Engine "runs on" after ignition is switched off

[markcro]

After a run my GT6 Mk. 3 engine will run on for up to 5 seconds after I switch the ignition off. This is bad as it is very rough and it knocking. I have started to dump the clutch in gear to stop it. What is the cause of this? Or more importantly, what is the cure?
Thanks for any advice.

[Dannyb]

heck your timing and make sure you are not running too lean.

[Velocita Rosso]

..and using the clutch too much to kill the engine, will eventually result in breaking the friction plate/springs
Your timing is out and or your mixture could be out and or you might need a de coke
Start with the simplest  

[Tinweevil]

Fundamentally running on is caused by your engine running like a diesel, i.e. ignition is a spontanious reaction to compression and heat.  In your engine the it is likely that there is a build up of carbon in the chamber, a spot on that is getting hot enough to be your ignition source.  Usually it's a sharp edge of carbon and the good news is that often it's on the plugs.

First thing to do is look at your spark plugs and replace them if thay have significant carbon build up.  Tune the engine (ignition and carbs) based on the colour of your existing or new plugs.  If all that fails then a de-coke of the head will be needed.

[WIMPUS]

Mine had it sometimes !
But that was because of the timing & co (dizzy was **** ).

my friend had seen a few weeks ago a TR4 (or was it a tr5  :B ), when the guy switch it off the engine started to turn the other way around  ??)  :o

[Kev W]

Mine does this if I don't run it on Shell V-Max or some such other high octane fuel (but it likes v-max the best, like Stella it's reassuringly expensive!), I know it's running a bit rich at the moment but I'm waiting for the guy at Mini Sport in Burnley to come back off the sick so I can get it on the rolling road (he's only been off for 2 months.....), I think he was last seen tuning one of our members GT6....  :X

[markcro]

Thanks guys as always! :-)
Unfortunately in Ireland we only get watery 95 Octane petrol. Stations stopped importing 98 a good few years ago as it was taxed too highly.  :'

[Steve P]

Mine  runs like a pig and runs on if i put 95 octane in it.Remember these engines were designed for 100 octane 5 star.

You might try some octane booster though some people think they are snake oil(i don`t).
Steve

[markcro]

I currently use a combined lead and octane booster bottle of snake oil. :-)

[PaulB]

I run my spit on 98 unleaded as it works out cheaper. As I get at least 10-15% more mpg and it is always less than 8% more in price.

[WIMPUS]

I and my friend that has 3 triumphs always get 98 octane here ..

I found out in the beginning he was running a bit better and had more power  :)
The garage where my spitfire was painted & co said to use shell v-power ..

Did it one time, idle was a lot higher then and the car seemed not so happy  :o

[don cook1]

My running on was due to the head needing a good de-coke.

[Chasj]

Re. snake oil additives.

I used to use Valvemaster Plus, but have now changed to Millers as it not only covers lack of lead and octane boost but also combats the ethanol issues.

Engine, Mk IV, runes better.

Charles.

[AGC]

Mine (Mk3) runs on a bit too. Have has full service and tune up recently so will try 98 octane fuel to see if that improves things.

[RobPearce]

Most of the previous replies are true and helpful, but there's one thing to add.

Idle speed.

If your car idles too fast it will be massively more prone to running on. Make sure the tickover is just comfortable and not fast. Neither my M3 GT6 nor my Mk1 Vitesse suffer from run-on when the idle is right, even on 95ULG.

[nang]

9555 wrote:

Mine (Mk3) runs on a bit too. Have has full service and tune up recently so will try 98 octane fuel to see if that improves things.

I almost guarantee that it will.but you may need to advance the timing a bit. (woot)
Tony.

[markcro]

9716 wrote:

Most of the previous replies are true and helpful, but there's one thing to add.

Idle speed.

If your car idles too fast it will be massively more prone to running on. Make sure the tickover is just comfortable and not fast. Neither my M3 GT6 nor my Mk1 Vitesse suffer from run-on when the idle is right, even on 95ULG.

Good point. She idles fast at 1000rpm at the moment. I'll knock it back to 800rpm

[ferny]

An air leak can also cause it to run on. Normal culprits are the breather hoses from the rocker box to the carbs. Make sure they have clamps on there and not simply pushed on.