Perhaps describe in detail how you have determined its running rich - and at idle, at cruise, full power, all of those?
What are the symptoms you see? Just the plug color but no actual running problems??
I run avgas pretty much all the time, but when I did run 95ULP for about 2 years, the exhaust went sooty black real quick, can't say about the plugs, never needed to look.
2-3 years ago I had rich flooding at idle and it was nothing to do with the carb. Checked all that, spoke to a trusted carb expert.
Carb float level perfect, and carb was not flooding "on the bench"- only in the plane when it was at idle. Faltering idle that would come and go.
Reading through all of the jabiru manuals there were clues.
Overhaul manual - "carb flooding - electric fuel pump giving too much pressure". I don't use the electric pump at all, so it could not be that.
The mechanical pump though?????
Overhaul manual describes fuel pump pushrod has a 0.10mm length tolerance. Why would the length change? It can't. "Effective length" can though:
I determined this was the problem with the mech pump by bypassing it, just ran it on gravity feed. No rich, no flooding.
Mech pump setup is (for mine at least) PUMP - GASKET - NYLON SPACER - GASKET - ENGINE.
My gaskets had crushed down from about 0.70mm thickness each, to 0.36mm T each. I had lost 0.72mm of thickness total.
Remember the 0.10 pushrod tolerance? It's set at that to give the correct fuel pressure I presume. I was way past that. It's just a stock quality automotive pump, nothing special.
My fuel pump was effectively 0.72mmm CLOSER to the engine that it should have been, the pump was over pumping - too much pressure (for idle at least)
(The pump lever has way more available travel than the pushrod can supply.)
Made new gaskets from 0.70mm quality gasket card - PROBLEM SOLVED INSTANTLY - nothing to do with the carb.
Why did the gaskets crush? Too much bolt checking? Shit gasket card? Both?