1982 Johnson 25hp occasional pre ignition. Previously running perfectly but now (after storage period) starts immediately and tickover is OK except occasionally fires way too early and either stops or just recovers. This happens approx every 10 to 20 seconds. Also will not rev, on advancing throttle engine speeds up a little then dies and same occurs if opening throttle by hand, (not via usual throttle lever). Carb stripped, cleaned but nothing found and tried adjusting mixture screw. Suspect Power Pack? but how to test - any suggestions?