are indoctrinated!!!! sandy

The difference here Sandy is that the OP stated the car runs well. This would not happen if he had the plug leads incorrect. If he had rough running, then definitely a case of sorting it out!

[el-saturn]

mine runs well and showed exactly those symptoms!? - result: one switch of wrong pin and it turned off.... sandy