Your title says "Breaks up on top end" your description says "won't rev up clear" big difference here, if it is breaking down at 600 RPM that is one thing, but if it is spitting and sputtering up to 3K, 4K, 5k, this could many other things. Clarify how your engine is working it will help.
I am with kart 43 on this though was going to say rich. either way if lean does not help go the other way.it revs up pretty far then starts popping and and breaking up I got it to rev up pretty good but not every time
Timing is set pretty close to stock
port matching highly unlikely
Carb to big maybe
rockers again unlikely
You did not try the needle?
so up till1/2 throttle all is good when you slamit down it starts sputtering?
or full throttle till it reaches say 4000rpm then sputters?
For a few years I've been chasing "splutter or pops through carb" changing mixture, carbys, coils, pumps, fueltank, just about all the guys at the track tried to help, and some saying it's not valve bounce. But it was, stronger springs, and it all went away. If you got doubt take your manifold and carby of and look inside from valveside and if it's black, it's sod from burnt fuel blowing back out through the carby. It will have a clean path in the carby where fuel comes out. You can rev the engine too with the filter of and it should blow fuel out, more than normal, relativ to the type of cam. Please let me know what you find, so I can rewrite my "book"It is lean up top you need to get the mixture at wide open richer. I don't think you are getting to a point where the valves are floating so rule springs out sorry.
Good one mate, yeah I guess mine can be edited too, actually it's writen with a pencil. I used to have GP960 but at about 8900 for one engine and 9200 for another, it would have a "missfire" that could be heard by others. The difference between the two was one had 1/4" stem valves , the other 5.5mm. Now I run GP 965 and rev to under 11000, to much I know. If I could just find a cam with the same power, at say 8500 would be great.He says it starts sputtering at 3/4 throttle, you could be right but there is not enough information given here to start changing parts. He has a 34mm motorcycle carb on this thing if it is not tuned to the engine he can switch all the parts he wants. He said he had dual springs installed did not state what pressure but I will guess, as he has nr rocker arms perhaps he has their dual springs at 65lbs. What weight springs will you suggest, you have as much information as I do. My book can be edited, it does not need rewritten.