The exhaust deposits are dry. The exhaust and gaskets are only about two months old. It's completely intact.
As long as there is no "ticking" noises coming from the the pipe to head interface, you are probably OK there. The dry deposits support the "too rich" theory.
As for the tach, I might have a non-stock one? I'm not sure. This is what it looks like, and what I'm going by when I state numbers:
Looks like stock and shows the 9200 red line. I can't imagine you actually had the engine operating at 10-11k.
I measured the float level mechanically. I haven't made a float level measuring device yet. I didn't measure the jets, but they do say what number they are. The needles are all in the stock position (4th from the top or whatever it is), as I put the clips on them myself. The carbs are stamped with 022A. I cleaned the emulsion tubes.
The dark spark plug deposits (along with equally hot head pipes) I interpret as a general rich condition. It is indicative of combustion temps not reaching the level where the deposits "self-clean" off the plug electrodes and center insulator. Extra fuel acts as a coolant, limiting the "self clean" action.
The surging you reported can easily be caused by those soot deposits, as the spark follows the soot/carbon trail, rather than jump the spark gap when the atmosphere between the gap makes forming an arc channel more difficult. (Electrical current wants to follow the easiest path available.)
You could try brand new clean plugs to see if the "surging" stops. But, if the mixtures remain rich, the that problem will return. That center electrode insulator is porcelain and white. A coating of black carbon defeats it's purpose. Since you already have hotter plugs, the condition points to over rich mixtures.
It is certainly possible that an old air filter is causing excess restriction (similar to adding partial choke) and making the mixture rich across the entire operating band. This would lead to sooting spark plugs and result in spark "misfires" where the current is channeled via the soot path rather than jump the electrodes. This, of course, would make the soot build up even faster.
Personally, I don't believe there is an issue with your ignition system. If new plugs cure the surging, and a new air filter doesn't cure the soot build up, then there is something in the carbs that is out of factory spec. You may have to check for non factory parts or some field alterations made to the parts you have. Have you considered using the UNI NU-4055 filter?
I thought I might explain the popping in the exhaust during decell. It is caused by the build up of combustibles inside the exhaust system being ignited by a glowing ember. An over rich mixture feeding the exhaust will pop when decell is followed by accell. And over lean mixture causes a missed firing cycle and those unfired charges are expelled into the exhaust pipe, where an ember from another cylinder's exhaust can light it off, or subsequent fire cycle from the same cylinder can. This latter situation was a real problem for 80's motorcycle "lean burn" carbs. And, the factory cure was to add enrichment valves to sweeten the decell gasses and cool them before an explosion occurred inside the pipe.
