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mikuni carb issue


hi all

after some advice if possible ! have 71 commando which has a single mikuni vm34 single carb .

never had any problems but now have .

starts,runs all ok but when in top gear at about 45mph to 50 mph it starts to split and is hesitant  any faster all ok just has the problem in the 45 to 50 range.

taken the carb apart and cleaned 3 times now but still have same issue.

do not have a sonic cleaner just been using carb cleaner.

any help would be great.

many thanks 



There is a funny old saying, that comes true very often, "if you think the carburettor is playing up it is an ignition problem, and vice versa"


Steve, since no one else has added a comment I will say more to try and be helpful and apologise if my last quip sounded flippant.

I had a Mikuni on my Commando and spent a year chasing a worsening carburation problem - spitting and running rich on a light to medium throttle at 40-50 MPH. Started perfectly, could go faster and it partially cleared, but the problem was always there when the engine was WARM.

Refitted the Amals and the same problem - the penny dropped. It was the infamous Boyer fractured pick up wires. A weak spark fails to properly ignite the fuel at certain throttle openings and it is running rich.

Not saying this IS your problem, but something to keep in mind.


... going to say what Norman said above. I chased a similar problem all week on my Commando in deepest France before I tracked it down. Why on earth they use those horribl crimped connectors I cannot imagine.


Mark your throttle, see if the symptoms are just at one throttle position, if so them look at the carb settings for that throttle position. If it is not throttle position related but fixed at a rev range then its more likely electrical and vibration related.  


I agree with the above, if you are up to over 40mph before the issues start its unlikely to be the carb. Check the wiring in and around the Boyer pickup and the board its self, cracks or a broken wire can cause these issues. On the connectors at the pickup, I chop these off and solder the wires together!

Regards - Paul.


Soldering wires is fine IF the join cannot move or vibrate.

Boyer's problem was the use of soldered wires onto the pickup board. They could vibrate just beyond the solder joint and thus the work hardened copper wire fractured. An attempt to control this was the drilled holes with tiny cable ties to hold the wires still. Some people Araldited it together. The proper engineering solution was screw clamp connectors, as used on a Pazon pickup board.

This bad engineering has led to many (hundreds of ?) Commandos breaking down away from home.

An old rocket engineer at Marconi told me that soldered wire joints on British missiles was prohibited for this very reason (probably a National secret so don't repeat it).


... heard a tale that when RAF engineers soldered a joint you had to be able to count the wire strands to check none had been broken.

I often hear people talk about soldered joints failing but I've never experienced it in over 50 years. Badly crimped ones, especially using those things Boyer used, yes.


No, the Boyer failure is always fractured wires adjacent to the solder. The PVC sheath holds it together for a while.


... fractured just next to the crimp. As you say the insulator made it look as though it was still intact and it started OK but as soon as there was any vibration it started popping and banging.


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