Are you sure it's not leaking around the o ring you really have to seat those carbs down tight before you tighten the pinch bolt tower engines have the same problem or the o ring won't contact the carb and engine block
you could take it too the hardware store and find an o ring that fits the carb but has a slightly larger thickness

Now as far as your low speed problem if it doesn't go away after you seal up the carb try richening the low speed while keeping a good transition don't worry to much about how low the idle is till it's on the plane
I think some complaints of idle problems come from people expecting too low of an idle as long as you can get it down low enough for your application that's all that matters I like to keep mine as high as I can get away with as long as the plane doesn't taxi away from me on the ground and will slow down for landing

When I set up my tower engine I open the carb wot then back the idle mixture out till there's a 1/8'' gap between the needle and the spray bar then run it and set the HS needle then throttle down to idle and work the lowspeed
if you set the HS with too small a gap on the low you will chase your tail and get frustrated in a hurry just something learned right before I was going to see what my 30-06 could do to a tower engine
another tip is don't mess with the low speed until the tank pressure has had time to bleed off at idle ohterwise your setting it with higher tank pressure then when it bleeds off your lean again hope that makes sense
GMS and Tower engine don't tune in as easiely as some of the other brands do but if you take the time to learn how to set them up you will enjoy a good running powerful engine.

And don't mess with a pump unless you want to move your tank back on the CG if you can get the tank height correct and the engine has no mechanical issues it should run just fine also run the smallest tank you can for the flight time you need