Coil testing

Hi

I own an 850 mk 1 (1973) with standard Amals and a Boyer and so far in about 10 years ownership it has generally started and ran well. The other day though it started to misfire at low revs so I supected the battery was on it's way out and the Boyer was defaulting to full advance, this was borne out by kick backs on starting. The motor would only run cleanly over 2k and would backfire badly below that with hard starting as well. Before that it ran really well and nothing else has been changed.

Checked the battery, making about 12.4 volts so put in amother battery that was showing a good 13.4 volts and still the same problem. I have checked the Boyer wiring (as it is an old one) connections to the coils etc and all seems as it should be, so I checked the resistance of each coil (Lucas 6 volt). The right one is around 2.2 Ohms, the left one 1.9, is this likely to be the problem?

If so any thoughts on what would be good replacements? Any advice welcome - thanks

That much difference in coil resistance is insignificant.

Has the timing changed? Check that the Boyer stator plate isn't loose. Also check that the wires attached to the stator haven't fractured (a common problem), and that the insulation on the stator wires hasn't chaffed where they enter the timing cover (another common problem).

Pull the spark plugs out and lay them on the engine (ground the threaded part) then kick the starter. Both plugs should fire with nice blue sparks.

Sure sounds like Dave has been there done that and me too. Bad smoke conductor somewhere, even brake light wires, but not likely in the coils. A bent out of tension cam chain tensioner can do this too. If Boyer wires not changed out prior to upgrad, guess where I'd snoop first.

Pilot Curcuit?

Thanks all for the suggestions. Checked the wiring and any voltage drop through the system plus checked for a spark. After checking the connections from the main harness to the Boyer I kicked it over again, this time making sure the plug was well grounded against the head, got a gazillion volts up my arm!

Put it all back together and all fine, did around 60 miles today no problems at all. As a precaution I have replaced some of the older spade connectors, I can only assume they were breaking down .

As the Boyer unit is at least 12 years old I did suspect that but as my Dad used to say check the easy bits first.