I have an 89 GL sedan 3 spd AT with 294,000 mi. on the engine that was doing the same thing (getting a mild pinging) at partial throttle. I connected the green connectors under the hood and checked my ignition timing. The ignition timing was right at 20 deg. BTC which was right on spec. I tried a tank of premium gas and the pinging did go away so I'm of the opinion that carbon buildup in the combustion chambers has raised the effective compression. I suppose the injector is running rich enough to cool the incoming mixture at full throttle to avoid the ping but not at partial throttle. My fix was to retard the ignition timing by 2 deg. to 18 deg. BTC and I'm now able to use regular gas again with out noticable pinging at partial throttle. My mileage is still in the 30 mi./gal range. And by the way, if the engine is running a bit hotter than normal, that could also contribute to engine ping. The EA82 may not be a high performance engine, but it's the most durable small engine I have ever experienced.