B&S Pulsajet carb needle doesn't do anything?

I have a B&S 5.5 engine (103000 series) on a splitter that with the air cleaner OFF, starts & runs at max rpm (well I think it's max) for about 30 seconds, then slowly starts to lose rpm's until it stops. I can get it to run longer when I replace the air cleaner. However, I noticed the main jet adjustment has no effect whatsoever. I replaced plug, diaphram, cleaned carb and the breather thoroughly. When I finally get it running the engine doesn't seem to develop the HP as it will often slow considerably when I get into harder wood.