I've got a mild 351, and when you turn the key off, it fires the motor round once more.
timing is 6 degrees befort TDC.
could it be a too small gap in the plugs?
would advancing it 4-5 degrees fix it?
thanks for your help in advance.

It is not uncommon for the outer ring of the harmonic balancer to have slipped so pull the number one plug and get the piston to top dead centre .
( can be done by feel with a screwdriver gently inserted while hand turning the motor . you will feel it rise to the top and then start to drop so take it back that fraction to TDC) check the mark on the balancer and you can be now sure you have the right timing or if not you may have to compensate. Failing this being your simple solution If it is an old engine an excessive carbon build up can cause it. Take the air filter off with the engine already really hot and bring the revs up to about 2000rpm and with a squirter bottly give a few good quick squirts straight down the carby and this will generally free up a lot of carbon by rapid cooling but is not enough cooling to hurt the head. Pull the dizzy cap and look down while turning the motor until you view one at a time the pig tail springs under the dizzy base plate. If these are really loose on the post then this or even a worn dizzy can also cause it. When you check your plugs if they have a hard brown crusty build up then your valve guides are worn and this could also have a bearing. build on plugs by worn guides or rings can sometimes be helped by talking to the local parts shop and stepping down two in the heat range but be careful if you go too cold a plug it is just as bad as it wont burn the crap off the plug. Hoping some of this may have helped you and didnt give you a headache. Take it easy.

Has the carby got a anti dieseling slenoid? these are suppose to prevent it, or what was siad above, if its that coked up, it can do it.

thanks for the replies.
the motor is 700km old, it has new plugs and leads.
carby is a holley with idle-stepper motor for a/c and cold starts.



what is an anti-dieseling solenoid?
I'll check the dizzy in the coming days, im willing to place bets that this is the problem.
It is one of the only things that hasn't been rebuilt or replaced, along with the coil.

Anti dieseling solenoid, is a device that cuts the fuel off to the bowl, when ign is turned off.