I have not checked the timing and do not know how to check an accelerator pump. When it is in nuetral and you rev it up it revs but is slow to respond both when you increase and decrease.
These bikes need a tuneup every 3000 miles. If you don't where the bike is on the maintenence schedule, then you should do a complete tuneup to establish a basis.
Tuneups include:
Air filter - clean or replace
Point gap - set repleace if worn.
Timing checked/adjusted
Tappet clearance checked/adjusted
Spark plugs cleaned gapped or replaced with properly gapped new ones.
Although oil changes w/filter should occur more often it is usually done at this time, too.
The accelerator pump is the extra doo-dad hanging down from number two carb. It has 3 screws on it.
It's function is to give an extra squirt of fuel into the intake runners and allow the engine to quickly pick up from low speed. The device is a diaphram pump that is actuated when you twist the throttle. There are interlinking tubes across and between the carbs so all carb throats get the shot of fuel. form the diapham pump.
The diaphram can rupture with age and this reduces or prevents that shot of fuel when you twist the grip.
If you can see into the carb throats, perhaps with the air filter box removed, you should be able to see the throat area really wet with gas when you twist the throttle wide open.
The slow to decrease RPM complaint puzzles me. Perhaps a vacuum leak in the intake runners, or something blocking the throttle from changing to a lower position.
Some cars have a dashpot interlink to the throttle to slow throttle closure. This give the automatic transmission time to disengage before killing the engine when you stop the machine. I haven't actually worked on a CB750a. So, I don't know if it is similarly equiped.
Good luck,
