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

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(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.