This is on a hitachi carb, from an '81. Probably slightly different to the USDM ones, the engine has NO (yes, absolutely none) emmision related stuff at all.

 

Anyway, i've been trying to set the idle mixture, by turning the screw 2.5 turns from closed, and then adjusting to give max rpms. Problem is, adjusting the screw isn't having that much of an effect on the revs, the engine keeps speeding up very slightly untill the screw is right out!! That can't be right...

 

The idle is rough wherever it is set, but the engine runs smoothly off idle, with plenty of power.

 

Any ideas?

Off the top of my head, maybe it's got a vacuum leak in the base gasket letting in enough air that the adjusment doesn't make a difference. I suppose that extra fuel getting in via a leaking top gasket could do the same thing.

But would either of those not cause the engine to run bad off idle?