Spark plug heat range is not about how hot the motor runs. It is about how hot the tip of the spark plug runs. The correct heat range keeps deposits burned off the firing tip and it’s tapered insulator, without getting hot enough to damage the insulator, or cause pre-ignition, igniting the gas from the hot tip before the spark gets there. If your J6s aren’t fouling, the heat range is not your problem
Going one heat range hotter (J6 to J8 like you did) should not make any difference in the way your motor runs, not hot enough to cause pre-ignition; and only after extended periods of high speed running will you be able to see a difference in the color of the deposits on the firing tip insulator. I’m thinking one or both of your J8 plugs are defective. I’ve heard of a lot of Champion plugs lately have been bad right out of the box, new. It has been speculated that the move to a manufacturing facility in Mexico created Quality problems. I’ve had a couple Champion plugs that didn’t perform well too. I have switched to NGK plugs with no problems.
It’s interesting that cross reference charts show both J6 and J8 cross to NGK number B6S, although one chart at Spark plug depot, after saying B6S, also suggests in the chart’s next line down NGK (PU) B6Y, for the J8 whatever that (PU) means.
If you get into using NGK plugs, as that number (the 6 in B6Y) gets bigger, the plug gets colder – opposite of the way Champion’s numbers work.
Now for your initial problem of the motor being slow to increase RPMs when you open the throttle, motors will often bog or lag initially when you open the throttle if the low speed carb mixture adjustment is set too lean. You need a slightly richer mixture there to start acceleration before the high speed jet kicks in. Try moving the low speed mixture adjustment richer 1/4 turn at a time until you can go from idle to fast without any lag, cough , or kill.
Dave
