Topi....
I had a hellova hard time with mine......same symptoms.
When the silicone fuel line is put on the nipple sometimes a sharp corner will carve out a piece of silicone from the inside of the fuel line. DuBro seems to be worse than other brands.
I reccommend that you remove all fuel lines and toss em'.
Then, remove the needle valve assy from the carb...unscrew the brass body....then back flush the carb housing with alcohol. Flush the body too. Be sure to backflush the nipple on the carb going to the head.
Back flush everything with alcohol...unscrew the top of the check valve on the head and clean up in there...those small silicone pieces are almost invisible and instead of just looking...flush it and blow everything with compressed air.
If it was running ok with the 2 diaphrams...then something happened....try flushing the pump too. The pump is not too difficult to take apart and you may find swarf and debris in there as well.
In addition...there is a piston on the bottom of the pump...you can remove it and flush out in there as well.
Replace all fuel lines....cut to size then flush before installation.
Replace O ring on throt barrel....they leak..