9 replies to this topic

[minimad74]

Hi Guys,

 

I have a 1977 1275 GT, which has just had a full engine rebuild and rebore to 1293cc. It now has a stage 1 exhaust/maniflow/K&N and new needle. It has also been fitted with a mk3 cooper s head. I have set the timing to the standard spec as per haynes manual of 13 Degrees BTDC, which the car runs great, and has no pinking etc, but when ignition turned off, it will run on for a couple of seconds. Would retarding the timing to say 15 degrees BTDC be better? Thanks.

[RODZMINI71]

Is your idle set correctly ?

[carbon]

Have you set timing as 13 deg BTDC as static or at 1000 rpm?

 

Changing to 15 deg BTDC will be advancing the ignition. As Rod suggests check the idle rpm, if this is set too high then the engine may show increased tendency to run on when ignition is switched off.

 

When you did the rebore what pistons did you fit? Reason for asking is the original 1275GT used lower compression pistons which gave 8.8:1 and this would be unlikely to show any running-on when switched off. If you have fitted 21253 or similar higher compression pistons then the advance ccurve in your 1275GT distributor will also not be correct.

[minimad74]

Thanks for replys, I have set the timing at 1000rpm, and I did disconnect the vacuum tube, but I forgot to block it off! Will check the idle next week, come to think of it, It may be a little high, 1050rpm roughly. I did all the work expect the rebore, which was done by a machine shop. They are non genuine standard replacements +20. Would the 13 deg BTDC, still be ok, after the other adjustments are redone?

[Cooperman]

When you switch off there is no ignition, so the timing alone will not cause 'running on'. However, if the timing is incorrect it can cause heating in the combustion chamber of carbon deposits which can, in turn, cause 'dieselling'.

Running on is very common in Minis with higher compression ratios.

[Spider]

When setting the Ignition Timing, as per the manual, you do need to disconnect the Vac Advance.

[dklawson]

My suggestion takes a different direction.  Assuming you are otherwise happy with the way the car is running (easy starting, good acceleration, good fuel economy...) address the symptom and leave the root cause alone.  

 

Get in the habit of doing the following EVERY time you switch the key off to stop the engine.

Push in the clutch and select 4th gear.

As you turn the key off, slowly release the clutch so the engine stalls as power to the ignition is cut.

 

This will stop the run-on before it starts.  If you do this every time you switch off the engine will stop smoothly every time without incident and no... it does not put undue stress on any of the components.  Alternatively you could try better fuel, a slightly richer mixture, changing the ignition timing, colder spark plugs, and in older engines... removing the carbon from the combustion chambers.

[Dusky]

Is running on actually dangerous?

[dklawson]

In run-on or dieseling the burning of fuel is not controlled.  Ignition can happen at times that create very high loads on the crank bearings.  Therefore, it should be stopped or prevented one way or another.  

 

You can address all the probable root causes and hopefully stop the run-on or you can address the symptoms using the stall method I suggested.

[Cooperman]

The big problem with dieseling is when the engine runs backwards If a timing chain tensioner is fitted the tensioner pad is given a sharp backwards load which can cause it to split

The method Doug advocates is what many of us who have higher compression engines use to stop this and it works fine in all cases.