Misfiring

Information relating to the Matchless G12 or AJS Model 31 650cc twin
Chessiegolf
Member
Posts: 54
Joined: Fri Dec 29, 2017 8:23 pm
Location: LINCOLNSHIRE UK

Re: Misfiring

Post by Chessiegolf »

Thanks for the replies, I'll have a good check of the connections and signs of cracking. I'll also try and capture a picture of the arcing, I've seem the arc stretch 1/2" though it doesn't seem to take the exact same path each time.
Chessiegolf
Member
Posts: 54
Joined: Fri Dec 29, 2017 8:23 pm
Location: LINCOLNSHIRE UK

Re: Misfiring

Post by Chessiegolf »

Here's a picture of the coil. The tracking appears to run from the nut of the LT connection to the rubber of the HT connection.

I'd have to remove the tank to get a good look for cracks on, what I imagine is, the Bakerlite cap
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MalcW
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Posts: 626
Joined: Tue Oct 24, 2017 12:05 pm
Location: DEVON UK

Re: Misfiring

Post by MalcW »

Have you tried swapping the plugs to see if the fault changes sides? Similarly, if they'll reach, swap the complete HT leads. If the problem moves to the other side with either of these tests, you have identified the culprit. if not, you have at least eliminated a couple of the options. If the problem swaps sides when you change over the HT leads you could then try swapping the plug caps. If the problem moves back to the original side it's the cap; if not it's the lead.

I'm guessing you have a distributor? Is that OK?

Malc
Chessiegolf
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Posts: 54
Joined: Fri Dec 29, 2017 8:23 pm
Location: LINCOLNSHIRE UK

Re: Misfiring

Post by Chessiegolf »

Thanks Malc, for the suggestions.
Yes, the bike has a distributor and points.
I had another new coil, so this morning I've put that on, though apart from a faint deposit of carbon around the LT connection, I can't see any signs of cracking.
The bike started ok in the garage but its wet today so don't want to go for a ride yet as the fault doesn't materialise until the engine has completely warmed up.
If the fault persists, then I'll do as you say and try swapping plugs and caps. I have some HT lead so could even make up new leads if necessary though the only fittings I have are the brass crimp type which I can never seem to make grip!
I'm hoping I've been unfortunate to have had two bad coils in a row, but we'll see.
Chessiegolf
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Posts: 54
Joined: Fri Dec 29, 2017 8:23 pm
Location: LINCOLNSHIRE UK

Re: Misfiring

Post by Chessiegolf »

An update.

Today had been a nice day in eastern Lincolnshire up to about 3 o'clock, so with dry roads I was keen to see if the replacement coil had done the trick. After 36 miles, never venturing far from home, and no sign of the old misfire though with the rain clouds gathering, I decided that was good enough for one day and put the bike away. I'll go out again as soon as conditions allow to try and build confidence but I'm pleased with today.

If the fault was nothing more than 2 duff coils in a row I'll be content, though it still seems unlikely to me to replace a duff Lucas coil with a new duff Lucas coil.

Once the bike was warm and seeming to behave itself, I sped around at the breath-preventing speed of 60 mph and all was well........apart from the vibration and starting to have to grip the handlebars more, 50 is far more relaxed, or am I just getting old!
SPRIDDLER
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Posts: 8542
Joined: Wed Sep 01, 2004 1:00 am
Location: WEST SUSSEX UK

Re: Misfiring

Post by SPRIDDLER »

Chessiegolf wrote:
If the fault was nothing more than 2 duff coils in a row I'll be content, though it still seems unlikely to me to replace a duff Lucas coil with a new duff Lucas coil.
Fingers crossed for you.
The Lucas brand name (and the Hepolite name) are owned by Wassells who manufacture much of their range overseas (China, India) rather than in the U.K. as in the past.
Many will slate Wassell's quality but (in my c.14 years experience as a Wassell distributor) I've found that generally the Wassell products are acceptable quality; a few are cheap 'n cheerful, most are very good value for money, but it seems that their quality control isn't always 100%.
Inevitably, the quality and the essential manufacturing volumes are driven by the market's requirement for low prices.
'There is a tide in the affairs of men
Which taken at the flood............'
Chessiegolf
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Posts: 54
Joined: Fri Dec 29, 2017 8:23 pm
Location: LINCOLNSHIRE UK

Re: Misfiring

Post by Chessiegolf »

I've now completed a 68 mile ride and unfortunately felt a few misfires once the engine had got really warm. The problem is not nearly as bad since changing the coil, but its there nonetheless. The misfire is intermittent and does not last long enough to be able to stop and investigate where the sparks are going or which cylinder is affected. Prior to changing the coil I could see sparking from the HT terminal to LT on the coil, so it seems to be that its an easier current path through the LT terminal (and presumably the points) to earth rather than taking the intended route through the distributor rotor and plugs.

I had renewed the HT leads using stranded HT cable (7mm). I've since seen that there are much more expensive silicon/carbon leads available which are sold as recommended for electronic ignition, although mine is a 12v neg earth, points set-up. However has anyone any experience of whether these cables are likely to perform better than the usual stranded copper type?

I've checked the connections at the coil, distributor and plugs, all look good and with no sign of arcing. When mounting the new coil I wrapped thin rubber sheet between the coil and clamp to electrically and mechanically isolate it and give it its best chance of survival! Everything is clean and dry, so its a mystery to me why the engine still occasionally misfires.

Its not a bunged petrol cap hole and doesn't seem to matter where the throttle is, or speed, though I've not gone more than 65mph. The only time it refuses to misfire is when the bike is stopped and the engine rev'd, then it behaves impeccably!

Any help would be appreciated.

Thanks
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