Man, you ask an old, feeble mind to remember about stuff that has cob webs all over it.
J (M, 1000cc) engines are a bit better, 998cc, instead of 1015, smaller bore, different cranks, rods, pistons larger valves, cams, larger inlet and exhaust ports, not a bad start over the older early Z series.
The ONLY issue we ever saw when Muzzy ran them (and I built them when I was in charge of R&D at RC Engineering) was the cam chains, cam guides, and cam chain tensioners. The early engines run roller cam chains, the later ones, link types (flat plates with connector rivit pins, like a timing chain in a car). The main problem with the link chain is that when it is in full operation rpm's, it tries to go round in shape, and doesn't change direction well at all. So, when the chain needs to "bend backwards" as it travels up, and back from the top end, it wants to literally force itself into the guides and tensioner parts, and is working against itself.
With the type of crank/cam orientation of these engines, old and new, that ain't good. When I worked at RC, the first J/GPZ crank I did was for an early GPZ1100, and I did a fast one with it. I used an old engine center pin, with roller chain sprocket, from the 1980 MkII engine (16 teeth, as opposed to early Z 15 tooth), machiined the cylinder and cam cover for rthe early cam tensioner/roller sprocket guides, and made a manual cam chain tensioner, eliminated the link chain guides, and made sprocket adapters for the later engine 2 bolt cams, to the early cam 3 bolt roller type sprockets. The owner was then a UPS driver in Cookeville, Tenn., and I spoke with him about a year ago, same engine is still going well, NO failures of anything, cam drive setup working just fine. The reason we got that engine in 1982 was, it tossed the cam chain and guides, broke the tensioner.
It is also NOT practical to attempt to graft a later J/GPZ head onto the early engines. The later engines have a lot more potential than the early ones do, and that is the series Muzzy did for the Lawson racers. With the exception of only that one engine I helped modify to roller cam chain setup, Muzzy used the stock J/M/GPZ link chains and guides, but changed them just about ever second AMA National race event, just to make sure they didn't get old enough to make problems.
I always wanted to do a different center in for the crank, and sprockets that were for gear drive, then make a drop in drive pod to do same, but, never got around to doing same.
Definitely look into doing a later engine if you can, they have a lot better potential from the start than the early ones do, and, when done right, about the only thing that beat them was the rider falling off the bike.
For cam profiles that work, you have an excellent source near you, Jim Doerr at Mega-Cycle Cams. His cams just plain work well in road race engines. From what I also remember, his wife Barbara, was/is super knowledgeable about cams as well, and is a nice lady to speak with.
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