xxv top vent kit sealing

for those with the above, how are you sealing the pipe (large) coming off the stove. I have a double walled large pipe used and kept the 18" smaller inside pipe on as well. I tried using a high temp caulk on the inside and still just at start up only get an ever so small amount of smoke. I have a carbon monoxide detector about 10 feet away and has not peeped. just curious as to whether or not this is a problem and how you might have resolved it. thanks.
jim