Engines:loosey-goosey for high rpms

A good 'J' engine will turn high RPMs, somewhere in the high 6,000 rpm range (although you can get them to turn higher, you will find that this shortens engine life and in all reality you wind up out of the power band of the engine.)In order to get your engine to put out those high Rs, you should do all the common things to pull max revs from the engine, including boring the engine 0.030 over; machining the cylinder block ports and head surface; plugging the appropriate water outlet holes (according to the APBA UJ rules); and thinning the lower unit gearcase to minimum dimensions (again according to UJ rules.)
Of course, you should balance the piston/rod assemblies, crankshaft and flywheel as best you can (if you don't know how to accomplish this task, check with someone from your local club). Another thing to try is not torquing the center two crankcase bolts to the maximum value. This will let the crankshaft turn a bit easier. Don't forget to the lower unit. Make sure there is the maximum recommended gear lash in the lower unit. Checking gear lash is not something for a neophyte so you might want to check with one of your local wrench gurus for this. The info is found in the OMC manual. Use a thin high grade gear oil to minimize gear drag (like 40 or 50 wt oil as opposed to the standard factory oil that is in the area of 110 wt.)
If you have a new or newly rebuilt engine, it is important to spend some time breaking it in. Vary the engine speed and temperature.

Remember to clean all the electrical connections, especially the ground connections. Dirty connections can cause poor engine performance or may prevent the engine from running at all. Next are the fine adjustments. Set the engine timing to the maximum position to get the highest RPMs. Use a hotter spark plug than the standard OMC plug which is a Champion L77JC4. I would recommend trying a long reach plug like L61Y, B8ES, or an N2C.

On some engines, the position of the restrictor plate behind the carburetor can affect engine performance. Try sliding the plate up to put the plate hole in the top of the venturi. Or, turn the plate upside down, and slide the plate hole to the bottom of the venturi.