Im working on my mates car and the main problem is the car will not rev past 4-5k when you drive or rev it in neutral. he starts getting a little bit of smoke when it lines up at about there and struggles to get to 5k after going past 4

im trying to wonder if it is his carbuartor as ive tried spraying the dual carb, ive been spraying the carb cleaner into the two sponge inlets underneath the air filter but i dont think its done anything. the sponges are black if that helps identifying
im hoping its something easy or so devastating that i cannot do without opening up something bigger. ive thought about taking out the carburator but im not too sure.

if it helps his engine is stock, his headers is stock
he has a loud exhaust mufflers and two hot dog 2" resonators.

help is appreciated
thanks