My star engine won't rev over 2600 rpm while in gear, but my port side works just fine.

557 engine with 750 Holley/8-71 blower/Accel Ignition box
So far, checked
Coil, rotor, cap, wires,ignition box, engine oil level, fuel filter,fuel pump,distributor, gaskets ( found out blown intake gasket) ,no blockage on the exhaust, compression are around 120 overall. what is next?

Ign timing/advance?? Throttle linkage??

check the shaft on the blower snout,if the splines hava a lot of ware they might not be engaged very good,i have seen it happen before.

Did the timing light today, 27 idle and 30 when in gear around 2100 rpm. However, the back firing poping noise is happening during the no loaded as well.

Have you checked fuel pressure/volume? Could it be sucking the bowls dry?