engine keeps running

[Bruce Bowker]

What are the possible causes that makes an engine keep running after the ignition is switched off? It stops in a few seconds but bangs and clangs and does not like it.

It is a '56 100M.

Thanks

Bruce

 

[aeronca65t]

Often, due to idle speed that is too high or vacuum leak in intake manifold.
And check timing.
Also, could be carbon build-up on head/valves (you may be able to reduce carbon build-up by giving it a good, hard run!).

 

[LanceLyon]

What octane fuel are you using? Lower octane (97) fuel will be more prone to dieseling, especially when hot.

 

[Dave Russell]

Bruce,
Nial & Lyon are both correct. Even small changes in fuel mixture, fuel octane, timing, can affect this problem.

see this link;
https://www.faqs.org/faqs/autos/gasoline-faq/part3/section-2.html
last paragraph of 8.2
Quoted below:
"Run-on is the tendency of an engine to continue running after the ignition
has been switched off. It is usually caused by the spontaneous ignition of
the fuel-air mixture, rather than by surface ignition from hotspots or
deposits, as commonly believed. The narrow range of conditions for
spontaneous ignition of the fuel-air mixture ( engine speed, charge
temperature, cylinder pressure ) may be created when the engine is switched
off. The engine may refire, thus taking the conditions out of the critical
range for a couple of cycles, and then refire again, until overall cooling
of the engine drops it out of the critical region. The octane rating of the
fuel is the appropriate parameter, and it is not rare for an engine to
require a higher Octane fuel to prevent run-on than to avoid knock [27,28]."

D

[ 03-30-2004: Message edited by: Dave Russell ]</p>

 

[Johnny]

A friend of mine had the same problem with a '56 100/4. He thought it was supposed to do that, even devised a method of compensating by placing car in 2nd gear and releasing clutch to kill the motor. After a thorough tuneup, including rebuilding the carburettors and adjusting the valves, it no longer "run-on." Even with 97 octane fuel the running on was minor after a long drive. Keep at it, you'll find out what's causing the problem.
Good Luck.

 

[Bruce Bowker]

Thank you all. I am using high octane. I'll check some of the other suggestions also.

 

[John Loftus]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Johnny:
devised a method of compensating by placing car in 2nd gear and releasing clutch to kill the motor. <hr></blockquote>

I was able to get most of the run-on problem controlled through timing but it still happened occasionally when the motor was very hot. The 'concours technique' for shutting off the motor in front of the judge

is to keep one foot on the brake, slowly let up the clutch while in second gear until you just start to feel/hear the motor lose rpm and then turn off the ignition switch.

Cheers,
John

 

[vagt6]

My GT6+ had engine run-on until I reduced the idle rpm about 150rpm to 750. The lower idle did the trick.

Don't know if this will work on your Healy, but it was an easy fix for my six.

I love easy fixes.

 

[Dave Russell]

As stated, when the engine is running on it is in a very delicate state of balance between the various settings as evidenced by it barely being able to run. Almost any slight change can possibly stop the condition. Idle speed, mixture, timing, fuel, or temperature. Strangely, my Hundred has never come close to run on (dieseling). I think I might try for a long time to find the exact conditions that would cause it, though it obviously does happen.
D