Join Date: Jan 2003
Location: Rapid City SD, Black Hills of South Dakota
I don't have experience with a custom setup like yours, but generally speaking conversions from R12 to R134a call for charging the system with R134a at 80% of the quantity of R12. So in your case, 800 grams (assuming the recommended R12 quantity is correct).
That, however, is a general guess. I would start with 600 grams and see what vent temps you get, then add a little at a time, wait and check vent temps again. Stop if you're not lowering the vent temp. Again, generally speaking, a little too little is better than too much, as far as efficiency goes. Hopefully, the oil type and quantity in the system is correct. Too much oil hurts the efficiency of the refrigerant.
From my personal experience doing a conversion on a '91 (R12) 164, the 80% figure was a little low, IMO. The 80% rule of thumb would call for 36oz of R134a, but I found through experience, that about 40oz worked a little better.
Also, unless the hoses you used are R-134a barrier hoses, you can expect a yearly loss of refrigerant and a top-off required at the beginning of the cooling season.
Last edited by Roadtrip; 02-09-2017 at 09:34 AM.