Skip to main content
000000 000003 000006 000009 000012 000015 000018 000021 000024 000027 000030 000033 000036 000039 000042 000045 000048 000051 000054 000057 000060 000063 000066 000069 000072 000075 000078 000081 000084 000087 000090 000093 000096 000099 000102 000105 000108 000111 000114 000117 000120 000123 000126 000129 000132 000135 000138 000141 000144 000147 000150 000153 000156 000159 000162 000165 000168 000171 000174 000177 000180 000183 000186 000189 000192 000195 000198 000201 000204 000207 000210 000213 000216 000219 000222 000225 000228 000231 000234 000237 000240 000243 000246 000249 000252 000255 000258 000261 000264 000267 000270 000273 000276 000279 000282 000285 000288 000291 000294 000297 000300 000303 000306 000309 000312 000318 000321 000324 000327 000330 000333 000336 000339 000342 000345 000348 000351 000354 000357 000360 000363 000366 000369 000372 000375 000378 000381 000384 000387 000390 000393 000396 000399 000402 000405 000408 000411 000414 000417 000420 000423 000426 000429 000432 000435 000438 000441 000444 000447 000450 000453 000456 000459 000462 000465 000468 000471 000474 000477 000480 000483 000486 000489 000492 000495 000498 000501 000504 000507 000510 000513 000516 000519 000522 000525 000528 000531 000534 000537 000540 000543 000546 000549 000552 000555 000558 000561 000564 000567 000570 000573 000576 000579 000582 000585 000588 000591 000594 000597 000600 000603 000606 000609 000612 000615 000618 000621 000624 000627 000630 000633 000636 000639 000642 000645 000648 000651 000654 000657 000660 000663 000666 000669 000672 000675 000678 000681 000684 000687 000690 000693 000696 000699 000702 000705 000708 000711 000714 000717 000720 000723 000726 000729 000732 000735 000738 000741 000744 000747 000750 000753 000756 000759 000762 000765 000768 000771 000774 000777 000780 000783 000786 000789 000792 000795 000798 000801 000804 000807 000810 000813 000816 000819 000822 000825 000828 000831 000834 000837 000840 000843 000846 000849 000852 000855 000858 000861 000864 000867 000870 000873 000876 000879 000882
English French German Italian Spanish

Low battery start on Commando

Forums

Hello All,

I have a 1972 750 Commando with Boyer ignition(bike is new to me) It runs fine with battery connected but when thebattery is disconnected the ignition light goes out after one second and the engine will not start. I have tested the 2CM capacitor as suggested in the Haynes Manual, indication are that the 2CM is OK, but when re-installed and connected still with the battery disconnected the 2CM voltage drops to approx. 1volt. Question do I require a replacement 2CM Capacitor or is it incompatible with the Boyer. Any assistance would be greatly appreciated.

Permalink

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.

Permalink

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.

Permalink

Previously malcolm_nicholls wrote:

Hello All,

I have a 1972 750 Commando with Boyer ignition(bike is new to me) It runs fine with battery connected but when thebattery is disconnected the ignition light goes out after one second and the engine will not start. I have tested the 2CM capacitor as suggested in the Haynes Manual, indication are that the 2CM is OK, but when re-installed and connected still with the battery disconnected the 2CM voltage drops to approx. 1volt. Question do I require a replacement 2CM Capacitor or is it incompatible with the Boyer. Any assistance would be greatly appreciated.

Many thanks Karl and John, you have answered my question the 2MC cannot power the Boyer. The reason for the question was only to be able to start the engine if my battery failed whilst out riding, but still I do have recovery if no hills are close to hand.

Regards Malcolm

Permalink

Previously malcolm_nicholls wrote:

Hello All,

I have a 1972 750 Commando with Boyer ignition(bike is new to me) It runs fine with battery connected but when thebattery is disconnected the ignition light goes out after one second and the engine will not start. I have tested the 2CM capacitor as suggested in the Haynes Manual, indication are that the 2CM is OK, but when re-installed and connected still with the battery disconnected the 2CM voltage drops to approx. 1volt. Question do I require a replacement 2CM Capacitor or is it incompatible with the Boyer. Any assistance would be greatly appreciated.

Many thanks Karl and John, you have answered my question the 2MC cannot power the Boyer. The reason for the question was only to be able to start the engine if my battery failed whilst out riding, but still I do have recovery if no hills are close to hand.

Regards Malcolm

Permalink

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.

Permalink

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..Undecided

2MC rulezzz!

Permalink

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.

Permalink

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

Permalink

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.

Permalink

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
Permalink

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.

 

 

 

Permalink

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.

Permalink

... 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.

Permalink

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).

Permalink

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.

 

Norton Owners Club Website by White-Hot Design

Privacy Policy