one cylinder misfire

1973 850 with a right cylinder misfire at tickover, which I have to keep set at 1500 rpm to maintain idle. The left cylinder is strong and steady, the right does fire fine but every 8th of 10th stroke there is a "cough", then a bit of hesitation, and if idle is set to 1000 rpm the engine will die. The left cylinder's momentum is keeping the right's piston moving. I have fiddled with the carb's airscrew, and have noticed that the right sparkplug is sooty. I have replaced the pickup wires on the Boyer stator. I have swithched sparkplugs from left to right, and also the sparkplug wires, and the low rev misfire is still there. I suspect the misfire also occurs at higher revs, as the right chrome header is much darker that the left, so I think there is unspent fuel being fired in the header.
Next, I will switch the coils, and see if the misfire moves to the left side. Otherwise, the bike performs very well.

Rather than swap components one at time I would pull the plug leads from the plugs only and swap that connection alone, if the problem doesn't move across, swap the plugs over with the same set up. If the misfire doesn't move then its not an electrical problem. You may have to loosen the coil bracket for the leads to reach.

Its a lot easier and less confusing than moving piece by piece.

Have you tried removing the left hand plug, resting it on to the head and running it on the right cylinder only ?

If the right cylinder doesn't respond to pilot air screw adjustment, that would suggest a blockage in the drillings.

My sparkplug wires are not long enough to reach from left coil to right plug, and vise versa. I might have to find a longer set, or as you suggest, loosen the coil mounts. I have an in-line spark tester somewhere, which should show any missing spark. Hope I can find it.
I will also try the left plug removal and run on the right plug only, but I'm sure the airscrew affects the right cylinder.
In my original post I said that the right cylinder "coughs" intermitently, but perhaps I should have said is "chuffs", as that is the sound it makes on the misfire.
I also put my hands in front of each exhaust pipe hole, and as the bike is running, you can feel the left side steadily puffing on the left hand as the cylinder fires, and the right side puffing steadily, as well, but there is also the intermittent "chuffs", where there is no pressure on the hand from a non-fire.
I will check my rocker clearances. Could this be a case where the timing is correct on the left cylinder, but not on the right? IE. the right is not in sync with the left?

singring said:

My sparkplug wires are not long enough to reach from left coil to right plug, and vise versa. I might have to find a longer set, or as you suggest, loosen the coil mounts. I have an in-line spark tester somewhere, which should show any missing spark. Hope I can find it.
I will also try the left plug removal and run on the right plug only, but I'm sure the airscrew affects the right cylinder.
In my original post I said that the right cylinder "coughs" intermitently, but perhaps I should have said is "chuffs", as that is the sound it makes on the misfire.
I also put my hands in front of each exhaust pipe hole, and as the bike is running, you can feel the left side steadily puffing on the left hand as the cylinder fires, and the right side puffing steadily, as well, but there is also the intermittent "chuffs", where there is no pressure on the hand from a non-fire.
I will check my rocker clearances. Could this be a case where the timing is correct on the left cylinder, but not on the right? IE. the right is not in sync with the left?

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Remove the coil bracket and flip it over.

The boyer stator plate fixes the relative timing of the two cylinders, its not possible to have one correct and the other out.
Easy to confirm with a strobe on each plug lead in turn .

If you've lost the airscrew setting put it to 1.5 turns out to start with.

Do only one change at a time and completely eliminate the electrics before you start playing carbs.

have you checked that your right exhaust nut is tight?

I'm new, but here goes.
I'll comment on fuel, leaving the sparky stuff to the experts. Assuming twin Amals, could be an obstructed pilot air channel
on the right side. Does the right side falter when you screw in/out the pilot air screw? If not that might be it.
Take the right Amal off and completely dismantle it - including screws. Get some carby cleaner and attach a tube nozzle to the can
and spray it in the pilot air screw hole, and see what comes out in the carby body (don't look too closely into the body
as you do it, if it's clear you'll get an eye full). You want to see the fluid flush out through the pilot air channel.
If it appears to be blocked, get fuse wire and work it through the air channel. It's tricky and you'll need to be persistent and patient,
but it will go. These channels can gum up if the bike's left sitting for lengthy periods. My 2 cents worth, good luck. Let us know how you go.

Try grabbing the carb and giving a good shake, mine was loose and caused similar problems. They can appear to be quite tight as the airbox gaiters will hold them in palce quite nicely. I know this will cause it to run lean but I looked at the plugs there was no real difference, but still it popped and spitted on over-run when closing the throttle. I finally noticed it when the left hand carb fell off 12 miles from home - still got home on the other cyclinder, and the left one even ran with little load on the engine when coasting. If it's too much fuel suspect the viton tipped float operated shut off valve is doing what it is supposed to, as if it fails it will just let fuel pour into the carb, causing the richness you find.

I am hardly an expert, but have been working on a similar problem lately. I kept fouling plugs and kept blaming carburation. However, I am convinced now that it was poor spark or no spark that was causing the unburnt fuel to foul the plugs. As a result I have been going through the wiring harness checking and upgrading every connection. This process has paid off for me.

95% of carburetor problems are electrical.

I get a similar problem once in a while. For me it has always been the idle mixture. Another possibility is a coil grounding internally. The stock coil clamps if tightened too much can crush the coil case and short them to ground. An easy way to determine this is pull the coil out of the clamp, wrap it in a plastic baggy the put it back. Start the engine to see if the misfire is fixed. The baggy insulates the coil body to ground. This never worked out for my problem, but was suggested by someone in the NOC in the early 80s.

Assuming Amal concentrics.
After setting the idle mixture I check the exhaust pulses with my hand near the end of the muffler. Sometimes one side will have a steady beat while the other intermittent. Usually need to close the mixture screw on the intermittent side by 1/8th turn and it clears up.
Off idle problems can be worn jet needle and needle jet.

Try to determine if the "pilot bypass" whole in the right carb is free & clear of any crud that may be causing a rich condition. that cauld explain the symptoms you describe if the ignition checks out to be OK.

A mate had this issue with his BSA lightning this week, he was fed up with looking for the problem and I offered to take a look. The mk1 amals were apart so I blew some air through the pilot intake and put it back together. It initially ran fine for a few minutes then the rh cyl started missing and would chime in sporadically. A quick check showed 11.5 volts at the battery and I suspected the boyer wasnt really happy as it seemed a similar symptom to one I'd had with my atlas (on total loss electrics) when the voltage dropped. Charged it up overnight and next morning the problem seemed even worse. Back firing unburnt fuel in the muffler, hard to start and not a happy bike, but appeared to be intermittent like an electrical problem. I even wondered briefly if I was facing a burnt valve!
Thankfully the ht leads were long enough to swap the leads over but the problem stayed on the rh ! yay progress. took out the pilot screw and blew the pilot circuit out backwards with compressed air and reset. Instant result! back to behaving itself nicely. 8)

"95% of carburetor problems are electrical."

I don't buy that at all.

It's more like 98%!

Pilot curcuits can be real mf'ers. You need a number 78 drill, cut off a q-tip and epoxy it in the end in order to reach in there. I finally drilled out the back tapped it and cut off an old throttle stop screw to blank it off. No more questions as to the blocked pilot.

use a tail lite soket and alligator leads accros the offendiing ign circut. wheb the points open ,, the bulb should be bright! When the points close, if there is a dull glow in the filament , this would indicate a bad, or weak, condenser. ( assuming stock points & coil on this year model)

I hope singring has it sorted since Oct '09.

Dave
69S