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Engine runs better with plug leads slightly off?


Davewillb
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Hi,

I've been struggling to get my 2.25 petrol to run without backfiring, spitting back through the carb etc. I've swapped the carb for a good spare, set the timing, replaced the condenser, HT leads and points and that's helped a bit but its not drivable still and wants the choke pulled out to run. Today I was pulling the spark plug leads one at a time with the engine running and realised that three of the four cylinders actually run better with the HT lead pulled slightly off the plug so that it arcs to the plug top cap - is this plug problems or the coil possibly?

Dave

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The only thing that occurs to me is that you are making the system reach a higher voltage before the arc commences, and I don't know why but I wonder if a snappier spark is helping - perhaps there is something that is leaking the charge away with the caps in place that doesn't get a chance when the cap is a little displaced?

I will watch this thread with interest since I would love to know the definitive answer :)

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The only thing that occurs to me is that you are making the system reach a higher voltage before the arc commences, and I don't know why but I wonder if a snappier spark is helping - perhaps there is something that is leaking the charge away with the caps in place that doesn't get a chance when the cap is a little displaced?

I will watch this thread with interest since I would love to know the definitive answer :)

You are right in what you say. Back in the "old days" a device called a spark intensifier was regularly advertised in various motoring mags. A small tube resembling an in line radio suppressor (when plug leads had real wire in them) was let into the HT. lead. This miracle device contained only a spark gap. Similar results could be obtained by cutting an HT lead and re-connecting via the opposite holes of a button. These tricks were usually done to clear persistantly oily plugs (cheaper than an engine re-build!)

A little useless info for you!

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You are right in what you say. Back in the "old days" a device called a spark intensifier was regularly advertised in various motoring mags. A small tube resembling an in line radio suppressor (when plug leads had real wire in them) was let into the HT. lead. This miracle device contained only a spark gap. Similar results could be obtained by cutting an HT lead and re-connecting via the opposite holes of a button. These tricks were usually done to clear persistantly oily plugs (cheaper than an engine re-build!)

A little useless info for you!

An old LR Series 1 engine I had, had them built into the plug caps that were fitted but I don't know if it was standard.

Marc.

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My guess would be that the insulation inside the plugs has started to break down. Some of the current that should be in the spark is being lost inside the plug body.

By pulling the leads off and introducing an external spark gap, you are reducing the voltage seen by the spark plug - by enough to reduce the internal arcing / loss.

Replace the plugs and it will go away!

Si

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You are right in what you say. Back in the "old days" a device called a spark intensifier was regularly advertised in various motoring mags. A small tube resembling an in line radio suppressor (when plug leads had real wire in them) was let into the HT. lead. This miracle device contained only a spark gap. Similar results could be obtained by cutting an HT lead and re-connecting via the opposite holes of a button. These tricks were usually done to clear persistantly oily plugs (cheaper than an engine re-build!)

A little useless info for you!

In the olden days , ie 1970s, you used to able to buy plugs with the gap built in even.

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All this talk about spark intensifiers has got me thinking. My 3.9 has an amplifier module strapped to the side of the distributor. Have I just paid LR £116 for what is effectively fresh air in a little black case?

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All this talk about spark intensifiers has got me thinking. My 3.9 has an amplifier module strapped to the side of the distributor. Have I just paid LR £116 for what is effectively fresh air in a little black case?

No - different kettle of fish. You've probably paid LR £116 for something you could have got from Maplin for £11, plus a box. My link

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My guess would be that the insulation inside the plugs has started to break down. Some of the current that should be in the spark is being lost inside the plug body.

By pulling the leads off and introducing an external spark gap, you are reducing the voltage seen by the spark plug - by enough to reduce the internal arcing / loss.

Replace the plugs and it will go away!

Si

The correct answer to the original post - and it wasnt even recognised.... ;)

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Or you paid £116 for a ~£30 item readily available from the motor factors:

http://www.v8forum.c...wtopic.php?t=78

Unfortunately, the quality of factored electronic parts is very patchy. The last amp I bought from a factors lasted ten minutes. As they come with that "open it and we won't take it back" disclaimer coupled with the fact that I was hacked off with the Disco riding around on recovery trucks, I decided to go straight to LR. At least if this one fails within a year I can take it back and they will replace it.

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