i had the same happen years ago on a well tuned stock 4k. turned out to be water in the fuel and lots of it.
try emptying out the filter onto the hot cement to find the fuel evaporate and the water stay behind. I'm sure there is another method but this was the tell tail sign for me. i found the filter half full no shit! if you find any water then follow what i did to solve the problem.
i drained all the fuel in to a couple of buckets, blew compressed air through the fuel lines, changed the filter, put 500mL of metho in then put the fuel back in. without the water of course which is heavier than fuel and will stay together in the bottom of your bucket.
this should get you going well but not perfect until the metho does its work on whats still in the system, still in the tank and more importantly in the bowl of the carby! i would keep a spare filter on hand for a couple of weeks and a little metho
turns out i had about a liter from the dodgy fuel station. six months later they dug the tanks out, funny about that..
good luck
neil