Pre-ignition

[Cheggers]

Hi All,

 

The Triumph has a habit of running on after I've switched off the ignition - It's been doing it for a while but is beginning to get annoying. I know the timing & mixture are OK as I checked them myself this morning...am I right in thinking the car needs a decoke, or is there another potential cause??

 

Cheers.

[red5]

Ignition, carb (worn/still drawing fuel), idle too high, carbon build-up causing dieseling or wrong plug. Saw more ignition/carb issues on old stuff, or a combination of all of these. What carb(s) it got on it?

[garethj]

Running on isn't pre-ignition, that's called dieseling. Some cars have a solenoid on the carb which cuts off the fuel supply when you switch off the ignition. If yours has one make sure it's working by pulling the spade connector off, switch the ignition on and then touch the wire onto the solenoid, if it's working you should hear it click.

 

If your carb doesn't have one, ignore all of the above

[Des]

Running on is down to modern petrol being a bit crap, the cure is to sort of detune your engine to suit, or better, if you can find one of those Tecalmit electric air valves the later rwd Escorts had.

[simmo]

Is this your Triumph 2000? SUs or Strombergs? could the carbs be out of balance? worn main jet or a float level?

[red5]

Stombergs iirc? Holes in diaphragms ? That poxy waxstat on SU's was also favourite...

[OwdChina]

Yup, agree with Des, a lot to be said about mixing old engines with modern petrol. Lead in petrol used to do two important things, lubricate the valve stem base and prevent pre ign. Does anybody remember cap guns ? The Hammer hits the cap and the cap explodes. This what happens with unleaded petrol in an older engine and it gave British makers a huge problem when we first had unleaded forced upon us.

Basically the lead in petrol used to stop the fuel detonating under compression, which prevented chamber hot spots, pinking and run on in a correctly tuned engine. There was a way around it, but it involved big cylinder head alterations, carb and ign timing changes + lots of money.

LRP was an even diferent ballache.

[25v6turbo]

Hi

Triumph 2000s are VERY fussy about what fuel you use,best try some high octane and see if it goes,my MK1 2000 did it for a while but shell optimax sorted it,are you 100% that the timing is spot-on,i,ve had loads of Triumph 2000s over the years and still own one now,great cars!A decoke is a simple job,but depends if you want to go that route...

[Cheggers]

Thanks chaps - It's a 2000 running on twin Strombergs (with no cutoff relays). I think a carb rebuild/ diaphragm replacement may be in order, and it's probably due a decoke anyway.

[25v6turbo]

If you have a small split in the diaphragm then they mostly run like a dog,i,ve suffed that,what plugs have you got fitted,are the gaps correct and have you checked the points gap,any other symptoms?

If only you where closer to me...

[Pete-M]

Modern petrol isn't that crap. I've run Rover P6 V8s (10.5:1 CR) jobs on standard Sainsbury's spec cheapo unleaded and they've been fine provided they were set up correctly.

 

However, if you ask the anoraks they'll always tell you "Retard the ignition to just before the Neolithic Era.. it's modern petrol, engines don't like it. Life was better with lead. Oh yes. Modern petrol is metric rubbish, sold in litres instead of gallons. Anything modern is crap. Leaded fuel, Lodge plugs, a magneto for the sparks, dynamo, cable operated drum brakes, starting handle and crossply tyres. The only way to make anything reliable. Nothing else works. ".

 

Check everything, dizzy, dwell angle if you're mad enough to still be using points, timing, advance / retard mechanism, plugs, carbs, manifold gaskets, the lot. Anything that isn't right isn't good enough.

[Cheggers]

25v6 - Funnily enough, there are no other symptoms at all. The old girl runs well, no misfire, no untoward noises, nothing...

[michael1703]

Does it do it every time? What ignition timing is it running? Does it do it with foot on brake pedal?

 

Some british 60's/70's stuff can run on Electrically, you turn off the ignition, the Alt is still turning and gives a bit of juice to the coil, also if you turn the ignition off with your foot on your brake tht can cause a run on also, very common on mgbs

[Des]

Modern petrol isn't that crap. I've run Rover P6 V8s (10.5:1 CR) jobs on standard Sainsbury's spec cheapo unleaded and they've been fine provided they were set up correctly.

 

However, if you ask the anoraks they'll always tell you "Retard the ignition to just before the Neolithic Era.. it's modern petrol, engines don't like it. Life was better with lead. Oh yes. Modern petrol is metric rubbish, sold in litres instead of gallons. Anything modern is crap. Leaded fuel, Lodge plugs, a magneto for the sparks, dynamo, cable operated drum brakes, starting handle and crossply tyres. The only way to make anything reliable. Nothing else works. ".

 

Check everything, dizzy, dwell angle if you're mad enough to still be using points, timing, advance / retard mechanism, plugs, carbs, manifold gaskets, the lot. Anything that isn't right isn't good enough.

 

Actually I think you'll find that no petrol is any good unless hand cranked at the pump and sold as motor spirit in £,s,d by a war veteran (WW1) in a flat cap and tie, smoking a roll up.

[Cheggers]

Does it do it every time? What ignition timing is it running? Does it do it with foot on brake pedal?

 

Some british 60's/70's stuff can run on Electrically, you turn off the ignition, the Alt is still turning and gives a bit of juice to the coil, also if you turn the ignition off with your foot on your brake tht can cause a run on also, very common on mgbs

 

Yes

8 degrees before TDC (have tried retarded back as far as 14 degrees )

Makes no difference

 

How would having your foot on the brake make a difference??

[dollywobbler]

I'm with Pete-M. My P6 3500 was very happy on 95RON unleaded, even supermarket 'shite', despite being designed for 5 star.

 

Our Mini went through a spell of doing this very badly. MOT test revealed terrifyingly lean mixture! Richened it up and it doesn't do it any more. Lean means a hot engine, and that can make the combustion chamber hot enough to ignite the fuel and create running on. The other issue can be a tickover that's too high.

 

Do you let the engine idle for a short time before switching off?

[Pete-M]

Actually I think you'll find that no petrol is any good unless hand cranked at the pump and sold as motor spirit in £,s,d by a war veteran (WW1) in a flat cap and tie, smoking a roll up.

 

Well, obviously....

[alf892]

8 degrees before TDC (have tried retarded back as far as 14 degrees )

Makes no difference

 

 

So you advanced it......it fired sooner.

 

As others have said......make sure the idle is correct and check mixture.....play about with the timing. Could need a decoke but not if it has been running on modern oils/fuel for a while as they are loads cleaner.

[barefoot]

Similar running on problem in Beetle.

 

Crap on unleaded, spot on on proper 4 star (you can still get it around here, but its hellishly expensive) but OK on super-unleaded.

[michael1703]

the brake pedal thing is that you can get a back earth through the brake light switch which keeps a bit of juice in the coil

 

the other thing is the alt and charging light

 

when you switch off does the charging light go straight out or glow brightly as it runs on?