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Pre-gap in spark plugs...

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I just learned something new!!!  Wondering how many of you fine fellows have heard of spark plug pre-gap?  Spark plugs from early eras did not have pre-gap which is something that modern plugs have.  The following is taken from the Harley-Davidson JD facebook group i belong to and i thought it might be of interest to you folks because it pertains to magneto ignition and it seems some of us experience (last sentence) oil fouling plugs.  (The author of the following is Steve Marks from JD facebook). 

i look forward to hearing your comments, experience, etc.

When the contact breaker points open, the voltage induced in the secondary winding of the HT coil rises very rapidly from zero volts. How far it rises, depends on a number of factors but will be thousands of volts. It will continue to rise until it reaches a high enough value to ionise the air between the spark plug electrodes and jumps the gap. On early magnetos and battery /coil systems, there was a continuous path from the HT coil to the spark plug gap. If the spark plug got fouled or oily, there would be a leakage path to earth which would affect the speed at which the secondary voltage increased. Introduction of an extra gap somewhere between the secondary coil and the plug's electrodes ensures that there is no leakage across the fouled plug. This extra gap might be in the distributor of a multi cylinder between the rotor arm and the cap. It might be in the plug itself as already mentioned. Another trick in the 'good old days' was to cut the HT lead and strip the ends on each side of the cut. Using a 4 hole trouser button, the end of one half of the HT lead would be threaded through two of the holes, the end of the other half of the HT lead would be threaded through the remaining two holes. This introduced the required gap in the HT path and produced a better spark! The HT voltage rises high enough to jump this new 'mid way' gap and is already high enough to then move on and jump the spark plug gap - often helping to burn off the oily deposits on the plug in the process.

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Steve, an old timer told me this in the 1970s and I used it to good effect on my first commando.

The idea was to lift the plug cap off the miss firing cylinder.  Surprising it didn't give you a shock.

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This has been around since the 1920's at least. I'll dig out an article I have in a 1920's copy of Motor Cycle. One magazine has an advert by the Lodge spark plug company for a well made auxiliary gap. They say quite clearly that they were selling them to meet customer demand but don't expect miracles!

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...the mag end of the HT connection.  Whilst theory says it could work, in practice if you have a resistive path at the plug, the new higher charge will still only dissipate according to the  resistance at the connection to your return path.  Intermittent firing never improves as the  charge builds.

 Might well clear a fouled plug but a leaky one is another matter.

 Only my theory, open to some education here...

 

Cheers

Jon 

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When Harley modernized their product range in 1929 with the sidevalve 45 c.i. they also replaced the magneto with a dual coil, waisted spark battery ignition. Most (if not all) electronic ignitions we put on our parallel twins also uses waisted spark. Which means that we have a pre gap. The two years I had a 1930 Harley as my means of transportation,  I never experienced an oily plug. Even when using the hand pump to give the engine more oil going uphill.

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... for your thoughts!  it was the button trick i'd never heard of, as a kid growing up on the farm, when a gas engine hadn't been started for some time, i remember Dad lifting the spark plug terminal off the plug terminal for what he said to "get a hotter spark."  As to wasted spark, the H-D brought that out in 1927.  i have a pair of '27's i built from parts...

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Mikael-How is the wasted spark considered to be a 'pre-gap'? The pre-gap is in series with the HT from the mag or coil to the plug. The wasted spark is a spark plug alongside ie in parallel .

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This is from "The Motor Cycle" Dec 13, 1923.

The price of a new Lodge plug was 6/6. That is three for one pound. The price of a pint of beer was perhaps 6d(?) Or 40 to the pound. So a sparking plug cost roughly the same as 12 to 15 pints of beer!

No wonder they wanted to keep "leaky" plugs in service and would try anything just on case it might help.

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Hi David,  Is your Inter fitted with a pre-gap  device ?.  judging from the  look of your plug ,  there must be  one in there somewhere for it to run as it does.  I'm still  going from weak ( clean plug ) to rich  (dirty plug)   while all the time getting  a spit back ,no matter what changes I make. Most annoying  and the source of much  ribbing from the boys .

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No!  Perhaps I'll buy a hotter plug one day to keep onlookers happy!  The PO was a brilliant engineer (anyone whose department managed to get 80mph out of a Scorpion Tank can't be accused of a lack of imagination anyway...) but he had almost an obsession with plugs.  Hot ("soft") ones to start, cooler ("hard") ones for racing...I'm a bit more cynical...whenever my plugs have given me issues it's always been about over oiling or fuel flooding, usually following trying to start with wrong carb or ignition settings.  But on the other hand - every manufacturer seems to make dozens if not hundreds of different ones nowadays so there must be something in it.  Unless it is (almost) entirely about marketing (which I suspect is very possible!)

As Al says - with wasted spark, the coils are in series but the sparks are in parallel.  I've not yet found the Lodge advertisement for their pre-gap device but they certainly wrote that they made no performance claims but that they were selling it for customers who wanted to try it out for themselves.  An extraordinarily honest approach in the days before the Trades Description Act - especially in a magazine with plenty of ads for "snake oil".

Spitting back sounds like it's too lean.  Or burnt valve..(or lack of tappet clearances - see how quickly the doubts set in?)

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They would be suspects but I have more compression than I can cope with. No I'm thinking its the by pass drillings that are too large  and passing  too much air , carb has been messed with . May work with a larger pilot jet .  If not will try another carb.  

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Al, first time I disagree with you and hopefully the last. The path is from one side of the coil secondary winding to the first plug, then through the cylinder head to the second plug. then back to the other side of the coils secondary winding. That means two plug gaps in series. One during overlap, where pressure is close to atmospheric pressure. where a thousand volts are needed to jump over the gap, The other plug gap needs some 10000 Volt to spark as the pressure is much higher. I regard this as about the same as the pre gap which started the discussion.

Edit: To clarify, I only talk about dual output coils, as the one  I had on my old Harley. But I believe that the same applies to more modern dual output coils. But you are correct that using two 6V coils in series will give a wasted spark but not a pre gap as the secondary windings act in parallel.

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Given that both Champion and Lucas were major sponsors of motorcycle sport for decades, they must have got a little irritated when, upon a competition bike breaking down, hearing the fault inevitably given out was "ignition failure"- after all, a bent conrod or snapped crankshaft does nothing to encourage a brand's sales....

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I’m glad this topic has generated some good discussion and that it’s ok to disagree; your dialog reflects the great group of fellows we are.

Michael, your commentary on Harley IoE coils is over my head which I’m ok with!  I had an old coil I gutted and installed a Tri-Spark with a condenser; it looks the part and works awesome with the electronic ignition on my ’27.

My understanding is the Lodge spark plug was well thought of; I know that the top national racers during the post war period through the 50’s used Lodge’s.  In fact, I have found a pair of HLNP pink Lodge’s that according to my research should be the correct ‘heat’ for my Inter.  If anyone has any dissenting comments or experience to the contrary, I welcome hearing from you.  I also picked up a couple KLG FE 75’s and have a couple Champions of the correct heat, so will give them a try.  I also installed a #15 pilot screw in the premier, but as yet have not had a chance to test how this alteration may affect throttle opening.  I still refusing to believe the engine is over-oiling, although that may be the case.

David, I would like to know what brand/heat range plug you are running since your Inter runs unfalteringly even with an oil soiled plug…?

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I suspect David gets away with it because he has a decent spark from his mag ,and  when on the slide cutaway or  needle jet the mixture  is  fairly good allowing  some measure of self cleaning. Would be interested to hear if the plug is a real  vintage one . I have a  large selection of  KLG  ,Lodge, Champion, etc  that were  bought in old packaging  but there are modern  "fake? old" versions about, dont know if these have old style glazed insulators?. I also have some expensive fine wire electrode plugs  that  did not manage any better . All the plugs need the engine to reach a good temperature for them to burn off deposits. I have an all alloy Ducati that  takes  ages to warm up , ( TT type carb)  and I use a warm up plug on that  for  slow riding.

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Currently using NGK B6ES. Possibly could try something hotter? Pic shows it after running home the last few miles in slow traffic with slow speed limits and trickling along local roads.

The plug was hard to remove...lots of "R" cooked on and gluing it in place. Like an old barbecue grill.

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...holy cooked bean oil Batman!  My plug doesn't look that soiled...  Your mag must put out a nice fat blue spark. 

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That plug mouth and insulator is a lot better than  what I saw previously , and probably is a fair representation of  what  you can expect  after  a well gunged plug  is run at a reasonable temp . If it self cleans up any better  I would be surprised.  I don't think it helps long term to run with a much hotter plug than recommended as it just  delays getting the real issues sorted. The motor can get very dirty and  I don't think "R"  is much good at collecting carbon. I have been running a detergent oil  and its telling me I'm making far too much carbon  in my attempts to clear the spit back.

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thanks for your comments, agreed, that a hotter plug is a vain attempt to avoid what is the underlying issue.  In my case, one reason for the fouling could be new rings that still need to fully bed in, my understanding is around 100 miles before they are able to begin doing their intended job...  I'd like to do a compression test, but i don't have an adapter for my hose to be threaded in the hole without removing the gas tank.  The compression is such that there's no way to move that piston without  using the compression release.  Aside from rings and having my different oil controls adjusted correctly, I'm not sure what else to consider other than logging some more miles. 

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We  had a new  250  Ducati  and were told by  the Shop  race mechanic  Bert   that it was a good one  and a bit "tight"  and was set up to be  a  production race bike ,but  they needed a cash sale. It took over 1500 miles before  it performed  at its best.  I managed an easy  timed 104 mph  which was pretty good for 1967 . I have one  from the same shop  batch that  won in the the 67  Barcelona 24 hr endurance race and came 3rd overall behind 2 big bikes.

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On a nearby topic, I suspect modern oils (including modern versions of "R") plus attention to giving bores cross hatched honed pattern have reduced rate of bore wear considerably nowadays even on old engines. I've now done 7000 miles since a rebore and, if the bore size was correct then, it has not worn enough to measure. Less than 0.0005" (half a thou).  (I didn't have access to an accurate gauge when it was done, but it's pretty much spot on the book figure now).

I'm looking at my copy of "Radco" (date 1986) and he describes using a home made cylinder lap using grinding paste and paraffin, followed by metal polish. Today I think we'd use 280 wet and dry...not polish...and the bore will last longer and pass less oil.

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I put a plug in the dishwasher once. I don't expect to repeat it. It came out just as black, with added rust.

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I have tried oven cleaner (nasty stuff!) ,  a home steam cleaner,  switch cleaner and  the gas hob, nothing worked.  Am considering  trying  my  plumbers Butane torch , what do you think ?. How about some caustic soda ?  also nasty stuff but might draw out the carbon from the insulator.

 

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