As Dave H said, it will damage the motor if it gets too hot. The trick is to find a gearing that will give you the performance you want, but not burn the motor up.

Try different gearing. If you don't have a temperature meter, buy one, or ask one of the faster drivers at your track for help and ask if you can borrow their meter; they'll for sure have one and most people are friendly enough to help.

What gearing are you running right now?

For 21.5 blinky in a buggy, try to shoot for a final drive ratio (FDR) about 5.5. Take the number of teeth on the spur, divide it by the number of teeth on the pinion, then multiply that number by the transmission's gear ratio (your B4's is 2.4). So, say you have a 72 tooth spur and a 26 tooth pinion, the FDR is 72/26*2.4 = 6.6. To lower the FDR, increase the pinion, or decrease the spur. So, try a 32 tooth pinion with that 72th spur, and now you have 72/32*2.4 = 5.4.