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As wierd one


Bigjohn

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After a perfect run to this weekend's event, we had problems getting the our Bedford MW to run properly from the start of the return journey.

Several stops and a lot of head scratching showed that there was no spark on the front two plugs EXCEPT (and this is the nutty bit) when we held the plug cap in close proximity to the terminal of the spark plus.

The spark could be heard arcing from the inside of the plug cap to the nipple on the end of the plug, when this was happening the engine ran fine. However as soon as the cap was pushed onto the plug the engine ran rough as the plugs stopped sparking.

To get home we secured the two plug caps just off the plugs with wire so they could arc to the top of the plug.

 

So why would a spark plug fail to work when properly connected to the HT lead but work well when there is an arc between the plug cap and the plug?

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After a perfect run to this weekend's event, we had problems getting the our Bedford MW to run properly from the start of the return journey.

Several stops and a lot of head scratching showed that there was no spark on the front two plugs EXCEPT (and this is the nutty bit) when we held the plug cap in close proximity to the terminal of the spark plus.

The spark could be heard arcing from the inside of the plug cap to the nipple on the end of the plug, when this was happening the engine ran fine. However as soon as the cap was pushed onto the plug the engine ran rough as the plugs stopped sparking.

To get home we secured the two plug caps just off the plugs with wire so they could arc to the top of the plug.

 

So why would a spark plug fail to work when properly connected to the HT lead but work well when there is an arc between the plug cap and the plug?

 

Did you remove these two plugs to look at them? They could be fouled, oil possibly. By holding the cap away from the plug it seems to intensify the spark, hence the old trick of a shirt button in the HT lead.. Something else you can do is to swap these plugs to another position and see if the problem moves with them.

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Yeah, we tried all kinds of swapping and the plugs were only wet with fuel as you'd expect.

My only thought is that the coil is on the blink - the plugs, leads, distributor and rotor are all new (less than 200 miles).

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If all the base parts are new, why are only the two front plugs giving trouble? The only place that could occur is at the distributor cap, the only place the spark goes to each seperate plug. I'd change it and see.

 

Though!! I had a similar problem on one of my Dodges, the cables on two plugs had touched and were fusing together, check the lines are seperated, but I'd still change the Dizzy Cap.

Edited by Tony B
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It could be a faulty new distributor but it seems odd that it suddenly went on two plugs. We were getting the occasional very weak spark on the no1 pot.

My hunch is that the coil is very old and tired and that all the sparks are weak and that plugs 1&2 are the worst affected.

It won't break the bank to change the coil and see what happens.

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Like Richard said there is a sort of coil booster that adds another gap in the lead up from coil to the distributor. My thought is that the voltage has to jump two gaps anyway. From rotor arm to cap , then across base of plug. You seem to be missing the first gap on the two front plugs.

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Hi Pete, they're copper core.

 

Providing the insulation is in good condition, not cracked/soaked in oil and the terminals are good they should be OK.

 

The reason I asked what leads you were using is that I have known the modern Carbon fiber leads give exactly the problem you were describing.

 

Pete

Edited by Pete Ashby
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My MW on initial start up the engine would run like it should ,stop it and restart it ,it would misfire , splutter and sometimes not even start...........NGK plugs swapped for J8C's .............cured it .

 

Now only have to cure the steam problem :undecided:

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NEWS FLASH!

 

This problem has gone all the way to the engineers at the BBMF. They say "it's the coil".......apparently they had a similar problem on a Spitfire recently although I suspect that it wasn't discovered mid-journey!

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NEWS FLASH!

 

This problem has gone all the way to the engineers at the BBMF. They say "it's the coil".......apparently they had a similar problem on a Spitfire recently although I suspect that it wasn't discovered mid-journey!

 

I thought a Spitfire had a magneto :undecided:

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