I tried inserting a nut in the bolt hold the 2 piecces of the nuffler and fixing an insert in the muffler tip to reduce the internal diameter. It seems that this has helped in increasing the back pressure from the muffler and the pressure of fuel coming from the tank to the carb. it takes the engine now 10-15 seconds to lean out when holed at 45 deg nose up position which is enough for me.

thank you all for your help.