Won these two rare carbs off eBay. They are the sort that have a vacuum port for the distributor advance rather than using the vacuum port on the inlet manifold. They were only used on the early rubber bumper (and the last of the chrome bumper) cars.
I do not set my ignition timing to the factory 10deg but use the vacuum gauge method that takes into account the modern unleaded fuel (that burns more slowly) and additives. This means the timing at tickover is about 18deg which makes it a bit lumpy when reconnected to the vac advance which advances the tickover timing even more.
This problem is resolved by using the vac port on the carb which gives zero vac advance at tickover – and hence a lot better tickover with no misfires.
These were the HIF4s that were fitted to the last of the chrome bumpers – the rubber bumper HIF4s had no vac port on the carb.
MGB tips mods and maintenance 