Well, the backfiring is back this morning.    So the timing belt didn't fix the problem.

Where do I go from here?

Pull the codes from the ECU. Check all round the inlet manifold and throttle body for leaks. Check all the vacuum pipes for perishing, pinholes and blockages.

ECU is currently not throwing any codes.  The light was on but flipping the dummy switch behind the speaker turned it off and it has not come back on.

Checked all the vacuum lines today at lunch and they all seem to be in good shape.  Tried disconnecting the electrical connections to the MAP sensor to see what happened and the idle RPMs dropped way down and the engine started smelling very strong like unburned fuel.

The other thing that both of my parents describe just before it backfires is the engine sounding like it is bogging way down almost to the point of dieing, then it will backfire or sputter and regain speed.  Seems to happen most frequently right after changing gears, but every now and then will happen as it's just cruising along.  Always happens in the lower gears though, never in the higher gears (above third)

Did an experiment yesterday...  Took the MAP sensor off my 1993 tracker that runs like a dream and zip-tied it on to the MAP in my dad's sidekick, swapped the vac line and electrical connection and took it for a test drive...  Ran smooth, no stuttering, no backfiring almost no hesitation.  Switching back to the original map, the engine has a totally different sound.  Sounds deeper, like it's bogged down and gasping for air or something.  There's also a definite fuel smell in the engine compartment with the original MAP that goes away when we switch to my MAP.  Disconnecting the MAP altogether causes the engine to run real rough, the tone goes even deeper and the fuel smell gets incredibly strong.  Am going to check the MAP and the various sensors tomorrow.

I am going to adjust the valves as soon as I get a feeler gauge, and going to tweak the timing as soon as I get a timing light. (Need to make another trip to Harbor Freight.  Those guys should really be paying me for advertising.)

It won't run too well without the MAP sensor, the ECU relies on that to calculate fuelling. There should be a small filter in the vacuum line to the MAP sensor, that could be plugged or damaged. If the filter is missing/damaged then the sensor will get contaminated from being exposed to fumes. I believe that the filter also provides some damping of the vacuum pulses.

I looked at that filter thinking it was a check valve, but it doesn't seem to be too restricted.  When I did the MAP swap from y tracker to his Suzuki, I did not swap filters.

If you buy a new MAP do those filters come with it or is there a separate part number?