14 replies to this topic

[sugarhart]

austin mini advantage
1987
my mini makes this horrible juddering whenever i turn off the engine. it use to happen not very oftern at all but now it almost everytime.

it only last a few secs but the whole engine judders asif it is still going round but there is no power 'asif i have turned the engine off to quickly or something' does anyone have an idea what this can be? it is getting worse and i am worried it is going to break something in the engine.

it's recently just had a MOT and the garage had replace the water pump but i think it was doing this before the MOT so i don't think this has anything to do with it.

[Cooperman]

It's known as 'run on' and it's very common with a lot of Minis. What happens is that after you switch off, fuel continues to burn being ignited by the residual heat in the cylinder or by a local hot-spot such as a piece of carbon in the combustion chamber. Higher compression engines tend to be the worst for this as you can get 'diesel effect' (i.e. compression ignition).
My 1310 cc 'S' almost always does this and I just stop it with the clutch down and 1st gear engaged, then let the clutch in immediately after switching off. That works evey time.
You may need to de-coke the engine by pouring Redex down the carb whilst running the engine at high revs - that's how it used to be done years ago before cleaner fuel became available.

[Nick Warwick]

i had this problem due to my mixture being too rich, try doing as cooperman said if it still happens try turning the mixture down a tad

[sugarhart]

Thanks to both your replies! i will give them a try!

[Ethel]

It's being too lean that causes run on - lean engines run hotter because of the excess oxygen. Have a look at the colour of your plugs after it has run on.

[ibrooks]

Seconded - usually lean running causes stuff inside the cylinders to get up to ignition temperatures. Timing can also have an effect.

Get it properly tuned by someone who knows what they are doing and the problem will probably go away.

If there is a large buildup of carbon then pulling the engine apart and cleaning it is the only instant fix to get rid of it. Redex down the carb is like snake oil. Once the car is tuned properly carbin deposits will naturally start to be burnt off/eroded so it will get better over time and since cylinder temperatures will have gone down it's less likely to be causing a problem anyway.

Iain

[Cooperman]

Seconded - usually lean running causes stuff inside the cylinders to get up to ignition temperatures. Timing can also have an effect.

Get it properly tuned by someone who knows what they are doing and the problem will probably go away.

If there is a large buildup of carbon then pulling the engine apart and cleaning it is the only instant fix to get rid of it. Redex down the carb is like snake oil. Once the car is tuned properly carbin deposits will naturally start to be burnt off/eroded so it will get better over time and since cylinder temperatures will have gone down it's less likely to be causing a problem anyway.

Iain

I've read before that the ignition timing has an effect, but how? Timing it the point relative to crank rotation at which the plug sparks. However, when you switch off there is no spark - that's why the engine stops - so how can that affect the running-on?
But yes, it's carbon deposits which are mainly to blame. Really high comp ratio engines do this due to 'compression-ignition'. The old Mk.1 'S' was (is) particularly prone to this.

[dklawson]

Redex is not a product I am familiar with so I cannot comment about it being snake-oil or not. However, there are ways to de-coke an engine without tearing it down.

A traditional method is to use water to remove carbon. Get the engine to operating temperature. Remove the air filter and bring the idle RPM up to about 3000 RPM. While the engine is running at high speed, use a pump water sprayer to spray mist into the carb intake. Spray as much as you can as fast as you can until the engine starts to loose RPM, then back off a bit and maintain the pumping rate. Keep this up for several minutes. Eventually you may be greeted with a cloud out the back of the car. The cloud will be a mixture of steam and carbon. Once you feel you have achieved something, return the idle speed to normal, re-fit the air cleaner, and take the car on a drive to evaporate any condensation in the exhaust.

Timing is by definition when the spark fires relative to the piston and valve positions during engine rotation. If the timing is way out then it may indeed influence an engine's tendency to run-on.

[Cooperman]

Redex is not a product I am familiar with so I cannot comment about it being snake-oil or not. However, there are ways to de-coke an engine without tearing it down.

A traditional method is to use water to remove carbon. Get the engine to operating temperature. Remove the air filter and bring the idle RPM up to about 3000 RPM. While the engine is running at high speed, use a pump water sprayer to spray mist into the carb intake. Spray as much as you can as fast as you can until the engine starts to loose RPM, then back off a bit and maintain the pumping rate. Keep this up for several minutes. Eventually you may be greeted with a cloud out the back of the car. The cloud will be a mixture of steam and carbon. Once you feel you have achieved something, return the idle speed to normal, re-fit the air cleaner, and take the car on a drive to evaporate any condensation in the exhaust.

Timing is by definition when the spark fires relative to the piston and valve positions during engine rotation. If the timing is way out then it may indeed influence an engine's tendency to run-on.

'Redex' is an old-type UK upper cylinder lubricant which used to be added to petrol (gasoline to you!) when petrol wa of poorer quality. It started after WW2 when our petrol really was of poor quality. I've used it in the past to de-coke a \Mini and it certainly worked. One poured it into the carb intake at c.3000 to 4000 rpm. It definately worked.

I'm still confused as to how the timing of the spark can affect the running-on when there is no spark during the running-on, unless incorrect timing has caused a build-up of coke in the combustion chambers and that is then causing the running-on. Running-on does not really harm an engine unless it 'windmills' backwards in which case if a tensioner is fitted it can break it.

I do like the idea of washing the coke out ands I've never heard of this before. Seems like a good way to do it. Could some washing-up liquid help there?

[ibrooks]

OK - ignition timing 101 as they say.

When the spark plug fires you do not have an instant explosion in the cylinder, you have a controlled burn. That burn takes an appreciable amount of time to actually heat up and expand which is the part of the process that we use. We want that expansion to occur just as the piston gets past top dead centre so we need to fire the plug a little bit before TDC. As the engine runs faster you need to fire the plug earlier and earlier to get the expansion happening at the right time which is why we have mechanisms on the dizzy to advance the ignition relative to engine speed (or we have an ignition map in a computer that does it for us.

Here's the relevant bit to this discussion. If the timing is too advanced the burn is going and expanding whilst the piston is still on it's way up the bore. This generates huge amounts of excess heat. In extreme cases you can hear it and it's known as pinking or more accurately pre-ignition (usually happens under heavy engine load). Let this go on too long and you'll melt a piston. However....... if it's not this extreme it will increase the cylinder temperature enough to cause the running on but without causing other obvious problems.

Incorrect ignition timing can cause running on or at least contribute to it.

Iain

[1984mini25]

Incorrect fuelling, a stuck/sticking float valve, incorrect ignition timing, too high an idle speed or running hot can all have an effect on whether the engine runs on.

[Cooperman]

Yes, that does make a lot of sense sense, but the problem with some A-series engines is, I believe, caused by the high compression which some big-engined (i.e. 1275) cars have and the amount of heat built up when the timing is optimised for max power at high revs. The old works rally cars were notorious for running on and the higher CR cars like the MG Metro had an anti-run-on valve. Some Minis might have had this as well, but I'm not absolutely sure. My 'S' has 11.2:1 CR and even though I have a very specially curved dizzy and it's really well optimised it runs on a lot, so I just put it in 1st and let the clutch up a second after I switch off. That works!

[xrocketengineer]

Here is a nice summary and explanation of all the possible causes:

http://mgaguru.com/m...ngine/ro101.htm

[dklawson]

There are work-arounds for run-on that involves no engine work. They involve Getting in the habit of stopping the engine using a different method EVERY time.

When you want to stop the engine, do not just turn the key off, try this instead.
Push in the clutch, set the handbrake, select 4th gear, press on the brake pedal.
Simultaneously as you turn the key to the off position, slowly release the clutch pedal like you were pulling away from a stop.
The engine will stall smoothly without any run-on.
This method works extremely well and does not put unacceptable stress on the engine or drivetrain.

A second method may work for you.
As you turn the key to the off position, press the accelerator to the floor.
Generally this works... however, I use the first method with the clutch.

For both methods, you have to do this every time. You cannot use either method to stop an engine that is already running-on.

[huw_jenks]

There are work-arounds for run-on that involves no engine work. They involve Getting in the habit of stopping the engine using a different method EVERY time.

When you want to stop the engine, do not just turn the key off, try this instead.
Push in the clutch, set the handbrake, select 4th gear, press on the brake pedal.
Simultaneously as you turn the key to the off position, slowly release the clutch pedal like you were pulling away from a stop.
The engine will stall smoothly without any run-on.
This method works extremely well and does not put unacceptable stress on the engine or drivetrain.

A second method may work for you.
As you turn the key to the off position, press the accelerator to the floor.
Generally this works... however, I use the first method with the clutch.

For both methods, you have to do this every time. You cannot use either method to stop an engine that is already running-on.

I've got an MG Metro which is bad for running on and I use the first method every time.

Also, if you are concerned about carbon buildup then I can recommend this stuff
PowerBoost

Edited by huw_jenks, 02 February 2011 - 04:21 PM.