"Spark knock" is actually detonation, and doesn't happen unless the engine is under load out on the road. What you might be hearing could be a valve sticking intermittently and slapping back onto the seat as it releases. Try this- - - -get some upper cylinder lube such as Marvel Mystery Oil or Wynn's Friction Proofing (if they still make the stuff!) and pour several ounces down the carburetor with the engine running at a fast idle until you get a heavy smoke from the tailpipe. Then dump a big slug of the stuff down the carb to stall the engine. If the engine doesn't stall, shut the ignition off quickly. Let it sit overnight, then start up and goose the throttle several times to blow away any carbon deposits on the valve stems that have loosened up during the overnight soak. I use KROIL for this procedure, but I buy it by the gallon.
The exhaust smoke cloud will be epic during this process, so warn the neighbors not to call the fire department!
Jerry
