Originally Posted by DoubleVision
This carb came off a 1979 chevy truck with a stock 350. However the throttle body came off a sean murphy carb. The throttle bodies between the two were identical except the SM body had the easily adjustable air mix screws while the factory body does not. There is no choke or fast idle assembly. I always remove them as down here where I live it don't get cold enough to need a choke. I did plug the vacuum coming off the side of the carb where the choke housing was. On the truck the carb was on originally the former owner told me when he drove it home it ran okay with the only issue being old gas.
When I got the carb from him it was still on the truck and hadn't been removed. He parked the truck when he found the frame was rotten.
When I disassembled it I was surprised at how good the condition was. It was clean with only a little fuel residue in the bottom which washed out with carb cleaner. How do I go about changing the idle orifices? Also the original throttle body wasn't worn out, it has very little play in the shafts I didn't use it only because of the difference in adjustable idle mixture screws. Thanks for the help.
The idle air bypass orifices are almost always in the base plate. There are holes above the baseplate connected to the orifices, but they are usually much larger than the orifices so the orifices in the baseplate are all that need to be enlarged or made smaller.
There are several different locations for the orifices, but they're all adjacent to the primary throttle bores of the baseplate. The sizes vary from no opening at all to around 1/8". Measure them w/a drill bit.
Carbs from bulk rebuilders will often have the bypass channels or orifices blocked off w/lead shot tapped into the casting. This can be fairly easily removed if it's encountered- but I doubt that's going to be your problem (no
bypass air). They block them in various places, sometimes the baseplate, other times in the carb body. If this ever comes up, follow the channels from the baseplate into the body and you'll find the holes I'm talking about. Often these rebuilt carbs will use cheap unplated brass metering rods (unmarked) and might even have unmarket jets.
The older style I call the 'tunnel' type (circles):
The later type I call 'tab' type. Some have two like on the left (right tab would be where the circle is), most look like the one here; one orifice is in the left tab, the other is just inside the bore (arrow):