detonation / knock issue on 340

Updates. Found the carb base plate is cracked. So I’m in search for a good replacement. I did do a compression test. Highest cylinder reading 180 lowest 165. The plugs I just put in 30 miles ago look like this. Guess I’ll continue the work after either I find a replacement base plate or a carb. I’ll follow up once I’m back into it

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Wow that is scary lean. Thankfully only 30 miles.
See how it runs with a new carb, try to get something in the 700 cfm range or an 800 thermoquad.
I am not sure you can run 91, see if it is OK after new carb. I run 93 non ethanol in mine timing 34° all in at 2800.