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48mm novi's are a little difficult to tune for a single pipe ultra/stxr. The carb single is very weak in the 48mm novi which makes it to where you have to run a low pop off. When I had mine I was running the jetplates in the back verses in the front which is the more popular way to run them. The return pressure also has to be way higher than you think if your at 120-to 100 your to low which this would probably work for a tripple the single needs more pressure.

I never could get my 48's to take off from idle on a dead punch but if slowely pulled in on the throttle is would leave and pull hard all the way to top at about 7280 rpm.
The 48mm carb IMO is just to much for a single pipe non ported machine.

The hesitation from decel is from the lose of fuel which is another reason to have a higher fuel pressure. Plus with the fuel pressure up the throttle response down low will get alot better and should be able to hear it too. Alot more popping around 2500-3000 rpm which is what ya want.

What jetting are you running in the novi's. 145 mains is too rich even 140's and at some point you might go upside down with the jetting. The specs most people give you are very safe and are mainly for tripple pipe applications.

The anti siphone valves are normally built into the fuel pumps of the carbs.

At some point I fianally gave up on the 48's.....while a good carb the overall size and massive air on bottom end the motor could not compensate due to lack of carb single and being a single pipe. It took about 15 hours of tunning with differant jetting and pressures and after being sold I figured I would go simple and find some 46mm buckshots which are a great carb. Are they as good as others maybe not but for a cheap and great running carb that tunes easy....well lets say in about a little over a hour they where tunned and running 7260 with the setup I had then. My R's are now lower since I have more a CC setup now and running less water in the stinger.

Well Tim said the same and still never came out.....I said come give it a shot I have tunned many a carbs and could not resolve the problem on mine but hopefully you will. I also had the modded 48mm novi with the ball and check valve ...which was for a tripple pipe application. But like before they have a very low carb signal so makes it a little tough. It was the same every time, I could feel the carbs just die out at about 1500 rpm or so....

My jetting was upside down too... 120 main and 135 pilot. 2.3 needle 95 gram spring. You need to run a 60 return jet and watch that thing come alive on the low and mid. take the 120 return and throw it down the street....!!