rough running when cold

[tomgok]

My 76 previously ran fine when cold. Now I set the choke, start and it feels like it is running on 5 cylinders. Any ideas?

 

[TimAT]

When the engine is cold and you set the choke, the butterfly closes. As soon as the engine starts, it needs more air so the choke butterfly is supposed to open (specs are in the manual somewhere) a little bit. The vacuum diaphragm, i.e., "choke pull-off", is supposed to take care of that. If the pull-off is dead, won't hold vacuum, it won't open the choke after start and will cause the engine to run really rich to the point of being near flooded. That can cause a cold engine miss that goes away once warmed up.

 

[gaC5]

How do you set the choke?

 

[TimAT]

If all is working correctly, the first pump on the accelerator pedal gives the carb a shot of gas (accelerator pump) and also allows the choke to close. The thermostat rod is retracted and is slightly pulling on the choke mechanism on the right side of the carb. Opening the throttle that first time, the fast idle screw passes over the cam and the choke is ready. The specs for both the minimum choke opening and fast idle are in the service manual. The specs for how far the pull-off opens the choke plate are also in there.
2 types of choke thermostats- one is called a "divorced choke". This one has a small tin cover mounted on the right side of the intake at the base of the carb with a rod up to the carb. The other type is an "integral choke". This one is easily identified by a round, usually black cover on the carb held on with 3 screws or pop rivets.