Season roast with salt and pepper; place over indirect heat on medium-hot
grill.
Stir together remaining ingredients and baste roast every 8 to 10 minutes
with mixture, until roast is done (internal temperature measured with the meat
thermometer is 155-160 degrees F), about 30 to 45 minutes.
Let roast rest for 5 to 8 minutes before slicing to serve.