Backfire is MURDER on a MC 2100 with power valve!
Overpressure in the intake/carb will blow out the diaphragm in the power valve and you are going to have problems with idle and fuel metering from that point.
One quick test to see if your power valve is cooked or not,
Turn the idle mixture screws IN, turn them together GENTLY, about 1/4 turn at a time until they bottom out.
DO NOT CRANK DOWN ON THEM ONCE THE BOTTOM!
If your engine is still running with the idle mixture screws all the way in,
the power valve is SHOT and you are siphoning fuel from the power valve enrichment circuit to keep the engine running.
If the engine ran well before you had the backfire, then it's probably something caught in the carb someplace.
Not uncommon to rubber or rust in the fuel lines to make it's way into needle and seat, passages, jets, ect.
That's why we are ALWAYS saying to use at LEAST two fuel filters!
I have a large metal one in the rear next to the tank,
An inline 'Clear' filter between pump and carb,
And there is a 'ROCK' (Compressed Brass Beads) filter in the inlet to the carb.
Two are disposable, and the carb 'Rock' filter can be cleaned with carb cleaner and reused,
And every time I take the top plate off my carb, there is STILL CRUD in the float bowls...
So you KNOW it's all through the carb!
Not like this guy!
He complained about no 'Idle' below about 1,500 RPM and 'Running Rough' when it brought it in here and was complaining about the ignition upgrade I did on it 1 year ago (with HIS choice of parts)...
I put a plug wires set on it, (MSD, $70)
And a $13 rebuild kit in the carb and it ran like a top when he left!
One main jet completely clogged,
Both idle mixture circuits completely clogged,
And I put a fuel filter on this carb last year when I did the ignition!
This is all stuff that got passed the filter, or in the carb through the bowl vents!