Low battery start on Commando

I have a 1974 Commando do I need to use a capacitor at all ? seems pointless if you can not start the bike without a battery.

A bad battery stranded me one night in my B44, took out the fuse, turned the ignition on and with one kick the engine started and off I went.

The Boyer requires more power than the capacitor holds in reserve so to speak. It is also good practice to insure the Boyer has a good supply and return to the battery as some types of these units can have issues with the ignition advancing at low speed due to low voltage. This can lead the kick back when kick starting and Commando's are said to have the ability to break bones when this happens if you are unlucky.

Why remove the battery??

If space is an issue fit a smaller battery but be aware it will have less in reverse so make sure the charge system is working well an the use of LED replacement lights is helpful to reduce power consumption thus help with charging the battery.

The 2MC capacitor is 'nice' to have it is not essential. If you try to start with a flat or non existant battery then it is very likely the bike will kick back if no 2MC. If even with a 2MC a Commando will not start then the alternator and/or regulator rectifier is suspect. and the quantity of amps from the RM21 is suspect. The Hi output 3 phase and the MKIII high output single phase will not start with or without the 2MC. as there is not enough amps generated at kick over.

... how the 2MC is supposed to work with a flat battery. Surely all the energy supplied by the alternator will be soaked up by the battery? I could understand it if the battery was disconnected the the capacitor could (perhaps) do its job.

With my Boyer power box, I used to kick start the Dommie (Lucas RITA) with a flat battery provided I disconnected the battery first.  Otherwise as you say, Ian, all the juice goes into the battery.  Having said that - I had battery problems recently and the ignition failed, and now I don't know if the capacitor in the Boyer has failed, or the alternator is losing its output...something I ought to test this weekend.  I'm sure Al has said before that the power box capacitor does not last forever, and mine must be at least 12 years old.  My ammeter is pretty hopeless (jumps about a lot).

I buy large capacitors off ebay, up to the size of a can of beans and fit these in place of the 2MC, they work well on singles with decompression levers as you can spin the engine before turning the ignition on to charge the capacitor and then the bike starts and runs well with boyer all day long in all conditions. The problems come with twins with no decompression levers so you can't charge the capacitor but you can bump start them down a hill.

I have had the fuse blow on my Boyer equipped Commando, the only way I found out was the next time I turned the ignition off and then back on to start it up it would not start, the fuse blowing takes the battery out of the circuit but not the 2MC. So with a fast running engine you get the boyer to run off the 2MC but at kicking speeds on a big engine the low revs are not enough generate enough to get the voltage required for the boyer, that's also why the bump start works. These random fuse blowings was traced to a duff harness and is now cured. I still fit a capacitor to twins but always with a battery, its good insurance as long as your battery fails at the top of a hill . With a brand new alternator rotor you might get it to start with a kick.

I experienced it the other way around..

The bike, Commando Mk2A fitted original 2MC and with the old black box Boyer, actually kickstarted better without battery than with battery.

It appeared the battery was shot..

2MC rulezzz!

Lucas RITA will fire quite easily with Boyer Power Box and flat or no battery. If it is flat then the battery should be disconnected until the engine is running or all the amps are lost in trying to charge it. Which agrees with what Bennie seems to suggest.

The Power Box must have a similar capacitor to the 2MC. But (unlike the earlier Boyer ignition) RITA does not advance at low voltage.

I believe there should be a capacitor built into the output of the powerbox but only to smooth the delivery of the charge. My trispark would cut totally for a second and that coincided with a momentary blank on the digital voltmeter I have fitted where the ammeter used to be. I removed my 2MC capacitor years ago. one less thing floating around under the seat. It's never tidy under a Commando seat and having seen under a Mk3 tank......Blimey

The powerbox may have a smoothing capacitor in addition but it does have an alternative to the 2MC as it is advertised as allowing batteryless running.

What you are overlooking here is the 'quantity' of electricity coming out of your alternator. All of these bikes are many years old and the magnetism in the rotor will 'fade away' in time, say 10 yrs or more. Looking at the battery/charge voltage will only give you a clue, but the real answer is to measure the actual amps (DC) generated by the charging system. Some details are on my web site of how to do this.If the reg/rect is in doubt an easy way to check is replace the reg/recy with a straight rectifier, start the bike and switch the lights on as a load-see how many amps DC you get from the rectifier? Should be 9 from an RM21.A O