1987 lt4wd surging and backfiring

My 1987 quadrunner lt4wdh is surging on idle and backfires when I let go of the throttle and brakes on the engine. The bigger the load the engine has to brake down (downhill or with trailer), the more it backfires. Also when puffing along with low rpms in low speed (below 12 mph) it tends to surge and backfire, but then only some dull puffs, not the gunshot type bangs I get when I let go off the throttle. If I give it some choke it runs smooth, but then won't keep the rpms down.Runs fine when I gun it, lots of power.

The carb has been cleaned out with new parts and the valves have been adjusted to specs. Adjusting the pilot screw only affects the rpm, nothing else. I think its sits at 2,5 turns now. Also adjusting the height of the jet needle did not affect the problems.

Where to start looking now? Valve- or carb-related? Valves have been adjusted, but not much and probably in the tighter end of the specs interval.

Thanks,
Simen