Sounds like a jetting problem to me.
Our 1985 4/4 suffered like this after I installed a K&N filter. Mike Duncan used to service that car for me and is rather an expert on carburettor engined Morgans (mine was a CVH). Mike re-jetted the carb for me and the car was transformed.

I found a jetting calculator here for a DCOE - not sure how this maps on to a DGV: https://www.carbparts.eu/calc

Could be worth checking.

Hopefully someone who actually knows what they are talking about will be along in a while.

Paul
Costock, UK
2014 4/4 Rolls Royce Garnet Red
Disco 5
Teddy - 17h1 Irish Draught cross