Consider the smog era M4M q-jet,
are there any 'negative improvements' that
can't be reversed with new rods/hangers.
.
(in other words, can a M4M be made to
operate like the older models?)

 

I think the main concern is the threads on the idle screws. They are so fine that you really cant over richen the motor. This doesnt really become a problem unless you have a huge cam. Mines a 214/224 with a 81 smogger qjet and i have no complaints.

 

The biggest problem with the "M" series and later carbs, is that the passages in the carb, are smaller than the metering orifices that they connect, need to be in order to make a motor with low idle vacuum run properly. So you can get the main metering system and of course the secondaries to run fine (WOT) with rods and jets and hangers; but low-PRM low-speed driveability is another matter altogether. It's just about impossible to get one of those to work well on the street with much more than a stock cam.

 

There's a couple of things in the idle circuit that need to be drilled out to fatten them up for acceptable performance on a warmed up engine. The changes are non-reversible but if you go in small increments you can dial it in good without going too far. Shoot me an email and I'll send you some write-ups I did that can walk you through it.

Like RB said- primary jets/rods and secondary rods all operate pretty much the same way as on the older carbs.