engine run on.....

[digger]

on my mates chevy when he turns the key off the engine runs on for 10-25 seconds, perfectly running then stops as it should.
we are baffled, i have had run on before but that usually is spluttering...
He has checked the wireing etc.

any suggestions?

[Plumpcars]

Been fighting this on a mate's small block since the heads/cam/carbs were changed.
Make sure the idle speed is slow enough first of all. Is the engine stock? What spec? Run on is normally caused by a hot spot igniting fuel so make sure you have the right plugs. We are currently waiting for a colder set to arrive from The States.

[Plumpcars]

Also what car/ignition/wiring/alternator is he using?

[Roscobbc]

An overly rich idle mixture can cause this, especially if a 'hot' cam has dictated the need for a richer idle mixture to enable the engine to 'tick-over'. The carbon deposits left by the rich mixture can create hot spots either in the combustion chamber or on the spark plugs, possibly change the ignition curve to get more advance at idle (retarded ignition creates heat within the combustion chamber) - perhaps a carb 'work-out' on idle jets & idle bleed jets (if a high spec carb)

[satan]

is it fitted with msd ignition?

[Col]

Are you saying turning the key doesn't cut the spark straight away, or that it is true running on, ie "dieseling" because they are caused by 2 different things?

[digger]

Are you saying turning the key doesn't cut the spark straight away, or that it is true running on, ie "dieseling" because they are caused by 2 different things?

yep this is the case....turn key off dosnt stop engine.

cheers for the responses

[Col]

In that case sounds like you have a "back feed" to the ign coil poss from alternator or maybe key switch itself. I have seen this when a ballast resistor was somehow wired through the acc position on keyswitch instead of the ign position although I forget the exact details, but coil was being back fed from the radio I think.
Re check the wiring to the keyswitch, the switch itself and the alternator warning light and make sure the feed to the coil goes dead as soon as engine is turned off (meter across from + of coil to earth assuming neg earth and std type coil ignition. Should go to zero when key turned off)

[Plumpcars]

Also what car/ignition/wiring/alternator is he using?

It would help to know this ? On a Chevy it's possible to get a backfeed from the alternator which can cuse certain ignitions to still run. As satan asked it happens with MSD units. If the alternator wiring is non standard it can cause it as a resistance wire can be missing.

[digger]

HEI ignition, 8mm leads, accel plugs, one wire gm.

How do you test the hei ?

cheers