Mower's Rpm dies when deck and collector is engaged

Hi

i currently have a countax c600h with a honda 16hp engine. The carburettor has recently been rebuilt and all the fuel lines been changed. However when i engage my cutting deck and the grass collector the mower nearly dies when I try to move forward to cut my lawn. The deck belt has been replaced. The mower idles at 3000rpm when stationery but won’t stay at 3000 or a high enough amount too cut my lawn, it dips down to 2100 rpm and nearly dies. How would i be able to fix this?

Any help would be much appreciated

James