High HC could be caused due to a number of different reasons. You recommend having a smog check repair mechanic diagnose your BMW 740il high HC smog check failure. Although your BMW only failed the HC portion of the tailpipe test the other numbers (CO2, O2, CO and NOx) will give an experienced smog technician a closer idea of where in the emissions system the fault is located, i.e. the ignition system, the fuel management system, the catalytic converter perhaps.
Hydrocarbon (HC) is basically raw fuel, otherwise known as gasoline. Among to other reasons your BMW 740il could have failed for high HC might be due to a defective spark delivery (ignition) system. Spark plugs and coil packs (and their harnesses) are constantly exposed to high temperatures and high voltages. You must ensure your BMW's ignition system is functioning properly by inspecting spark plugs and coil packs. If any of the plugs look worn out or have excessive carbon build-up replace them immediately. You may use any brand of product designed to work in your BMW 740il.
Another problem to look into is a condition called "Rich Misfire" (or "Lean Misfire"). When for any reason the engine control computer is unable to properly control fuel delivery to the combustion chambers the resulting exhaust emission will contain high levels of HC, CO and/or NOx. If your BMW 740il is producing high HC because of a fuel control problem, repairing the fuel control fault should level out all three harmful emissions your BMW is being measured for during the smog test; HC, CO, and NOx.
