ok first thing
are you sure the tester is reading parts per million because 100 to 150 parts per million when calculated into percentage is EXTREMELY LOW for co readings
be sure it's not a percentage reading you have
if it's a propane engine then you dont want to see more than 1% at the tail pipe
thats one percent not 0.1%
usualy hydrcarbons are read in parts per million, be sure its a co tester
tell me these things first then we can tackle any emission problems you have