Recurring weak spark SQ4 Mk2

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berniejones
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Recurring weak spark SQ4 Mk2

Postby berniejones » Thu Jun 21, 2018 7:56 am

Since my rebuild I've had a number of experiences of difficulty or failure to start which I've traced to a weak spark.

Each time I've replaced the coil and the problem has been solved for a while but then seems to reoccur.
I've tried new Lucas and others and also some old Lucas originals from other 6V bikes I have and every time it seems to fix the problem but only for a while.

I'm struggling to think of any reason that the electrics could be breaking down the coils in any way but that seems to be what is happening,

Any thoughts on what might be the root cause of this behaviour please - or other ideas on improving the ignition?

Many thanks,
Bernie
'56 SQ4 project
'52 BSA M21
'61 Dommie 99
'62 BSA A65
'81 T140
'54 Sunbeam S8
AOMCC# 4631

john.whiting
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Re: Recurring weak spark SQ4 Mk2

Postby john.whiting » Thu Jun 21, 2018 12:30 pm

You may be getting carbon tracking in the dist cap...or the capacitor may be failing.Try one outside the diz body.I assume you have made sure there are no supressed components in the HT circuit,and you have copper wire core leads.....A worn diz will also cause problems as the point gap can vary.Its very difficult to fix the pivots inside the advance when they get worn.Moisture condensing inside the diz also causes non starting.

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paul.jameson
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Re: Recurring weak spark SQ4 Mk2

Postby paul.jameson » Thu Jun 21, 2018 9:51 pm

Replace the condenser. If that doesn't work, replace the rotor arm. Neither is expensive, both will give you useful spares to carry in the tool box. If neither works, look very carefully at the strap which goes round the outside of the points plate from the connection on the side of the distributor to the points pivot. The connection stud should be soldered into the strap. If it has become unsoldered, the resulting problems are a swine to trace. If you have a lot of play between the cam and the distributor shaft the easiest solution is electronic ignition.
Paul Jameson
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), 54 4G Mk IV (project), Healey 1000/4 (project)
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berniejones
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Re: Recurring weak spark SQ4 Mk2

Postby berniejones » Sat Jun 23, 2018 9:24 am

Many thanks, parts ordered as suggested. I'm still thinking about electronic ignition options and see that there have been a number of previous posts on that topic.

If I do go that route then I think a 12V conversion and a system that uses an electronic advance/retard will be best as there is quite a lot of slack in my original setup.

Cheers,
Bernie
'56 SQ4 project
'52 BSA M21
'61 Dommie 99
'62 BSA A65
'81 T140
'54 Sunbeam S8
AOMCC# 4631


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