Modern Vespa

Hello community,
I have been having issues with my Sprint backfiring. I noticed that the backfiring is very minor when the tank is full and increases as the tank empties When the tank is near empty (one bar on fuel gauge) the backfiring is excessive, and sometimes stalls the scoot. I also notice that when the tank is that low and I go to unscrew the fuel cap there is a big gasp of air of air that comes out, almost like a big exhale. Figured I would mention that in case that's a clue.
Things I have done so far to mitigate the issue.
1) New spark plug (OEM) correct gap
2) New air filter
3) Cleaned fuel injector and verified proper operation.
4) Tried running on REC 90, but no change.
Has anyone here experienced anything similar and have ay recommendations as to what can be causing that?

Any help would be greatly appreciated.

Backfiring is the result of unburnt fuel in the exhaust system igniting there instead of inside the cylinder. This is due to either bad fuel or too rich fuel/air mixture. Maybe your engine is not getting enough air.

10 Reasons Why A Engine Is Backfiring

Poorly Adjusted Ignition Timing
Crossed Spark Plug Wires
Bent Valve
Faulty or Damaged Spark Plugs
Rich Fuel Mixture
Leaky Exhaust Valves or Manifold
Faulty Fuel Injection System
Faulty Carburetor
Overheating
Faulty Oxygen Sensors

Thanks for your reply! I replaced the OEM filter with a Malossi filter and lulbed the filter. I should prob check to make sure that the filter is still in place and didn't get sucked in. The scoot has normal acceleration and power so don't think its bent valves. Only one sprk plug wire, so no crossing there. I did check the spark plug and it is dry. Perhaps I can have the timing checked. Not sure how to verify if the O2 sensor is the culprit.

maybe you lubed the air filter too much.

Evaprectomy and block off your SAS. Your venting system is full of fuel. You have a vapor lock situation.

That sounds like that can be the issue. Are there any instructions on this forum on how to block off the SAS. Or can anyone provide instruction?
Thanks

Stick it on the center stand and use a rag to block the end of the exhaust. Does the engine die? If not, that's why it's backfiring.

So I put a rag on the back of the exhaust. The engine bogged down a bit but never shut off. Does that mean I need to block the SAS or purge the vent system of any fuel? If so, can anyone point me to a thread or link that gives me instruction on how to do that?
Much appreciated!

That means you have an exhaust leak. And that's why it's backfiring.
Was the rear tire replaced before this started?

I havent replaced or removed the rear wheel. Beside where the exhaust connects to is there anywhere else I should look for an exhaust leak?

It's been a fairly common problem on our Sprint/Primaveras, it's very annoying.

This past week I bypassed the evaporator system. It was a total success! I feel like I'm driving a new Vespa again. No backfiring, no stuttering upon initial throttle. Just perfect in every way.
Thanks to all that chimed in and revealed the solution to this issue.
Kudos