Hi All. This is my first post. I have away from the trade for 10 years, and am having trouble with the new refrigerants. I hope someone can help me.

My situation is as follows:
I have a freezer (1525 btu) that was designed on R12, running a tecumseh tin can, cap tube system. The refrigerant was changed at some point in the past, and apparently many technicians have looked at this with minimal success in making it work.

I am not 100% certain what refrigerant has been dropped into this thing, but I am guessing 409a , however it is acting like there is a restriction. I am running 150# head, and 6# suction, with a 74f liquid line and a 40f suction. I am interpreting this system as having 38f subcooling in my LL and 40f SH. I can't get the box temp below 40f and with the 40f superheat, my evap is starving.

There isn't a TD across my LL filter drier, so I am ruling out that it is plugged (however I will be replacing it when I open the system).

I am viewing the cap tube as the culprit, and have been attempting to determine;

1./ What size and length should it be for this system.
2./ For the future, what impact will changing refrigerants have on cap tube sizing

I have spent the day on the net researching this and have obtained contradictory information. According to an engineering study posted on scienceasia.tiac.or , R12 requires a longer cap tube than R409A.
According to Supco exactly the opposite is true. (taken from the sizing charts in their catalogue)
The danfoss cap tube part sizing chart bears almost no similarity to the rest in terms of sizing and length relating to capacity.
The trade text "Modern Refrigeration and Air Conditioning" by Althouse posts a chart that doesnt go as high in terms of capacity as the rest, however they have different sizing relationships again.

I realize that I can simply try it out and shorten the tube until I get some performance that will be acceptable, however I would prefer to approach this with a full uunderstanding, rather than a trial and error. If this is a consideration for future retrofits, I would like to know this going in. If I am missing something I would like to know that too.

The main sources I have found I have listed below. Sorry... as a new member I cannot post links.

scienceasia
danfoss
Supco
Modern Refrigeration and Air Conditioning text by Althouse.