Single cylinder status idles great, but...

... twin cylinder idling questionable on my 1973 850 Interstate with Boyer analog MK III ignition. I have noticed lately that at idle the left cylinder fires intermittently, like maybe 40% of the time and "chuffs" the other 60%. The right cylinder fires every stroke, and "carries along" the left cylinder when it is not firing. I adjusted the valve clearances, checked all ignition wiring, switched sparkplug leads, confirmed that carbs are synched, but still the same issue on the left cylinder. Now here is the oddity: I removed, and grounded, the left sparkplug. Kicked the engine over and the bike idles well running on the single right cylinder, and spark appears to be good on left sparkplug, so I adjusted the right throttle slide stop position so engine idles at 1300 rpm. Then I removed and grounded right sparkplug and installed sparkplug on the left cylinder, kicked the engine over expecting to have lousy firing on left cylinder but was surprised to find left cylinder is firing just as strongly as the right cylinder did, and noted that spark is also good on the right sparkplug. What the ...? OK, so I adjusted the left throttle slide stop position so engine idles at 1300 rpm. Then I installed the left sparkplug so engine will run on both cylinders, kicked the engine over, and now left cylinder is back to firing only 40% of the time, and I also noted that rpms are now down to 1000 rpm, and struggling to run, when running on both cylinders. I had to raise the throttle slide stop position on both slides to keep the engine from sputtering out. The engine does run, and ride, well as throttle is applied, but as I ride and back off the throttle approaching a traffic intersection, for instance, the engine deceleration is not smooth at lower rpms and the engine, and bike frame, shudders. I am inclined to think that the left carb is not feeding fuel at a steady rate, but could the ignition also be a possibility in that the spark is weak to the left cylinder when engine is operating on both cylinders?
Any suggestions are appreciated. Thanks

Boyers are notorious for causing one cylinder to misfire somewhere less than 3000 rpm. Runs great above 3000 or so. Check the wires from the Boyer pickup plate. Check with a meter, they fracture inside the insulation near the pickup plate and the fracture is not visible. There's a ton of info about this problem and how to fix it on this forum.

Thanks Jim...I had checked the resistance of the pickup coils, and continuity of wires on the stator plate earlier this summer. They showed to be fine, but will do again.

singring said:

Thanks Jim...I had checked the resistance of the pickup coils, and continuity of wires on the stator plate earlier this summer. They showed to be fine, but will do again.

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If you can do it after the engine is warmed up, it would be a better check. Also wiggle the wires while you check the continuity. The theory is that the wire strands are being held in close proximity by the stiff, cold insulation. When it gets hot it softens and lets the wires separate far enough to screw up the timing.

Pull points cover to inspect wiring. If all good ,work back towards battery. Check fuse corrosions and determine battery strength with voltmeter. How old battery ?

I believe I would at this point, make sure both carbs are equal in reguard to slide/ idle speed screw. Back off throttle cable, back both idle speed screws out 'till slides are bottomed in body then 2 or 3 turns in after you feel the screw contact the slide; each carb. As it is now you really don't where the slides are in relation to each other and the one that's open the most will carry the load. If this proves to be the issue you can now go back and adjust mixture screws. At any point the engines idling too fast, back out both idle speed screws an equal amount. If all is going well at this point adjust both throttle cables so slides lift and drop together.

there is a very fast, conclusive, and easy way to verify if you really do have a carb problem, or not

simply swap the carb themselves left to right, just take them off from the two manifold to head allens

you should not even need to remove the tops of the carbs to swap the slides as there is enough cable
slack to swap them straight up

quick and easy way to eliminate the carbs as the problem

All great suggestions...the battery is new last year, and was fully charged before my adjustments. Not sure about switching the carbs as they are left-side-right-side specific, but for a short trial it might work to proof them. Also, I did have the carbs properly synched before, but no harm in doing it again.
thanks to all