91 3pg marathon puff back through carb

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I have a 91 marathon that I have rebuilt entire engine with long block. New drive clutch, rebuilt driven clutch with high torque spring and new ramp buttons and new drive belt and new gen/starter belt. All gas line ok, all throttle linkage working correctly, govenor tight for more rpms, Carb removed and total cleanup and adjusted to specs. Cart starts well and goes nice and fast. Had trouble with hills but hope that has changed with new spring but not tested yet.
Now for problem.. I seem to have a slugish cart at low rpm. Seems to take a bit for rpms to climb when starting out to go. I also have at times ( after warmed up) a puffing back fire through carb or muffler, not a bang but a puff when I put pressure on peddle to start going.
I have been leaning to the muffler being plugged even though I can blow air into it with compressor but then this little engine is not a compressor and the muffler seems to be a bit heavy for its size.
Sorry for long winded explanation but hope I have not left anything out. Can not find a single thread about this type of puffing sound.
Any help or thoughts would be great. Going to take muffler off and run cart next to see if engine has more power.
Wondering if plugged muffler is holding unburned fuel that is ignited after warming up and running a bit.
Timing was set at time of rebuild also and new plug wire and plug.