Running on - what causes it?

[nbracken]

My car has started to run on when I kill the ignition, but on an intermittent basis. Timing is spot on. I also believe that idle screws are set correctly (I have a SB Chevy ZZ4 with a Holley 600 vac secs). Tickover may be a tad high at about 900-1000.

Are there other things I should be looking for or is it just the tickover now at fault?

Tks

[STEVE G]

Mine does it too occasionally,only for a second or two,but I think its when I've give it a thrash and the plugs may be glowing abit.I heard that a top fuel engine burns out the plugs within a second and runs like a diesil until the fuel runs out(or engine detonates).Sounds possible we have a similar but less terminal effect.
Steve G {-}
GE427-Rover 281ci.(300+BHP)

[r0lex]

Normally running on I think is caused by timing being to advanced. If you get to much of this you can blow holes in the pistons. oops

Paul

[SimonP]

Nigel,
Have you altered anything on the car - including the fuel?

It could be that the last fuel you filled up with was a little below spec or something... or have you had more than one tankful from differing sources and it's still the same?

If your engine will cope with a lower idle speed, slow it down a little. That may be all that's needed.

Other than that, look at idle mixture - aim for about 2.5-3.5%CO.
Check for any vacuum leaks - eg vac. advance line detached, or a leaky brake servo.

Check the colour of the spark plugs after a run - that should point to any incorrect fuelling condition.

Is your engine running even the slightest bit hotter than it used to?

Also you could have some buildup of carbon, which is glowing a bit. If it's not severe you may be able to clear it with something like neat Redex.
If you have access to a compression tester, you can check if your compression has gone up a bit, which would point to deposits.

Does this help?

Simon.

[nbracken]

Hi guys thanks for the tips - to answer some of the specfics:

Fuel is same as usual 95 octane.

I will slow down the idle and see if it helps. When I think back the prob started after I altered the idle when it was too slow - I almost had to have my foot lightly on the throttle to stop it stalling.

Vacuum is OK and the engine is running no hotter than normal.

I will take a look at the plugs and maybe do a swill through with some Redex. My feeling is the engine does tend to run on the richer side anyway and so their could be some build up starting to occur.

Thanks again.

[osgood]

Contrary Mary

Nigel, running on is caused by something still glowing inside thus igniting the fuel. Retarded ignition also causes this as the engine runs a lot hotter.

There is an anti-running on valve which can be purchased if the problem cant be checked. The valve opens when the engine is switched off, draws in air and kills the engine regards Eric.

[kdavies3]

I think your biggest problem will be the high idle speed.
My 383 is the same unless I bring the idle down to around 750 rpm.
It makes it lumpy with the high profile cam, but I'd rather put up with that than have it run on every I stop anywhere hot.

http://animatedgif.net/flags/-flags-uncat/Wales.gif

Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales

[rocket]

Run on has nothing to do with idle speed.

It is either timing or maybe vac leak.

If you engine is very old carbon build up sometimes causes it (still glowing)

Rgds

Roger

[tallyho]

Originally Posted by rocket

Run on has nothing to do with idle speed.

It is either timing or maybe vac leak.

If you engine is very old carbon build up sometimes causes it (still glowing)

Rgds

Roger

Along similar lines - I took the beast out for a long run today for the first time and noticed after about 100 miles of motorway cruising when at lights idle had gone up from around 1000 rpm to 1500 rpm. At first a quick blip on the throttle seen to reset it, but after a while this didn't work. After switching off I did get some run on. But on restarting it was back to 1000 rpm. Any ideas please??

[mylesdw]

Originally Posted by tallyho

Along similar lines - I took the beast out for a long run today for the first time and noticed after about 100 miles of motorway cruising when at lights idle had gone up from around 1000 rpm to 1500 rpm. At first a quick blip on the throttle seen to reset it, but after a while this didn't work. After switching off I did get some run on. But on restarting it was back to 1000 rpm. Any ideas please??

Probably ice in the carburetor.