Morris Minor Owners Club

It is no good just changing the carb and hoping for the best. The needle size and profile has to be determined otherwise the mixture will be all over the place.
The crankcase vented through the carb (as marked in diagram above) is a suitable way to vent the crankcase and as an example is the way the 1275cc Midget/Sprite crankcase is vented. The 1275cc Midget/Sprite crankcase is vented via the canister oil separator/filter on the front of the timing chain cover direct to the carbs. The pipe splits into two just before the carbs and connects to both carbs.
There are several different needles fitted to the MGB - for example MB, AAU & ACD. The Minor 1098cc standard needle is AN. The needles may look all the same but they are of completely different profiles.
I have in the past resorted to reprofiling the needle to obtain the optimum performance when fitting larger carbs.

Agreed, correct spring, jet and needle are essential if you don't want to get in a complete pickle.

I've read a fair bit about the different needle sizes myself, as well as how to check them against the SU specifications for the carb, so I'll be doing that. Perhaps then, if I get that bit right then venting to the carb will be okay. I'd rather do that than try to fit a vent to an air filter housing.

Useful point from moby about the orientation of the float chamber though. I guess any changes need tweaking a fair bit until everything is running right, or as best as it can.

I believe you said you would be using an HIF carb in which case you do not need to worry about float bowl angles, this only applies to the HS (and earlier)

Yes you're absolutely right. Sorry; not got the carb yet so my plan is to order that, order the manifold and then start sorting it all out before trying to fit. Several carbs on ebay around the £80 mark but I'm expecting to service it first, check the needle, get hosing to blank off the two lean outlets etc etc. I'm sure there will be lots else to figure out as I go...