Carter BBD 1 1/2 Adjustment Help Please

[rds]

Trying to find a base/factory preset adjustment for the fast idle screw and curb idle screw pictured. I can't find anything anywhere and I absolutely don't understand any of the instruction sheets. The carb is off the car. It's a 1971 with automatic transmission. 2 barrel.
Simply how many turns do i back them out after screwing them all the way in. Something like that. Thanks!

 

[MoPar~Man]

The fast idle screw is set to give you a comfortable smooth (but high rpm) idle when the engine is cold, possibly in gear with your foot on the brake. Worst case situation for a cold start. A situation that doesn't last long as the engine warms up and choke pull-off will dial the cam to lower settings as the engine warms up and you end up on the idle screw. It's a pretty in-exact science here. You want the lowest RPM that will keep a cold engine running, that's how you adjust that screw.

 

[CBODY67]

If you are trying to do a preset off the car, on the fast idle speed screw, you can manually open the throttle as you also close the choke plate manually, too. This would put the fast idle cam in its highest speed position. Then slowly move the fast idle cam down to its last step before it is completely out of the picture. At that point, on its lowest step, set the fast idle speed screw to where it has a little bit of preload on the throttle lever. From that point, also screw the main idle speed screw so that when the fast idle cam is released (with the choke fully open), the throttle closes about 1 threads-worth of distance to hot base idle speed.

Or you can put about 1/2 turn preload on the fast idle speed screw (on the lowest fast idle cam step). Then set the main idle speed screw to match that position, then back it off about 1/2 turn for an approximate hot base idle speed level with the choke fully open.

This is kind of working backward of sorts, from full fast idle to minimum fast idle to hot base curb idle speeds. Of course, preset the idle mixture screws 1.5 turns out from lightly-seated, too.

Of course, once the carb is on the engine and the engine is running, set the hot base idle speed and tweak the mixture screws for best idle smoothness at about 600-650rpm in "P", or whatever speed the tune-up specs indicate. Such "smooth best idle" should also result in the highest intake manifold vac levels, too.

With the hot base idle speed finalized, then check the fast idle speed on the lowest step on the fast idle cam. Might need to lower that speed a bit so that step on the cam is close to 200rpm more than the hot base idle speed.

Seems like that with the engine fully to operating temperature, that highest step on the fast idle cam results in a speed of about 1500rpm? But even with a cold engine, that might be a bit high, so little changes to get it to where it starts and stays running, reliably, in your coldest weather, but does not result in an engine speed too high to cause issues on icy roads when stopping. Which is another subject.

Hope this might help,
CBODY67

 

[rds]

Thanks guys for the help. I'm going to put the carb back on the car and try to adjust accordingly!

 

[MoPar~Man]

Thanks guys for the help. I'm going to put the carb back on the car and try to adjust accordingly!

Make sure that when the engine is cold, with the engine not running, you press the pedal down (or pull the throttle linkage back and then release) that the choke linkage pulls the fast idle cam to it's top position (the position giving the highest rpm). You might want to first test the choke diaphram by disconnecting the rubber line from the carb and sucking on it (easiest done with the carb off the engine or the diaphram removed from the carb). It shouldn't leak, and the diaphram moves easily and pulls in. The short rubber line might be hard and brittle and it might be a good idea to replace it (any auto parts place sells them by the foot).

When the engine starts, the fast idle screw will be (or should be) on the top cam position. When the engine is running, the diaphram will want to pull the cam to it's next setting but won't be able to until you press a little on the pedal or pull on the throttle linkage and then release. Or maybe I'm confusing that with the choke getting pulled off right away. As the engine warms up, each time you press and release the pedal the engine RPM should back down as the thermostat linkage continues to rotate the cam, until it's fully rotated out of the way the fast idle screw is no longer working to control engine RPM.

The length of the thermostat link rod will be critical to this working correctly. And that has to be combined with the right gasket between the manifold and carb. As far as I know, the correct gasket for these BBD's is somewhat thick, like 3/8 inch thick. Buy a new gasket, as you need a good seal here between the carb and manifold, but make sure it's a thick one.