I see a bigger issue here. If you are riding with a lot of weight on your handlebars, then you are doing yourself a disservice and asking for a crash if your bike ever gets out of control. No one is perfect, and I regularly find that I am riding with weight on my handlebars, at which point I do me best to relax my arms and hold myself up with my body. As soon as I do that, my hands feel 100% better and any numbness starts to wear off. I'm willing to bet that you are riding with a lot of weight on the handlebars, which is causing your hands to go numb. Changing where you apply that weight (throttle aids and big foam grips) can help alleviate that numbness, but the real issue of overloading your arms still remains. That being said, a carb return spring can be too stiff, which will contribute to the issue.
Back to the issue. You don't want weight on your handlebars when you are riding. Handlebars aren't there to hold you up, but rather to hold the controls for the bike and to give you something to turn the wheel with when you are coming in and out of a turn. If you keep your weight on the handlebars when you are turning, then you end up forcing the front end DURING the turn which is counterproductive to control, suspension and your safety. This also applies to braking, which you should NEVER do with full force on the handlebars. I'll explain the braking situation and let you figure out the rest of it on your own...plenty of good books out there about how to ride a motorcycle.
Braking on a motorcycle can get dangerous if your front tire looses traction. In that situation, the tire will slide. If you have no weight on the handlebars, then the angle of the tire will remain constant in comparison to the motorcycle since there really isn't much of a moment to make it want to turn. In this situation, the bike will stay in a straight line and come to a stop (unless you have it leaned over pretty far). However, if you are leaning on the handlebars, then the wheel will turn immediately in whichever direction it is turned the most causing you to endo or high side the bike. Similar effects cause discomfort and ambiguity in the handling of the bike when turning. Any racer on this forum has learned this concept and will keep weight off the handlebars as much as possible. Do this by clamping the bike with your legs, use your elbows on the tank, thighs on the sides of the seat, chest on the tank... you get the point.
A lot of riders talk about how many years they have been on a bike and how much they have learned. However, if you aren't actively learning how to ride a bike, then you are just reinforcing bad habits every time you get on the bike, which doesn't help you improve. Learning about body positioning and correct weight distribution on a bike makes the bike incredibly more safe, you more comfortable and the ride more enjoyable.
I hope this helps you figure out a solution to get over the numb hand issues.
Camelman