Oscillating idle.

Just guessing here.

You could have too much electrical noise on your ignition pickup wiring and it may be confusing your ignitor.
I changed my pickup wire years ago to a shielded cable to help prevent that.

This may be helped with resistor wires or plugs.

You could lower your idle to see if it still happens. A lower idle usually results in a weaker signal which is more suceptable to noise.
Raise the idle rpm to see when the oscillating stops. Higher rpms usually result in a stronger trigger signal. Also, check your pickup gap.
It could also be a weird vacuum leak.

Got a bit tied up with general shit today so didn't get a chance to see if coil voltage is oscillating inline with the revs. Tomorrow I should have some spare time to check that. I'm also going to drop plug gap from 32 to 28 to ensure I have sufficient coil output voltage.
Failing that I might just go ride it and not care 

A voltage oscillation needs to be viewed on the oscilloscope. and i would be looking at the pickup trigger signal.
The best thing you could do is narrow down what may be causing it.
figure out if it happens:

when the engine is hot or cold
at what rpm does it go away
does a vacuum leak change the outcome
do you have a vacuum petcock
if you kill one cylinder's spark does it still happen?
is there a pcv valve on the bike?
 

F64 wrote:

A voltage oscillation needs to be viewed on the oscilloscope. and i would be looking at the pickup trigger signal.
The best thing you could do is narrow down what may be causing it.
figure out if it happens:

when the engine is hot or cold - can't tell as when on feathered choke its revving at 1500 rpm and doesn't happen then. Once warm enough for no choke it's apparent but only on tickover. 

at what rpm does it go away - any throttle input eliminates it. 

does a vacuum leak change the outcome - its not anything to do with having a vaccum leak. There aren't any. First thing I looked for. 

​​​​​​
do you have a vacuum petcock - no

if you kill one cylinder's spark does it still happen? - not tried it and don't see the relevance? 

is there a pcv valve on the bike? - no. 

 

Sticky float valve in the carbs?  As the fuel in the bowl rises and falls it changes the pressure necessary to force fuel up into the carb throat, which maybe drops the flow a bit?  

Okay, embareeeeeeessssed   I'm blaming Jack 
Whipped the plugs out to regap down to 28, or so I thought!
Don't ask me how but they were all around 10. Surprised they didn't coke up immediately on choke.
RE gap to 28, fired up and almost steady idle!!!

Your transparency is admirable, we all have these moments 

Nowt to do with me Guy. My body but under Jack's mind control. 

Hey, at least you were consistent.

Glad it is sorted. Have had a few "obvious" things fool me over the years but got there in the end. Proves in a way we do know what we are doing.

Rob  B
UK