My Commando won't start

Yes, this is one of these threads. I am now in the unfortunate situation that my 1970s Norton Commando 750 won't start. Hopefully someone on this forum will have a suggestion or solution.

First, I want to give a bit of history on the bike because I think that might be important information. I got the bike last year from dad. For the twenty-odd-years he has owned it it has never been an easy starter. The bike has been very picky on the exact starting procedure (ignition off, tickle carbs, two kicks, ignition on, give it a couple of kicks and hope it starts, if not wait a few minutes and try again). At some point I remember my dad had so much trouble starting the bike that a neighbor insisted on blessing the motorcycle. The blessing was a nice gesture, but it didn't make it start. He ended up fitting new plugs and finally got it going.

During the winter I have fixed, serviced and upgraded quite a lot on the motorcycle as I have documented here. Starting the motorcycle this spring I had severe problems getting it to start. We are talking spending up to an hour on and off trying to get it to start. When it finally started it would run fine and it would start on one kick when hot.

Last week I decided that I had to fix this problem. After doing a bit of reading I figured that the pilot jets in the carbs could be clogged. I removed the carbs, cleaned the jet and carbs and fitted them back on. I synced the carbs, set all the adjustments screws to initial positions and tried to start the engine. Long story short, it won't start at all now.

The the spark plugs do ignite, but the timing seems off. Plugs are wet when I pull them out. I have tried to give a little squirt of petrol into each carb, but the bike wont ignite at all then.

I have tried to diagnose the problem. What follows is information on the motorcycle:

1. It is fitted with a Boyer Brandsen Micro MKIII connected to two Lucas 12 V coils wired in series.
2. It has a modern solid state rectifier and a Zener diode.
3. It is run exclusively on 98 octane petrol which in Norway is ethanol free.

And my observations and measurements:

1. I have fitted new NGK BP7ES which both measure to 0 Ω.
2. Turning the ignition on and off will sometimes produce a spark. Spark plug is grounded to head for this test and other plug tests.
3. Touching the yellow/black and white/black leads on the Boyer will always produce a spark on both plugs.
4. Checked that leads from stator plate are correctly connected.
5. Kicking the bike over will produce a spark on both plugs.
6. Resistance of left and right coil is 3.6 Ω.
7. There is no continuity between coil shells and any of the terminal connections.
8. Resistance of spark plug caps measure to 5 kΩ.
9. Resistance of coils on stator plate is 129.7 Ω.

I have performed a load test of the Boyer following the instruction from here using a freshly charged battery.

1. Battery measures 12.6 V with ignition off.
2. Battery measures 12.29 V with ignition on.
3. Positive battery terminal to negative coil terminal is 12.14 V.
4. Positive battery terminal to positive coil terminal is 0.03 V.

I have not strobed the ignition, but i do have a strobe light. Neither have I checked the compression. Tomorrow I plan on connecting the Boyer directly to the battery terminal using some fused test leads. I have also drained the petrol tank and will be putting clean fuel into it.

Any and all suggestions are welcome. The weather is nice on the West coast of Norway and I would very much like to be out riding.

solbero said:

cleaned the jet and carbs

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How, need more details about what you used. Did you use a #78 or 16 thou drill to clean out the pilot jet as an example.

Why did you use two 12V coils with the Boyer when 6V coils wired in series are the recommended coils for use with Boyer.

kommando said:

How, need more details about what you used. Did you use a #78 or 16 thou drill to clean out the pilot jet as an example.

Why did you use two 12V coils with the Boyer when 6V coils wired in series are the recommended coils for use with Boyer.

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Jets were cleaned using a 0.4 mm cleaning needle used for cleaning 3D printer nozzles.

The bike had the 12 V coils when my dad bought the bike in 2000. I suspect they are the original coils fitted on the 1970 model. Boyer writes that two 12 V coils can be used in series on low compression engines. But I suspect they might be part of the problem.

A Norton Commando is not a low compression engine. Get a pair of 6V coils and follow the Boyer instructions for their use in series unlike the 12V which are in parallel.

6v coils in series are recommended...the primary coil resistance will be additive in a series connection, so if each coil is 3 ohm your are making the boyer work through 6 ohm total...which is not good.

Your point 3, touching wires of boyer stator gives plug firing...that implies there is a poor connection or damage on those wires....recall opening the circuit to the ign coils is what triggers the spark...so if wiggling the boyer wires causes spark you are momentarirly opening the circuit...theres a broken conductor in the wires.
Early boyers had issues with wirings at the stator plate developing cracks in the copper from vibrations. Difficult to see and diagnose.

kommando said:

A Norton Commando is not a low compression engine. Get a pair of 6V coils and follow the Boyer instructions for their use in series unlike the 12V which are in parallel.

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I would like to change the coils for the recommended 6 V coils, but getting the parts shipped to Norway is expensive and I would not like to start replacing parts without being sure that that part needs replacing. And just to clarify, the bike had the Boyer and the 12 V coils when my dad bought the bike in 2000, this is not something fitted by him or me.

Tornado said:

6v coils in series are recommended...the primary coil resistance will be additive in a series connection, so if each coil is 3 ohm your are making the boyer work through 6 ohm total...which is not good.

Your point 3, touching wires of boyer stator gives plug firing...that implies there is a poor connection or damage on those wires....recall opening the circuit to the ign coils is what triggers the spark...so if wiggling the boyer wires causes spark you are momentarirly opening the circuit...theres a broken conductor in the wires.
Early boyers had issues with wirings at the stator plate developing cracks in the copper from vibrations. Difficult to see and diagnose.

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This procedure is described of touching the stator leads together to produce a spark is described on the official Boyer fault finding page, look under I Have Sparks On Switching On And Off But Not On Cranking heading.

"And just to clarify, the bike had the Boyer and the 12 V coils when my dad bought the bike in 2000, this is not something fitted by him or me."


...and it was always hard to start.

concours said:

"And just to clarify, the bike had the Boyer and the 12 V coils when my dad bought the bike in 2000, this is not something fitted by him or me."


...and it was always hard to start.

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Yup, I know. In hindsight I would suspect coils could be the reason for the bike being hard to start. But it started most of the time, and being a British bike and all that my dad never looked more into it. My work during the winter seems to have upset the delicate balance it was in where all the little faults seemed to cancel each other out.

The bike had 12V coils from the factory, when the Boyer was fitted it was fitted in the cheapest way not the best way.

kommando said:

The bike had 12V coils from the factory, when the Boyer was fitted it was fitted in the cheapest way not the best way.

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Seems likely.

solbero said:

Seems likely.

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From the factory 12v coils would not be wired in series but in parallel as the points ignition (each plug fires independently) is not a wasted spark setup as is the Boyer (both plugs fire together).

Check the resistance reading at the coil +/- terminals with nothing else connected to the coil. 12v coils are usually near 3 or 4 Ohms each. 6v's are near 2 Ohms each. Having 12v in series means 6-8 Ohms of primary resistance in a moderate compression ratio engine...not ideal at all.