Have a '93 B2600i that I bought as a non-runner with a bad ECU. Got it rebuilt by AvePro (excellent service and one-day turnaround) and the truck runs now. Great on the highway, but around town this engine vibrates a LOT at idle. Can't believe this is normal. I've gone through and looked at connectors to make sure of solid connections and no corrosion. Vacuum seems tight. This is a rebuilt engine the previous owner had put in, and it does have new motor mounts. One peculiar thing it does is the idle fluctuates a bit. Sometimes it idles consistently low. So I get out, screw the adjustment screw until it's 800rpm or so (5 speed manual, BTW). Then everythign's fine until I drive it next time, whereupon sometimes the idle will be too high! Any ideas what the idle thing is, or why the engine vibrates so much at idle? Oh, when I got it I replaced plugs, rotor, cap and wires. It also has a new fuel filter and is timed correctly.

I had a similar problem with a 2200.
The EGR valve had a bunch of buildup in it and would cause the engine to run erratic.
Once I cleaned out the gunk, it ran like a champ.

Thanks! I'll give that a look.

Could be cam timing, ignition timing, malfunctioning injector, faulty 02, plugged air or fuel filters, egr valve stuck open, plug wire shorting or high resistance, plug wires mixed up, MAF, temp sensor, IAC (can't remember if it has one), various other problems.....

Apparently mazda distributors with magnetic pickups are prone to rough and variable idle with misfire at low rpms, don't know if the hall effect are prone to the same symptoms but that might do it too. Unfortunatly without taking it to a dealership or a really good mechanic the only way to test it is to have a good eye and a timing light, or swap in a known good one to see if it changes.