#1 make sure everything is well maintained, tire pressure, chain condition/adjustment, wheel bearings, keeping oil changed, valve adjustments as needed, i'm sure theres more...
#2 might look into timing advance modifications, i dont know if its possible on your bike or not though. otherwise try to run the lowest octane your bike is designed for, typicly more btu's in lower octane i belive
#3 jetting you could try installing a one size down pilot or main jets, if your needles are adjustable you could raise the clip one position, you could try different needles. if the jetting is not stock, see what affect going back to stock settings has.
on my own bike i replaced the aftermarket needles, that were already installed when i purchased the bike, with factory needles. results - runs smoother/ more power off idle to 6k rpm, slightly less power 7-9k rpm, 9-11k rpm same, 40 mpg with the aftermarket needles, 44mpg after installing stock needles without any change of riding style.
#4 have your carbs synchronized, you can buy gauges, or have a shop do it
If you change your jet size make sure you take an infrared thermometer to your headers- If you lean it out too much it can get too hot and burn your exhaust valves.
An easy way to get better mileage is to change sprocket size. If you go bigger in the front and/or smaller in the rear it'll lower your cruising RPM's. You'll lose some OOmph... but everyone knows that GSX-R's are for corners, unless you're in the midwest (...).
Also, you can crouch down and rest your chin on the tank all the time. I always consider doing it, but I feel too 'cool.'
Might be able to leave the mains. If you're gonna try to get better mileage, you're gonna use small throttle openings which will work with the pilot jet, needle, etc. There aren't a lot of needle options on the OEM carbs because they are CV carbs, and the ones available are going to be developed to increase performance.
Before we were required to run catch pan lowers in racing, we didn't use lowers as they were more of a windbag that developed too much frontal area. You'd have to experiement. All depends upon what you want to do.
I think the old Suzuki Samuri used a similar set up, but I might be wrong. You'd have to build manifolds, etc.
The '89 model 750 is kind of unique. The 88/89's had big bore/short stroke compared to the 86/87 and the 90/91 models. Not particularly heavy on the torque side compared to those others. The carburetors, as I remember them, were larger than the 1990 models. I think Suzuki decided that they wanted big intake ports, and they struggled. Probably doesn't help economy. Might help by going to a 1990 CV carb, but I can't say for sure. I'm not sure how handy you are either.
What kind of mileage are you getting now? As it is, it's probably a 100RWHP bike with a weight of about 410# or a little more.
I haven't rode it in 10 months or so.what if i did some grooves behind the carb on the carb body? would that help any?
With a CV carb? I can't say that it would accomplish anything significant.
You might be better off looking at weight reduction and friction reduction. Going to a 520 chain and sprocket conversion would allow you to change the gear ratio to something that might suit your riding and reducing cruise RPM's. Additionally, it would reduce the weight of the drive train.