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Odd fault & Repair


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Hi Folks,

over the last couple of weeks my reliable TR4A has taken to doing a little cough now and then.

Something is causing the sparks to go AWOL just a tad.

 

In my mind I deduced it to the ignition. High revs above 2500 it all runs very well.

At low revs 1500 - 2000 and it will miss about one cough per minute.

At very low revs 800 -1000 it coughs much more - not enough to cause it to stop.

 

Starting has been getting worse during this period.

 

Today I was going to change parts one at a time.

The rotor arm has been insitu for about 10 years or more and looked OK.

I replaced this with a NOS rotor but not a Dizzy Doc one. The big brass rivet looked nice and clean :o

 

The engine started easily and running at mixed revs was trouble free. It actually felt smoother.

 

My theory is that at the lower engine revs the coil has a little more time to generate a decent (higher voltage) spark.

As the Rotor arm is beginning to fail this higher (low Rev) voltage was arcing through and the spark fails.

At the higher engine revs the coil voltage is a little lower and so stays where it is.

 

As I was not inside the dizzy cap I can't guarantee my theory s spot on - but the replacement arm appears to have sorted it.

 

Roger

 

 

 

 

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Hi Roger, It could also be the plugs' centre electrodes run hotter at higher rpm and so allow the spark to fire across their gap at lower voltages. So the rotor arm 'leak' cannot compete at higher rpm.

Peter

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Hi Folks,

on my long drive to the AGM today I have found yesterdays diagnosis is tosh - unless another problem has arisen.

 

Tomorrow I will replace the plugs and see what happens.

It would appear that things need to get really warm before the coughing.

 

Roger

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Hi Kevin,

it is a 20 year old Lucas :o Gold sports coil.

I haven't ruled it out but will get round to it in a logical order so that I end up knowing which component actually is responsible.

 

I'll be ordering some parts from Martin Jay tomorrow

 

Roger

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Hi Roger

Coughing engine which is worse at low revs makes me think of a leak of air on the intake side

think check the manifold gaskets of carb/inlet manifod

I have chatted about replacing manifold gaskets with high temp silicone before - but it cured my similar problem before

Good luck either way

Michael

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Are you running points Roger, of so perhaps a condenser thats breaking down when hot?

 

I once spent weeks looking for a fuel problem on a v8 in a boat that ruened out to be the condenser.....

 

 

Steve

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Hi Folks,

I hope I'm not feeling smug too soon. B)

 

I changed the plugs today and checked the manifold nuts (all nice and tight).

Not sure what I was expecting. It all went well whilst warming up. :)

 

When fully warmed up it drove faultlessly. :blink:

 

Even taking off from a standstill in 2nd there was no coughing.

 

Could it be that simple.

 

I'll keep an eye on it and report back with any feedback

 

Roger

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Ello Rodger, if ye still got yer olde plugs,

get a magnifying glass, and have a good look at the porcelane area.

 

specificly for a corona frae the metal end, and also for a wee crak /slit in it

 

as plug get hotter, the crak get bigger, so earth oot

 

It took some finding on mine, crak was barely see able and they wer a newish plug

butt, whenst run int dark, all sorts of things come to light,!!!

 

https://www.google.co.uk/search?q=cracks+in+a+spark+plug&client=firefox-b&dcr=0&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiUsazf56rXAhXiL8AKHadeCg0Q_AUICigB&biw=1368&bih=806&dpr=1.25#imgdii=jdGtBS77YZ9usM:&imgrc=mqKVLEfC8l-7EM:

M

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Rogger your diagnostic qualities are Manifold as they say, I would have had the carbs in bits, the Cam out, and head gasget changed before going for the Rotor Arm.

 

Sue must have seen a good catch a bit ago!

 

Glad its up to speed again.

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I've been chasing a missfire on a non TR.

 

I was getting a very hot ignition coil, if fact two failed...to cut a long story short, it was the voltage regulator that was regulating to 18volts. Not only did two coils fail but on their way to doing that, the extra HT was randomly breaking down various insulation in the ignition chain.

 

It's a good idea to measure the resistance across the primary side of the coil......3.2 - 3.4 ohms is good for a non ballasted coil.

 

Anything lower is a symptom of failing insulation inside the coil.....if you are down to 2 ohms, as one of mine was, then you will be pulling 6 amps instead of around 3 ish....Amps burn!

 

Also measure the voltage with the engine running above the cut in speed.

 

Dick.

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