detonation / knock issue on 340

shimsham1 said

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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Be sure to hang on to that paper with the cylinder pressure readings.

If all else fails and you cant get a tune without detonation, you may have to pull the heads to check for hot spots or carbon build up. If so, the highest pressure cylinder chamber would be the one I would look at first. Polishing the valve faces, smoothing the chamber, and knocking down sharp edges on the head would likely get you back to good if needed.