Re: Motor revs up when I shake it back and fourth?
Well, I am not an expert on your particular model but I do have a few questions that may help narrow down the culprit. When it bogs down, does it seem to die on you or just move along at a significantly reduced power?
If the answer is that it runs OK but with reduced power, I would suspect you are indeed running on only one cylinder. If this is the case, try pulling each spark plug at a time to see if it dies (use insulated plyers to pull boots). If it dies you can be sure you were only running on one cylinder and the working one was the one you just pulled. Now try switching the spark plug boots and clips and do the "pull one spark plug boot at a time" test again to see if the problem stays with the same cylinder or moves to the other one. If the problem moves to the other cylinder, then the problem is most likely ignition related and if it stays with the same cylinder, it is more mechanical (compression, leaf valves, crankcase seals, head gaskets, etc.) or the spark plugs themselves.
If the problem is, that it wants to die when it bogs down, try pumping the primer bulb to see if that revives it. If it does, then the problem is most likely fuel pumping related. This could be your fuel pump, fuel hose, tank venting, air leaks or carb intake valve sticking. Instead of going into how to isolate all of those it is best you report back to save time.
As for the piston hitting your spark plug, I cannot see why it would happen with the J4Cs and not the J6Cs, so perhaps this is your overall issue. J4C is the right spark plug so it is a mystery to me at this point, why your pistons would be hitting them. You may need to pull the cylinder head and take a look in there. A finger nail of carbon is probably not too much but it doesn't hurt to clean it up. I don't know how you did the seafoam decarb but I personally like to just spray large amounts of pure seafoam right into the carb throat while the motor is running at a fast idle. Then I spray it until the motor stalls. Take out the spark plugs and spray good quantities into the plug holes while manually turning the flywheel. Let it sit for 1 hour. Then start it up and spray some more through the carb. Then take it out and run it hard to blow out the crud left behind. I prefer that method then the "seafoam in the gas" method, since sometimes the "in the gas" method dislodges stuff that might be in the carb jets, fuel hoses and/or fuel tank, causing new problems.
