have you tried changing the arb itself?

has it always done this or is it a recent thing?

the 2wd 28mm arb would be worth fitting, just measure yours first to see if it's not the same though, the granada 24v used a 29.3mm arb which is a direct replacement fit if you can get one, improves the front end on even standard setups so worth investigating if you can get one for cheap enough