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Gt6 mk3 misfire


Dolomitejohn
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Hi again. It's never ending....... ... I have now clocked up 70 miles and on 2 separate occasions it has developed a misfire but only when engine gets thoroughly warmed through.  I thought I found the culpret last week (corroded King lead connection in dissy cap). But today it did it again after 6 miles and an engine off soak. I recon its electrical as the misfire is very sudden and kinda sharp. 

Spark plugs look new, coil is my old dolomite gold lucas sports cv oil, dizzy cap looks new as does points and condensor.  Ht leads...... Look OK and look new but I have not measured resistance yet. 

Any specific gt6 causes? 

Cheers all. 

John 

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Hi tony

Yes was thinking this might be culpret, but have never had one fail before. 

But the issue arises when hot ad usually after a switch off and soak for say 45 mins.  So it makes sense as condensor would take a while to heat through. 

Cheers

John 

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Just had a thought. I fitted my used but good dolomite sprint sports coil (for ballast system) as the coil fitted on the gt6 looked rough. 

Does the gt6 mk3 have a ballast ignition system?   Have I fitted an incorrect coil which is breaking down due to heat and over voltage? 

John 

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GT6 Mk3 should have a ballast system. Early ones had a ceramic resistor bolted next to the coil. Late ones use a resistive wire in the loom. The changeover was around the same time as the swing spring, though I wouldn't guarantee it lines up exactly.

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In general I reckon the GT6 has a  ceramic resistor next to the coil  to drop the voltage

Later spitfires have the dropper dull pink white feed wire

 Ic you have neither then its a fair gamble you are over loading the ballast   type coil   you need to check the coil feed voltage with the 

Points closed 

If you do have a 6v coil on 12v then   you sort of double  the HT beyond the cap and rotor capacity and the doubled amperage at the points burns the contacts and cooks the condenser

And it  will get excessively hot 

If that helps 

Pete

Andnthisnsilly tablet doesnt loadnrepliesn  ornhave spaces

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Late GT6 can have a white/pink resistive wire which does not go to the coil directly. Instead of joining the white/yellow wire at the coil it joins the other end of the white/yellow on the starter solenoid.   

Now might be the time to consider electronic ignition, but beware that some of the cheaper ones don't work with ballasted systems. I have an Aldon, which is specifically for a late GT6 ballasted system.

Doug

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HI Doug.  I am not in front of the car right now, but I know from a photo of the solenoid that there is a total of 8 wires connected (includes the large cables).

From the photo I can;t see white / pink.   I can see white / red (or brown)  and white & another colour (not able to clarify from photo).

Both of these wires are on opposites to each other. See photo attached here .

My Chassis no is KE22191  so not sure if this should have a Ballast or not.

I was planning on ordering a new coil at lunch, but I may have to wait till I can double check at home....unless you can advise from the photo.   (photo taken prior to bulkhead refurb and new soleniod being fitted. )

 

Cheers Doug.

JOhn

IMG_20181220_114722[1].jpg

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The "white and some other colour" that's rather hidden in the photo is the ballast bypass. I would expect your car, being a late one, to have the resistive wire in the loom. It may be thoroughly hidden, or it may join the white/yellow at that can't-quite-see-it point in the photo.

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hot coil is a lot easier to swap,  certainly tight valve clearances will cause a few hic ups  but the engine has no idea if it has unleaded exh.  seats or not so thats a myth

burnt valve/ seat will miss all the time when its gone its......gone 

Pete

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Hi All again.

So I have checked my car and I reckon I have a ballast resistor.  The Haynes manual shows the wiring and this seems to match what I have.

I have measured the resistance of 3 coils.

1) Coil currently fitted to GT6   (previously removed from my Dolomite Sprint and known to be ok on the Dolomite).  Sports coil Lucas.  Gold in colour.  3 Ohm

2) Coil currently fitted to Dolomite Sprint. Badged as sports Lucas coil. New this year.  Silver in colour.  2 Ohm.

3) Original Coil removed from GT6. Very old. Working.   Lucas. Silver in colour.  2 Ohm.

So would the 3 Ohm coil run hotter and potentially be overheating and causing the misfire when the engine has thoroughly warmed through?

I measured the voltage at the end of the wire that connects to the + on the coil on both cars. In both cases this is battery voltage.  I assume the ballast resistor limits the current when in use..

Thanks All.

 

 

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As a definitive answer to whether you have a ballast system or not you need to measure the voltage at the positive terminal of the coil with the engine running. Anything under 12v indicates a ballast system which needs a coil with a resistance of around 1.5 ohms. The resistor reduces the supply voltage to the coil after starting so a 3 ohm type would give a weak spark

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Actually you should not measure when running. Turn the ignition on but don't start the engine. Check the coil -ve (contact breaker) is at 0V and measure across the coil. If you see around 8V then you have a non-ballast coil on a ballast system. 12V means it's a non-ballast system. 6V means a correctly assembled ballast system. (All assuming the battery is at 12V). Since you've measured the coil as 3 ohm it's not going to be 6V.

Having a non-ballast coil on a ballast system won't make the coil overheat. However, it will lead to weaker than intended spark and a hot engine is probably more susceptible to misfire because of that.

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Do make sure no spark plugs have a R (resistive) suffix

Guess youve read the problems that seem to be appearing on non glazed plug ceramics giving some grief.

The fact the coil is hot is not fully indicative of a fault,  when its engine mounted it takes on engine temperature apart from waste energy

Are you sure its not running a tad lean ??

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We came across a spitty non starter with a new look alike lucas45d4 where the points were not make and breaking

These are the clip on leads nothing to incorrectly fit but   showed as permanently  closed when obviously open

Examination and meter checks revealed no fault but as fitted do not break,   fitted some old set and   varvar room..

These points look genuine as per lucas, with no  reason found to explain the closed circuit, and its not the condenser shorting

Pete

 

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