Sparking Coil

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

Sparking Coil

Post by Chessiegolf »

Having rewired to a 12v negative earth system, which of course entailed fitting a 12v alternator and new coil, all seemed well when I went for a ride. About 6 miles into the ride I slowed for a left hand corner and on opening up the throttle, the engine started to misfire, it wasn't too bad and I got home again (another 6 miles). On investigating I found that the coil was sparking from the HT terminal to the distributor LT terminal. The misfire only seems to be on the right hand cylinder.
The plugs are nearly new and I checked the gaps to make sure the spark wasn't having to jump a too large gap (set at 25 thou).

Before I delve any further by removing the seat, tank and testing the coil resistances, has anyone any idea why the spark should appear where it does.

The fault started suddenly and may simply be a coil that has failed early on in its life, its covered just 124 miles, not that its failed completed as its producing a good spark, albeit from the wrong place!........a mystery to me.

John
Mick D
Posts: 2886
Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Sparking Coil

Post by Mick D »

Hi

Have a look at the insulation around the HT lead receptacle on the coil, check for cracks.

Also check the integrity of the HT lead, plug cap and plug - the spark should 'prefer' to jump the plug gap rather than the much larger one on the coil, it sounds like either the path from the coil to the plug is damaged or the coil external insulation is poor.

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

Re: Sparking Coil

Post by Chessiegolf »

Ok Mick, thanks for the advice.

When I removed the plugs to check the gaps I should have swapped them over to see if it transferred the misfire to the other cylinder.

I'll check for cracks and make sure the leads are well pushed home. If the fault is the same as before then there's probably no alternative than to remove bits to get at the coil.Tucked up under the tank its hard to get a good view of the black cap.

Thanks again

John
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