I had the same problem with my Ultra last year after installing a 3.4 pulley on my SC & doing a PCM re-flash. Fuel pressure checked 45-48 psig on prime & 45 @ idle & increased when burping the throttle! I soon developed a clogged cat, so I removed it and installed a bypass pipe. That helped regain the HP, but I still had the 'drop dead' feeling @ 5300-5500 rpm.
About a week later, I found a hire-line crack in the plastic return fuel hose, so when I fixed that problem, I was hoping that was the solution. It did help somewhat, but did not totally resolve the problem. At that point I was thinking the fuel curves were off on my tune, causing a lean condition and hitting KR. We modified the fuel curves again & did another flash on the PCM. Next, we recorded KR on the scanner @ 5500 rpm and found retard @ 2-4 deg, but I knew this was not enough to cause the engine to fall flat on it's face.
I had a lot of other things going on during this time, so I was forced to live with the problem for about 2-months. Then one day I was driving to the our shop & the engine just shut off. I was able to coast into a side street & called my son to bring the trailer & a fuel pressure tester. When he arrived & tested the pressure, it had 10-lbs, so we loaded it on the trailer & towed it to our shop to replace the pump.
I ordered a Carter pump which arrived with "Walbro" stamped on the side of the pump. When discussing this with Carter technical group, they informed me that they use 'Walbro' as a supplier for their pumps. (Which was fine with me) After removing the tank and old pump, I noticed the electrical connector was different, the Walbro pump had larger spades, so I had to make the terminal connectors using female spade connectors. You can order the correct terminal for the Walbro, but that would take several days to arrive. When I finally had the pump ready to install, I noticed the fitting in the pulsator was not very tight, but out of desperation, installed it anyway. Everything went back together as planned and fuel pressure was restored to 46 psig prime. I thought this was a little low and suspected the pulsator was leaking!
I drove the car for about a week. I did notice some improvement, @ 5500 rpm, but still had some 'drop-dead' felling now @ 5800 rpm. The next day, I called Walbro seeking advice and they asked me if I eliminated the pulsator & installed the rubber hose supplied in the kit. Of course, my answer was NO! They said that the Walbro pump was designed to operate without a pulsator & advised me to remove the pump & install the rubber hose in place of it. I do have a rack, so removing the tank again was less than a 30-min job, and once complete, fuel pressure jumped to 52-psig @ prime # 48-psig @ idle & with throttle burp, it jumped to 60psig. Now I was ready for a test drive knowing my fuel pressure was much better than before. Bingo, problem resolved and full power every time up to 6000 rpm!! (Yes my PCM fuel shut-off was moved to 6200 rpm during the reflash)
In summary, check the cat first and if that checks ok, you are starving for fuel @ high rpms!
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