When mine backfired and wouldn't start, it turned out to be the vacuum hose from the bottom of the air horn to the Idle Control Valve. The hose split and wouldn't stay connected, but luckily there was a guy in a van that had some electrical tape and I was able to tape the hose to the connector, plug it in and drive it home.
Reach under the air intake throttle and feel around to see if the hose came off.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible