A lot of marinades are very high in sugar. Sugar burns quite quickly when it is at a high temperature. To reduce the likelihood of the marinade burning, remove the chicken from the marinade and use a paper towel to remove the extra marinade. Once the chicken is almost finished grilling, add the additional marinade to the chicken for the last couple of minutes. This will add the flavor you are looking for but since the extra marinade has not had as much time to cook, it is less likely to burn. The same goes for any high sugar sauce like BBQ sauce. The longer it cooks, the more likely it is to burn. But....a little charred flavor is soooo good. :)