carrier ultra xl freezing suction pipe

[w8awhile]

Hi Guys,i have a 45ft reefer with a carrier phoenix ultra xl.
the problem is the suction pipe is freezing up and the unit is only getting down to -6c
we have changed the sv3 & sv4 valves,also the tx valve..the system has no leaks,the evaporator is clean as is the condenser,had it on vac pump 18hrs,the only other thing is we have put the unit on M50 hydrocarbon instead of 404a...
any ideas would be great...cheers

[cadwaladr]

My first question is why did you change it from 404a,and did you make sure you pressurised with nitrogen and vacced it with all solenoid valves in the open position if you did then pull the charge and put it back on 404a and see how it performs.