i have the same machine and ext. i had the same problem and got tired of the roof top ext also. i posted on another forum and an individual responded with taking a bolt and running it through the housing with a nut on both sides to act as jack bolt. position the bolt on the part of the flapper that is inside the housing. adjust the bolt just before the alarm goes off so it takes just a little pressure to set off. i did my that way last winter, but have yet to try it out.
kevin
kevin