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British Motorcycle Classics

 

British Motorcycle Ignition

Provided by Brian Holzigal

On most bikes up to '79, ignition was supplied by either magneto or points coil. Magnetos were phased out mostly at the end of the pre-unit era, 1963. Triumph and BSA twins had gone to 4CA type points which used the spark quenching capacitor/condenser on the points plate. No capability was designed to individually adjust each cylinders timing and adjustment could only be found via the points gap. Truly horrendous. The next design 6CA were a lot better and allowed the individual cylinder adjustment by adding two banana shaped plates to the main round plate. The points and banana plates were both adjusted by offset screw. The condensers were relocated under the tank. The last 10CA style were used through '68-'78 and had the points adjustment by a 'V'using a screw driver. Triumph introduced the Lucas electronic ignition in '79. No other British bike was factory supplied with them until then. Today the market has a few electronic systems to choose from, the most prominent being Boyer, Pazon and Tri Spark are two more and all offer units for twins, triples and in some cases unit singles. They can also be got in analogue or digital form. Digital, being the best for electric start, where the voltage may drop to 8 volt or so and can cause a misfire and kick-back. The cheaper analogue units are fine for most cases and if you have a good original or upgraded charging system then there are few to no issues. It pays to keep any bikes on a proper trickle charge/tender when not using the bike often. As for timing these ignitions they are straight forward via supplied directions but in the case of the Boyer the twin static timing may be advanced and will cause minor kick-back. If so, retard slightly then check with timing light if possible on your model.

With points, the 4CA type are a disaster and not worth the effort and if you must persist then good luck. With Triumph twins, the exhaust cam which carries the mechanical advance has a locating pin so timing is easier. Once 38 degrees or 10mm BTDC on the firing cylinder (both valves closed) has been found and a quick check of points gap at 12-15 thousandths, use a set of pointy nose pliers to grab the cam and turn clockwise against the return springs. With ignition on and spark plug in its wire and lying on the head, look for spark at the end of the travel. This will be as the points are just opening. If there is more travel after spark then adjust the banana plate in the clock wise direction.

There are no locating pins on the Norton and BSA twins or the triples. Find before Top Dead Centre (BTDC) timing mark or measure 28 degrees for the Norton. This unit spins anti-clockwise. Put the advance unit into the taper and nip up close to the required cam ramp to points. Check the cylinder, points and coil are correct with the wire colour. Turn over a few times and set points gaps. Find timing marks again on the cylinder with both valves closed. Try with pliers for points opening. If a points break can't be found with adjustment return banana plate and main plate to centre positions. Move the advance unit slightly. Repeat until successful without using up all adjustment. Check other cylinder timing. A similar process is required for A50-A65 and they are also anti-clockwise except the 36 degree BTDC. The full advance degree required is due to the piston dome height. The 28 degree Norton has a shallow combustion chamber and flat top pistons so flame travel is quicker than the Triumph and BSA hemi designs with high dome pistons. They require earlier ignition to get the job done as the flame front is impeded by the piston crown. This applies to the triples as well. Timing the triples is similar to the non-pinned Norton and BSA but clockwise. There is good marking on the rotor to assist.

The easiest, of course, is to go electronic, set and forget. I still run points in some of my bikes but these are offset crank 'V' fire with modified points 10CA plates as I am too stingy to go special ignitions for these. Some have over 2,000 miles on them and I haven't checked the points as they should be good for double that between services. The biggest problem with points systems is the mechanical advance wearing and causing poor idling due to gap changes.

With magnetos some replacement parts are available if you wish to have a go yourself but I prefer to send mine to a professional. Timing these is relatively simple as the points fit in a taper with a locating notch. The drive side is tapered so with the timing cover off and the magneto bolted back in, be sure of drive wheel travel in relation to engine travel, with the spark plugs in the leads and resting on the gear box or something metal find 38 degrees BTDC. Cylinder not really important at this point as HT leads will be fitted to correct cylinder after timing. Turn the points in the correct direction until just coming onto the ramp. If auto advance, wedge advance open (springs tight) this will be full advance. Fit onto taper of magneto loosely then do up nut loosely. Using thin paper (cigarette is good) place in points. Move points to find just open (cigarette paper releases) then tap advance unit onto taper with small hammer and socket to seat, firm up bolt. Remove wedge and check that rotating advance unit sees points just breaking at full travel. This may take a few times to get right. With a manual advance via the cable and lever type magneto figure out full advance at the points end. Move cam against points direction which makes points open earlier (advancing) therefore moving cam ring away from rotating direction has the points opening later (retarding). Push or pull? Set the BTDC height, check the points are just opening with paper and tap the wheel onto the magneto taper, fit nut and tighten. Recheck.

Twin magnetos have cam ring openings 180 degrees apart but not always. Wear and manufacturing tolerances could see a few degrees difference between cylinder firing. This can be seen on single carb machines when one pipe is blued and the other ok, indicating incorrect timing on the blued pipe cylinder. Check the timing on both cylinders and adjust it slightly to compensate and get the ignition on both a little out instead of one cylinder way out.

With mag/dyno seen on most Singles up to 1963 the points are opened by a rubbing follower that pushes open the points. Like the Twin manual advance mag find the direction. Then move cam plate against that direction with cable with 12-15 thousandths points gap, (it can be closer as the spark at the plug is weak) and cigarette paper just releases. Tap on the drive gear. Big Singles typically use 7/16 to 1/2" BTDC.

Most magnetos are available nowadays with conversion to electronic ignition. The main problem with this is that the charging system may need to be upgraded to cope.

Good luck
Brian Holzigal

 

 
 

 

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