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Commando Quits

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Hi everyone.  I've been an NOC member for some time but this is my first forum post, so apologies in advance if I'm contravening etiquette somehow.

I have a 71 Commando 750 production racer, bought new.  The bike has run well over the years but recently has developed an annoying habit:  the bike starts and runs fine, but will come to a dead stop after about 10 miles riding.  I can't re-kick or bump start it, until it sits for at least 30 minutes, then it starts and runs fine.  This feels like an electrical problem, because there's no fuel starvation feeling in the stop.  The bike is quite original.  After this problem started to occur, I fitted a Boyer Mark III ignition and it still happens.

Does anyone have any advice as to cause?  I had consdered replacing the 2MC capacitor and zener diode, but before I do that I'd like to know I'm chasing the correct causes.

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Check connections or hot wire battery to ignition. Are coil bodies insulated? At nearly 50 years old 2MC is probably time expired.

I had an intermittent Boyer black box.

 

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Do you have a voltmeter handy?

When it dies, put the voltmeter on the coils and kick the engine over slowly. You should see volts on the coils come and go as you turn the engine over. If you do not, you need to work your way back to the supply to the boyer. If you do, it is either not a electrical fault, or the coils are faulty.

Note:- As you have two coils in series, one side of one coil is connected to ground, the coils are connected together and the other side of the other coil goes to the boyer.

At the boyer side you should see 12V and on the join, about 6V, if the middle voltage is substantially different from half of the 12V then one coil has gone sick.

Start with this test and report back.

Tony

 

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It is possible that the vent in the fuel tank cap is plugged.

Try opening the tank if it shuts off on you again.

Hope this helps

Mike

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+1 for Mike Sullivan's suggestion on tank venting

the other Mike Sullivan :-)

If all else fails, my mate had a similar problem on his A10.  It would run and start fine but die in the same village on his test route every time.  It turned out to be closed down tappet clearances.  Bit of a long shot but worth checking.

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Did you put new coils on when you did the Boyer, I have had a few coils over the years fail when hot and then starting to work again when cool.

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Thanks al for the advice.  I will try the voltmeter-on-coils suggestion tomorrow and report back.

To answer another question, however, there were new coils put on the bike when the Boyer was installed last year.  Since this problem occurs before and after the Boyer and the new coildwnet on, I'm thinking component failure is a long shot.  Still,  I'll run the tests....

Yes, that does seem to make faulty coils seem unlikely, unless they are standard type and have been overtightened. Still worth checking, but too much polish on the fuel tank cap now sounding more likely - can you blow through the hole?

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I had a very similar problem many years ago on my Commando. Starts well but woyuld fail after several miles. The fault was the wire from the points housing to the frame, which fatigues and breaks over time. When the wire is cold, it holds the broken ends together but, as it warms up, the connection is lost.

Working in aerospace I had no problem getting one of the sparkies to solder a new multi filament wire to the stator plate of the Boyer ignition and no problem since. The original copper filament wire doesn't survive fatigue particularly well, so it's worth a check. Disconnect the LT lead - attach a meter between the free end and the stator plate connection set to resistance and wiggle the wire - particularly in the are that would bridge the gap between engine and frame. I found I could detect an open circuit very easily!

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Mark. can you conform  that you have checked your tank breather?

Simple stuff first  - I thought my stator was fried again - turned out to be a broken wire!

Cheers,. Mike

 

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