Urgent Overrun problem

[mrtim]

So the car has to go through the MOT tomorrow and it has started to overrun when I turn it off with the key.

I have let it go for about 60 seconds and it still runs (albeit badly) when the key is turned off. I now have to stall it to stop the engine.

Please tell me what I can do to fix it.

spec
1.6 bottom end
1.4 sr standard head, cam and carb ( the webers were givin me problems)


The auto choke was sticking on so i disconnected it.
I have tried with the timing set at both 5 and 10 deg BTDC does it at both.

Now 24hours left till the MOT.

mrtim.

[nutsaboutnovas]

this will sound stupid but are the lights left on when you turn the engine off?? with my dads car his engine would run normally as there was a deformed bulb and the whole car was running off this bulb as it was shorting out.... could be an earth problem....

[Manta Mad]

Check your timing, if this has gone 'out' then it may cause your running on. Might help if the timing is retarted a bit.

[mrtim]

i dont think its electrical the lights all go off.

i dont think its the timing either. ive tried adjusting it.

my mate reckons its the wrong plugs.so im gonna try them now.

Any other ideas???
Ive got about 3hrs left to fix it.

[Manta Mad]

One of the causes of running on is carbon build up in the combustion chambers, this causes remaining fuel to ignite and runs on. Unlikely though if your engine has recently been rebuilt.

Might be worth checking the mixture CO %

And on your auto choke which you have disconnected, did you just disconnect the electrical connection to the autochoke? Reason I am asking is, alot of autochokes work on the bi-metallic strip, which needs the electrical current to heat up the strip which in turn moves the choke flap to 'off' - just a thought.

Only other thing I can think of is, I remember reading a while back that some Fords had trouble with running on and Ford fitted an anti-run on valve to prevent this - only thing is, its so long ago I can't remember where about the valve was to be fitted.

[Chris LR]

My SR runs on becasue a non-GM head gasket was fitted, this casues the bits of head/block that arn't covered to get very hot, they in-turn keep igniting the fuel/air after the spark has stopped.

How did the MOT go?

[mikeoxford]

i wouldnt have thought overrun would be an MOT failure..........?

[Chris LR]

No, but in the hands of a 'dodgy' mechanic it would be.

[mikeoxford]

is this sometimes called pre 0 ignriotn as well
:?

[mikeoxford]

pre igntion even :o

[Chris LR]

Like I said I get it from time to time, about 1/10 times, It only lasts for about 5-10 seconds, 60 seconds must be sorted out.

It can happen on all cars after a very long trip, but it means that their is a combustion fault if it happens all the time and on short journeys.

[mrtim]

Hooray!!!

Car passed the MOT.

Im in N.Ireland so it was done at an MOT centre and I didnt tell them about the overrun. I just very slyly stalled it with the foot brake on. Worked a treat.

Car still does the overrun thing but I reckon its carbon build up from when the webers were on and runnin v rich. I dont care now though I'll get the standard carb off and put the webers back on.

Thanks for all your help.

mrtim

[Dicko]

run the engine as u would normaly, before you turn off the key, get open the bonet, as soon as u turn off the key go and pull the plug off the coil (be careful, maybe use an insulated tool, but i never do ;)
if it stops then its an electrical problem, if it carries on then you are getting pre-ignition, so its running like a diesel where the heat of the engine is keeping it going by pre-igniting and creating a spark/flame to ignite the fuel, this is usualy down to timing or worn parts

[mikeoxford]

:lol: :lol:

visions of mechanics shooting across garages throughout the nation

[Baz318]

Im havin same over run prob with my 1.2,i just got her,and the timing is off,so i take it that is the problem,she splutters for a few secs and then dies,must be northern ireland car problem,lol

[Aragorn]

1400 head on a 1600 bottom end will be giving higher compression than normal

not at my own pc so i cant work it out but i know CP did this and it used to run on too

i'll get the figures for you later - maybe try running it on optimax or super and see if it stops???

[mrtim]

Ive got the 14 head off and the 16 back on with the webers.

Never fixed the over run problem. Interested to know why tho.

I hav to take the 16 head off again tho. Think ive blowin a ring. Carbs need rebuilt too.


(Started this topic in december. Funny how it came back up again.)

[Aragorn]

gah shite mate

didnt notice since baz up there had pulled it onto a new page :/

[CP]

As aragorn said its cos you massively raised the CR - like up to about 13:1!!!! The smaller 1.4 combustion chamber combined with the longer stroke 1.6 engine raises it up about 3 points. At this compression the cam timing, ignition timing, combustion chamber design and temperature, general crud in the chamber from being run normally and the lowish octane rating of modern petrol will make over run pretty much inevitable.

Compression ratio is the ratio of the total volume above the piston at BDC divided by the total volume above the piston at TDC.

ie long strokes, wide bores and small combustion chambers = high CR. Bearing in mind that the 1.6 is basically the same block as the 1.4 only stroked and bored out to give the extra 200 cc its easy to see that the CR is increased massively.