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Correct plug gap

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Correct plug gap

Posted by George Phillips at September 14. 2018

I've moved this post across from "Singles" as it is more electrical than mechanical.

Looking through past posts there seems to be a variety of opinions on the correct plug gap for a mid fifties ES2 ranging from 15 to 25 thou.

One "Book of the Norton" 6th Edition 1951 states 15 - 18 thou but 15 thou recommended. A later 8th Edition 1957 that I have states 18-22 thou. Then the 9th Edition 1963 states 20-22. A further Maintenance Manual - undated but cost 2/6d at the time and looking like mid fifties - states 15thou for a KLG F70.

James Smith's excellent manual "Classic M/cycle Electrics Manual" notes that modern plugs are supplied "pre-gapped" and "it is preferable not to change this."

Does modern petrol affect the plug gap?

Also, the CR increased throughout the 50's from 6.45 through to 7.1. Would this have a bearing on it?

So where does that leave us? Trial and error and lots of plug washers?

I think on balance I'll go for 18 (sitting on the fence). Any views?

George

Re: Correct plug gap

Posted by David Comeau at September 14. 2018

Gap for old bikes is whether  points/coil or magneto. I too had seen norton wanting .015 and BSA wanting .014 for mags. That is mainly to enable easier kick starting.

Before I learned this, the bigger gap plug if used on mags caused misfired at a very slow idle with one side dropping out. Mags fire positive voltage on one plug and negative on the other, one side being a little hotter...forgot which one. The hot side would keep running, the cold side dropped out until you rev the engine.

It would then be a sign of a weak mag, probably a bad cap.

Re: Correct plug gap

Posted by ian_soady at September 15. 2018

I always use .025" for coil systems and .018" for magneto.

Re: Correct plug gap

Posted by Alan Osborn at September 17. 2018

Previously David Comeau wrote:

Gap for old bikes is whether  points/coil or magneto. I too had seen norton wanting .015 and BSA wanting .014 for mags. That is mainly to enable easier kick starting.

Before I learned this, the bigger gap plug if used on mags caused misfired at a very slow idle with one side dropping out. Mags fire positive voltage on one plug and negative on the other, one side being a little hotter...forgot which one. The hot side would keep running, the cold side dropped out until you rev the engine.

It would then be a sign of a weak mag, probably a bad cap.

Despite the love of mags by a lot of people the 15/18 thou as opposed to 25 for points tells all, ie the more voltage you have the larger gap you can run, the smaller gaps tend to foul sooner. Now do we know that the double ended coil a lot of people (don't mention Honda 4s) like/insist on, also has this one good spark and one cooler spark. So if you get a bit of intermittant-one sidedness then again a double ended coil can be the culprit, or even part of the agrovation. Your choice.

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