Main air bleeds

[73ss]

 I have noticed every chevy q-jet that I have seen from the early 70's into the 80's all have the large main air bleeds & rich jetting. I have seen a few from the late 60's with the smaller bushed bleeds. Most every BOP q-jet that I have seen from the same era had the smaller MAB's & leaner jetting. Does anyone know why this was done on the chevy castings? Smog? less nozzle drip? I have yet to find a large casting chevy carb with the small MAB's, Other then the "teardrop" design. It seems the large casting BOP carbs with small MAB's are quite common. Anyone know why this is?

[Cliff Ruggles]

Correct, most Chevy units from 1970 to 1980 have the large MAB's unless they were the single MAB style. 

Some have pretty big jets in them with pretty small metering rods. 

When we set them up here for non "stock" applications, smaller bleeds are installed with smaller jets/MR's.....Cliff

[73ss]

Do you know why chevy did this?

[Cliff Ruggles]

Most likely ever tightening emission standards.  Using a lot of emulsion air leans up the entire curve, not just one part of it.

The large MAB models can be made to work OK, but when used outside of the stock application, much more effective to install smaller MAB's and completely recalibrate them....IMHO.....Cliff

[73ss]

Several years back, I bought a new edelbrock 1901 carb for use on my 68 vette. stock L-79 327. After going through the book I noticed the 1901 had the bushed MAB's in the air horn & the large .125 MAB's in the main body. I pulled the MAB restrictors from the body of the original '68 carb & installed into the 1901. Stock edelbrock jetting was 69/39b rods which was close to the original '68 carb. This crispened up the throttle response buy a bunch. The car always felt lazy off throttle before I did this. Ever see bushed MAB's on the airhorn & Large in the body?