asuming that this engine is broken in and has been running fine for awhile before acting up ,several things could be the cause.
1.crossed fuel lines.
2. new tank fuel pick up too close to rear of tank should be about 3/8 inch away from rear wall.
3.clogged fuel filter if used.
4.dirt or forien matter in spray bar.
5.loose rear and/or rear cover screws.
6.pinched fuel/pressue line
7.foaming fuel tank.
8.bad fuel/alcohal absorbs water like crazey so if jug was not kept capped tightly in a humid/damp area could be contaminated.
9.bad glow plug/just because it glows does not mean it is good,element could be covered with a clear glaze preventing proper ignition or element could be deformed slightly.
10.if muffler pressure is not used vent line could be plugged.
long list but there are alot of other things that could be the cause,listed the most common and some of the easiest fixes.let us know what you find.