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Festive Triumph misfire

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I've taken the Toledo 150 miles south for Christmas, but am now faced with a bit of an issue.

 

As a present, the Toledo has given me 3 plugs firing, 1 plug not.

 

The engine (1296cc Spitfire MK4) has started misfiring at idle. It's only the one plug, and it seems to start firing at higher revs - I've had the spark plug light tester thing on it.

I've tried a few different plugs, a different HT lead and a different rotor and dizzy cap - but still exactly the same on that cylinder! Any ideas?

It's still drivable - I just have to not let the revs drop at junctions - but it's certainly not ideal!

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I've taken the Toledo 150 miles south for Christmas, but am now faced with a bit of an issue.

 

As a present, the Toledo has given me 3 plugs firing, 1 plug not.

 

The engine (1296cc Spitfire MK4) has started misfiring at idle. It's only the one plug, and it seems to start firing at higher revs - I've had the spark plug light tester thing on it.

I've tried a few different plugs, a different HT lead and a different rotor and dizzy cap - but still exactly the same on that cylinder! Any ideas?

It's still drivable - I just have to not let the revs drop at junctions - but it's certainly not ideal!

 

Try swapping the plug with one of the others........

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I'd also suspect valves: you've changed all of the ignition system components to that plug, and you still have the same consistent problem.

 

Not hard on that engine to whip the rocker cover off and see what the clearances are on that cylinder.

 

However, if that plug is actually not sparking at idle then you've not found the ignition component causing the problem. I would go through changing the components one at a time again, methodically, and also try a different condenser if you have one.

 

Is the distributor body tightly clamped, ie can it be turned? Could the timing have slipped in that way?

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Guest Hooli

Give the engine to Captain70s & he'll diagnose it in the months to come.

 

More seriously, valves sound a good shout to me too.

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Seems alright - the rotor doesn't wobble if that's what you mean?

Yes, I was thinking that there was something that was causing the distributor cam/points follower heel to ‘wobble’ at the same spot on each rotation.

 

With the engine running can you get the lead to the non-firing plug to spark? This would eliminate the ignition as being the source of the trouble.

 

Squirrel2

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Yes, I was thinking that there was something that was causing the distributor cam/points follower heel to ‘wobble’ at the same spot on each rotation.

With the engine running can you get the lead to the non-firing plug to spark? This would eliminate the ignition as being the source of the trouble.

Squirrel2

Yep - when the lead is swapped to a different plug, it fires every time. Just doesn't seem to like this plug/cylinder combo.

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The distributor cap? Each cylinder uses a different section of the cap - is the stud for that cylinder damaged?

 

If the points gap has closed up it MAY be that tolerances between the 4 cam lobes allow 3 sparks but not the fourth and just at low speeds?

 

Me? I'd check the valve clearances next.

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I’ve had similar issues with my old chod caused by shit quality distributor caps where the contacts inside rapidly wear and the minute metal particles off these make misfiring and running problems, the last time it happened was with the Triumph 2000 and the car was hardly driveable, a new cap sorted it...

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I'd also suspect valves: you've changed all of the ignition system components to that plug, and you still have the same consistent problem.

 

Not hard on that engine to whip the rocker cover off and see what the clearances are on that cylinder.

 

However, if that plug is actually not sparking at idle then you've not found the ignition component causing the problem. I would go through changing the components one at a time again, methodically, and also try a different condenser if you have one.

 

Is the distributor body tightly clamped, ie can it be turned? Could the timing have slipped in that way?

OK. I'd still do the things I suggested in my previous post.

 

What's the battery like? Is the engine earth lead in good condition? Is the battery earth lead good?

The distributor cap? Each cylinder uses a different section of the cap - is the stud for that cylinder damaged?

 

If the points gap has closed up it MAY be that tolerances between the 4 cam lobes allow 3 sparks but not the fourth and just at low speeds?

 

Me? I'd check the valve clearances next.

Thanks all, I've checked the valve clearances and they all seem good.

 

It's got an electronic ignition, so no points gap to be checked.

 

I've tried a couple of spare dizzy caps, same end result.

 

All engine earth cables are good.

 

My thinking is it must be the grounding of the spark plug (based on the video below attached), as it fires when grounded elsewhere - but what can I do apart from clean the thread (which I've already done), try a different spark plug, check connection between HT lead and plug is good, check plug gap...? Confusion...

 

 

 

UPDATE: having written this out and waiting for a file to upload, I cleaned the plug holes again and cleaned the end connections on the leads...now it's all good! Took for a test drive and it quickly relapsed, but another clean and all good again...think I'll be replacing those leads (again)...

Thanks all and merry Christmas!

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