Right cylinder rich burn

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Sep 25, 2007
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Hi< I bought a 1969 Commando with a 1973 850 engine. It has a single 36mm mikuni with K/N filter. I immediately had to replace the intake boot because of cracking . I now have problems with my right spark plug fouling black in a matter of 10 minutes riding and eventually it stops firing. I have been through two plugs to confirm this. Also I put replaced the fuel lines and drained the tank to make sure no crap got in the carb. It also has a inline filter. I am not very knowledgeable about the mikuni and need some advice. Thanks
 
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Ignition coil? have you tried swopping coils and or leads? Do you have points or electronic ign?
 
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Are you still running the stock Norton coils? Are they 6 volt stock coils or been replaced with 12 volt ones as the years went by ? Boyer's don't like that. Are you still running .025 gaps on your spark pugs for points ing.? Boyer's don't like that. How are your wires to the plugs?Would you grab on them in a rain storm? Hope the wires are made of copper and not fiber with carbon. Boyer's don't like that. Load tested your battery lately?Boyer's don't like volts below 12. If you have more that 10,000 miles on the boyer have you fixed the leads from the coil plate were the points use to live?Boyer's don't like that bit of maintenance left out. When the Boyer was installed did the person add some grounds to the system ? They are really needed. Lashed the valves on the bike at least once a year? The list goes on and on. Hows the kill switch? hows the key switch? Hows the Mikuni jetted? Clean air filter?
 
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Norbsa pointed out all the stuff you must check anyway to eliminate all possibles. Does sound like a specific problem to that cylinder and if the other one fires OK try and swop parts over. Better still change both leads and caps anyway, check the coils for badly crushed cases (can short internally but will work if temporarily detached from frame ground).
Bad Boyer connections will stop any kind of spark from happening as although this system fires both coils together, triggered by either magnet you need the circuit to be complete via the board mounted coils for either plug to fire. So if one cyl fires OK I doubt the problem is there.At least with only one carb your problems are simplified. if it works on one cylinder it must be electrical or at worst tight valves, leaking valves etc. Is compression good on the bad cylinder?
 
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Rich burn

Are you still running .025 gaps on your spark pugs for points ing.? Boyer's don't like that.

What plug gap is proper whith a Boyer?
 

L.A.B.

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Re: Rich burn

zotz said:
What plug gap is proper whith a Boyer?

Boyers work fine with the standard plug gaps as far as I am aware, and as I recall, they certainly used to recommend using the standard plug gaps with their systems?
I have always used the standard plug gaps with no problems at all.
 
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As you open the gap on the plugs you can put heat to the coils nothings free. I found some power and better starting playing with coil, plug and plug gap, I don't like going backwards from there as always do what you want you will anyway. The best gap is the one that yields the best spark that doesn't overheat your coil. Hope you find your problem.
 
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