Based on the information you gave, the motor is definitely detonating because that is the only thing that will shatter a plug. If it is something that has just developed over the years, then more then likely the problem is an air leak.
If you blipped the throttle, would the rpms comes right down or do they come down slow?
The first test I would do it to see if there is an air leak. This is very east to do with just a few parts. First you will have to remove the pipes and carbs. If you have a stock bumper, take the rubber ends off and bumper, get a hose clamp that will fit over it and then put those ends on the exhaust flanges making sure to tighten down the hose clamps. Now go to your local hardware store and by a set of electrical pipe ends that are close to 28mm or 1 1/8" inch. You can also use a PVC end that also has an OD of 28mm or 1 1/8". Now buy a schrader tire valve(the kind that have nuts on both ends and then drill a hole in one of the caps you bought to stick in the intakes. No matter what size you choose. Now make a hole to you can screw the.
Now normal case pressure is between 4-7 pounds, this is not really important but it is good to use it as a guideline so you don't pump in to much air when you check to see if the motor is sealed. Now that you have the intake and exhaust sealed, pump up the cases with about 6lbs of air. Now wait for at least 15 minutes and then check the pressure again. If it is still at 6lbs, then you have no leak. If it loses any air within the 15 minutes, then you have an air leak and will need to find it. The best way to find the leak is to remove anything that covers seals(side covers, flywheel and the area where the to case half's meet) then pump it back up to about 6lbs and spray with soapy water and you will find leak by looking for spots that soap bubbles are forming.
Now that you have found the leak, you will need to fix it and that depends on what is leaking.
If there is no leak, then my second guess would be that the gas you are using is bad, probably from sitting over time. It may also have low octane and the motor requires that higher octane fuel be used.
If octane is the problem then the fix is the easiest to fix. Just by better gas like some race fuel. If your motor is not set up with that much compression, you can also get away with a 50/50 mix of regular.
If you do these two tests and nothing changes, then there must be a more serious problem.