I need to know what the pressure onf the fuel schroder valve should be

sounds like a fuel delivery problem, have you had the OBD read ?the backfire could be a symptom of timing out or misfire so need to check a few things

IAC - idle air control problem possibly .( You can drive it if you use your left foot to brake but keep the gas pedal down to keep the RPMs up.) Wouldn't be a fuel filter , if fuel filter was clogged partly it would idle ,but would lack power ,low fuel pressure at higher speed's . Do you know what IAC valve is ,an how it works ?
Idle Air Control Valve
Or maybe the EGR valve pintle is covered with carbon an sticking open .
Or possibly a dirty or bad MAF sensor - mass air flow sensor - the PCM - engine computer needs info from this sensor so it know's how much fuel to feed the engine .
Hooking up a professional type scan tool to view engine sensor data cut's down on guessing as too what the problem is .

007 Idle Speed Problems Scan Data for Idle

SKM, fuel pressure regulator is on top of throttle body. This could be the problem, could also be vacuum leak, faulty EGR, etc. You may want a good Tech, to diagnose and estimate repair. Check the attached links,instruction and guides, Good luck
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TBI 220 Fuel pressure regulator change 1
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May hav a fuel pressure prob. Turn key off n on three times. On 4 start car it shud run....u builtup enuf pressure. Probly somethin w the oneway chec valve in pump....its lettn gas return

Sounds like too much fuel, could be a leaking injector, faulty pressure regulator, or blockage of air intake. Did you replace the injectors and pressure regulator when you worked on the throttle body ? Can you see normal fuel spray on both injectors when the engine is running ?

is the gas tank vent recirculate hose clogged?
bad gas cap?

sounds like a bad throttle position sensor or a fuel pressure regulator. also check the idle air control vavle

Vehicles: All post-1996 (OBD-II) vehicles experiencing the above symptoms

Check your IAC Valve (Idle Air Control Valve), and TPS (Throttle Position Sensor), and throttle linkage.
You could even be having problems on the spark side of the equation - the ICM (Ignition Control Module), spark plugs, coil pack(s), etc.
On the fuel side, you may have an underperforming fuel pump (leading to low fuel pressure), or bad fuel pressure regulator, or even clogged fuel filter/fuel strainer.

You'll want to query your car's computer to find what OBD-II DTC (Diagnostic Trouble Code) that the car's computer has stored. The presence of this stored DTC code is the reason that your Mercedes has lit the CEL (Check Engine Light).

One can get a free DTC code check (in the US) at AutoZone/Advance Auto Parts/Pep Boys. When you have this code (they may look up a brief explaination of the code for you), you'll at least have a good indication of which engine system has been found by your car's ECC (Engine Control Computer) to be performing erroneously.
OBD-II = On-Board Diagnostics II (applies to 1996 and newer model years)

You know the timing is right because you checked it with a timing light? The distributors can go bad causing this also it could be caused by low fuel pressure, bad fuel pump.

Check your fuel line pressure-use the schroder valve(looks like a tire valve) on the main fuel rail just before the injectors. If it doesn't spurt out when you press it briefly, you have no fuel pressure. Bad fuel pump-clogged fuel filter? Do you smell gas anywhere like its leaking out somewhere or not getting fire when you crank the starter?
If you DO have fuel pressure, then I would check timing or spark circuits.

Hope this helps.