15deg.F temp? Did you roast outside? What do you mean by turn temp 340deg.? I always understood turn temp to be when the original temp (drop the beans,) then bottoms out after about 1:30+ min., and then starts to reclimb in temp. Temp turns to rise to finish roast.
Is there a P.I.D controller hooked up to your popper? Or modified some way to read temps? 520degrees?
After 28 min. the beans should be charcoal. Unless you did roast outside in that temp.
Years ago, living in New England, extreme weather made it difficult roasting outside. I built a wooden box for my
old West Bend Poppery 1. Worked good in the Fall. But Winter was tough, so I smoked up the garage doing a number of roasts.
I had good results, but would not say great. Still better then buying roasted beans around my area.
I roasted outside for the 15 degrees F. For the 340deg turn was my first roast using the Hottop outside at 20 degrees ambient hum. 45%. The 340 deg turn was the internal barrel temp at which I noticed the beans turning brown. I used a laser thermometer to guesstimate the temp of the popcorn popper and it registered 520 degrees outside and when I brewed the black beans, it tasted slightly burnt. As for my 28 minute roast, that was the 15 degree outside ambient. I was using the popcorn popper, a nostalgia popcorn machine 1,095 watts. I used the same popper for a Vietnam bean and it only took 5 min 49 sec to get full city roast at 41 degrees at 68% hum.
My hottop roasting takes awhile because, a chime goes off at barrel temp of 167 deg F, but the manual says to put beans in at 225 deg. Once I put beans in, temp drops to 160.
Manual says to turn fan on 25-50% and drop temp to 50% once it reaches 200 deg for 30-60 sec to dry out beans.
Once temp reaches 259, drop fan down to 25%.
At 280 degrees turn heat up to 70-80%.
At 300 deg, turn heat up to 100%. If smokes, turn up fan.
Around 4 more minutes about 360 degrees watch for browning.
At 370-375 deg, turn heat down to 50%.
at 400 deg - 1st crack should occur, turn fan up to 25%.
1st crack ends - 1:30 - 2 minutes.
After 13 min 30 sec, turn heat down to 30%.
What I noticed about their instructions is it takes a long time to roast the beans. It seems like they are baked instead. All the youtube vids show the guys not using the fan, but they get a lot of smoke. When I use the fan according to the manual, it keeps the smoke down. My last Hottop roast took 21 min 33 seconds to achieve light city roast. I started with 250grams of beans and ended up with 226grams. Started roast at 168 deg, noticed browning at 350 and finished roast at 420 deg. in the garage it was 32 deg at 67% hum. First pop was 16 min 58 sec, ended 17 min 50 sec. 2nd pop was at 21 min and 3 sec. 1st pop was very heard to hear. 2nd pop was more predominant. I brewed it a half hour later and the beans seemed small and light when grinding. The coffee was watery. I primed it for 30 sec and saw a lot of co2 and brewed for 4 min with proper temp 6 oz water at 11g of grinds. Either it was the roast or the beans are crappy. I'm going to try to roast the Vietnam beans and see how that came out. I loved the flavor on the Vietnam when I used the popcorn machine.
And I would like to add, there is no such thing as "tasty Vietnam beans". They are firmly seated at the top of the bad coffee bean list.