Roller Camshaft Rpm Range
pic is supposedly worth a 1000 words :)
woah, woah buddy. we don't need AJ up in here...
anyway. these might be helpful to OP
(DV's stuff purposefully left out)
so just off these charts, that cam has a theoretical operating range to about 5~55
so running out of steam around 5K seems about chalk. upping the carb might net some R's if the exhaust is up to the task. also, if it's a knock off air gap that too may be an issue. heck, even the ignition may play a role there too.
there's a whole mess of variables here and any single or combination of them could result in the scenario.