I have a northstar 5500 with a B/S engine it runs for about 4/5 min then backfires and stops

How many hours on the meter?

Likely overdue valve adjustment. The Kawasaki OHV engine requires periodic valve adjustment. Schedule is every 300 hours but due to symptoms adjust regardless.

Other general maintenance; fuel filter, air filter, spark plugs.

Less likely causes; fuel solenoid, safety interlock module.

OHV engines require periodic valve adjustments.

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Try running it with a loose fuel cap. If it now runs fine find and clear the vent hole in the cap.

Might have jumped timing

carburator needs to be cleaned,and put new fuel in the tank as well as a gas stable product

replace the fuel filter

Had the same problem with an almost new Craftsman w/ Intek 20.0 HP. Check all the "usual suspects" and had to have a tech come out. He had me start it. It ran for a few minutes and stopped (per usual). He immediately went to the Anti-Backfire solenoid. It is an electrically operated "plunger-style" fuel line blocker. They are famous for going bad. The part is about $65 and its sole purpose is to starve the engine of its fuel supply at shutdown so it won't backfire. If it is disabled, the user must throttle the engine down for 10 seconds or so before turning the key off in order to avoid backfires. Gee... let's see, 10 seconds of my time each time I use the mower vs $65 part ... No contest! Remove the solenoid, take a pair of electrician side cutters and snip off the tip of the plunger and reinstall it. Problem solved ! ! !

Remove the gas tank and clean it out, look for signs of water and or dirt in the tank also look for a pinched fuel line between the tank and the carburetor

If that is carbureted, and has a mechanical fuel pump, try running a length of fuel line from inlet side of pump to a gas can that has fresh, clean fuel, and if it runs good that way, you know problem is most likely in the tank. Removing the tank & cleaning it out is best solution from there. I'd even be tempted to add a small bottle or 2 of Lucas Fuel Sytem Cleaner to first fill in clean tank, just to help clean lines as well as carb.