Jump to content

Sooty plugs and running on


Recommended Posts

Having a slight problem with the running of the engine.

After a reasonably long run the engine runs on after turning off the ignition and coughs a bit out of the recyling valve on top of the rocker cover.

Also the inner two plugs seem to soot up whereas the outer two are light brown as they should be.

I have checked the mixture and timing but they seem OK.

Any ideas?

Also has anyone fitted a catch tank?

Cheers

Link to post
Share on other sites

Could be crankcase oil/fumes coming though the crankcase breather and igniting by compression ignition.

You could modify the crankcase emissions system, address the source of the crankcase pressure  (engine condition?), or just stop the engine after a long run by intentionally stalling it (bring up clutch with foot hard on  brake).

Link to post
Share on other sites

When was the engine last decoked?   
Could be a lot of carbon on the back of the valves causing the running on.

image.jpeg.cdb5f350e592bf7d48aa8b126327bd17.jpeg

image.jpeg.1a7b5afb25913fa42823137ce6645576.jpeg

Edited by BlueTR3A-5EKT
Link to post
Share on other sites

You could have a head gasket problem across the two centre pots. If it’s dryish black soot it’s probably fuel, but if it’s a waxy, deep brown scaly soot it could be oil or carbon build up as per Peter’s image above. Does it burn oil?

Kevin

Link to post
Share on other sites

Hi David,

I had the same issue from time to time, more with the engine (coal) hot,

a 71°C thermostat helped sometimes, different ignition setting or different petrol did not help.

Finally I fitted 2 "petrol stop valves" direct in front of the main jet like other producers did in front of their idle jet.

1350291036_antirunonvaves-b.jpg.b3d1a40969182b09ea4bba7dde627a7a.jpg

A hughe improvement was to fit a pair of better HS6 carbs instead of the carbs which have been on my TR4A.

And the "new" carbs did nou have that little spring loaded valves; so I cannot tell you what finally helped. 

NdVT9saMjX2WT0TzhPDD4YjidDMc-2JVzrC838W-

But with the "new" carbs I currently do not need to use my "petrol stop valves" anymore.

Ciao, Marco

Edited by Z320
Link to post
Share on other sites

Having the outer cylinders a bit weaker than the inners is quite normal and is probably due to the intake geometry. Nothing to do with burning oil or blown head gaskets, it's just the way this engine seems to work. If the  inners are too black, perhaps you may need to weaken the mixture just a tad.  

As for the running-on , what spark plugs are you using?  They can affect this. 

 

 

Edited by RobH
Link to post
Share on other sites

In my opionion: if you switch off the ignition, and the engine indeed want's to stop because there are no igmition sparks anymore,

but does not stop, it does not matter how the ignition was or is adjusted before you switched it off?

Is there anything wrong in my thought?

Edited by Z320
Link to post
Share on other sites
1 hour ago, Z320 said:

In my opionion: if you switch off the ignition, and the engine indeed want's to stop because there are no igmition sparks anymore,

but does not stop, it does not matter how the ignition was or is adjusted before you switched it off?

Is there anything wrong in my thought?

No.  
The hot carbon/coke/coal deposits in the engine glows hot after running and ignites the incoming fuel/air mixture.

Edited by BlueTR3A-5EKT
Link to post
Share on other sites

Running on must be one of the most common complaints!    A search here finds nearly 1300 posts, not including this thread, dating back twenty years!

Again, for some reason I cannot post a link, but go the 'Search' button and use "running on", in quotes to search with.

It's such a common complaint that a special page might be worthwhile, listing all the found causes and their cures!

John

Edited by john.r.davies
Link to post
Share on other sites

Running on increases with hot engines, the ignition is without a spark only from the hot parts/coal AND petrol.

This is why most produces fitted a "fuel stop" or "air stop" valve in the line to the idle jet - sadly SUs don't have one.

For example the air cooled VW beetle always had a "fuel stop valve" (laud to Ferdinand Porsche).

In my case worn SUs (I never believed it) caused the problem.

Edited by Z320
Link to post
Share on other sites
1 hour ago, Z320 said:

In my opionion: if you switch off the ignition, and the engine indeed want's to stop because there are no igmition sparks anymore,

but does not stop, it does not matter how the ignition was or is adjusted before you switched it off?

Is there anything wrong in my thought?

If I interpret your question correctly the answer is this. If the ignition timing is retarded too far the engine cylinder temperature is increased, this can cause the engine to run on (dieseling) when the ignition is switched off. Advanced ignition has the same effect but usually if the ignition is too far advance you would hear the engine "pinking" under load.

Link to post
Share on other sites

Hi Phil,

you are absolutely right: a hot engine because of a wrong adjusted ignition can cause running on.

But with a ignition adjusted "about" correct some degrees difference causes nothing. 

I played on the screw for a long time...

Ciao, Marco

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.