My ZDZ 60cc engine developed starting problems after backfiring through the carb and stopping it now will only give a few pops on choke and nothing else. I checked the ignition battery first which was getting low so recharged that and tried again but nothing so checked the spark and found none so I exchanged the ignition for a rexcel unit I had which is new and unused this got my spark back but apart from the pop on choke the engine still refused to start but I have noticed fuel is running back out of the carb through the trumpet and today I also noticed air bubbles going backwards along the fuel line back towards the tank. Would this be caused by a carb problem or is it in the engine itself. I removed the engine backplate to check the rotary valve and that seemed to be ok with no damage or swarf inside the crankcase I also checked the carb diaphragm's and filter and they seem to be ok but I didn`t remove the needle. Up until 2 days ago the engine was running fine with very easy starts and loads of power so after all my checks I am at a loss to know what is wrong. The carb is a Walbro WJ-71 fuel is 40 to 1 premium petrol. The engine as it is has been good for the last 3 years with no problems. I have also noticed than if I turn the prop anti-clockwise past TDC then rock it back to TDC it seems to be popping in the carb as if it is blowing back out where as I thought it would have sucked in? if I plug the overflow pipe while doing this it pressurises the tank which doesn`t seem right?

Any help would be much appreciated