I’m afraid that the flo-jet carburettor is a pig to strip and clean as, in theory, the main jet has to come out before you can split the float chamber. On a new carb this wasn’t a problem but the jet is in a drilling with the needle valve, under the float chamber and collects any moisture in the fuel, which then corrodes the threads below the jet. First try screwing the needle in, counting the number of turns, if it does screw in you are lucky, then unscrew the needle and the hexagonal gland housing round it together. The main jet is a long way up the hole and should be able to be unscrewed. You must use a screwdriver that is a good fit and if it won’t unscrew you are in trouble. If you can get it out a few turns, this is enough to enable you to split the float chamber at an angle but don’t force the chamber apart or you will bend the jet. Screw the idler needle at the top of the carb in, again counting the number of turns, then remove it. Blow all holes jets etc, out and, whilst you are blowing things out, remove the petrol tap from the tank and clean the filter, removing the pipe that you mentioned and blowing this out too. Re-assemble, screwing the needles in and back out the same numbers of turns as they were originally, a bad thread in the main jet hole will ruin this method so trial and error, warm up the engine then, on half throttle, screw the main needle in until the revs drop and out until the revs drop then set the needle half way between, is the way to go. The gland housing needs to be tight enough to prevent leaks but not so tight as to prevent the needle from being turned. The flo-jet is fairly forgiving of dirt so, if the tank isn’t rusty, and everything is now clean you shouldn’t have problems for a while.