what does it do to change timing on a 21.5?

the easy answer is changes the power curve and rpm. Which will change the needed gear ratio.

I think most who use a ESC that doesn't have timing advancement are CRANKING the timing up, but those who are using timing adjustable ESC's (LRP , TEKIN and others ) don't need to advance as much (if at all) and use the ESC's timing to get the effect.

The bigger question would be -- what would YOU be looking to improve, that TIMING can improve for you?

I Run The Tekin Rs Which Is Adjustable And Those Motors Still Work Best The Timing Pegged As High As You Can Turn. And Then Fine Tune It From There With The Speedo

iam using the gtb, so will turning the timing up make it go faster?

crank it and forget it!

thought Novak GTB was not adjustable.

It's not. I think he was referring to the timing on the motor. The GTB is one of the lowest timing speedo's around. Therefore mechanical timing on the motor needs to be maxed with the GTB. If your club allows it, using a GTB, you'd probably be better off with motor that can do more timing than a Novak (12 degrees of timing).

which direction do you turn the timing ring to tun up the timing?

When looking at the rear of the motor (pinion away from you), losen the 3 small flathead screws, and turn the timing ring counter clockwise as far as it will go.

how much will it change my gearing?

if your gearing was exactly right then maybe a couple teeth less on a 170' carpet track