So how does a choke work?

[AustinJeep]

1045336

I'm trying to figure out why my Jeep doesn't stay running at idle. For example, it dies when cold and hot (pushing clutch in at stops). I plan to clean the venturi's, but I don't think the choke is working. I have a 1980 CJ7 with stock 258. When the engine is cold, the choke is supposed to hold the butterfly valve open for more air, correct? Then the spring warms up and the valve slowly closes as the engine warms. But why does my choke have 12V going to it? What happens if it doesn't get the 12V? I found this unhooked when I bought the Jeep.



thanks!

-chris[addsig]

 

[83CJ7]

1045368

I think 1980 may be an uncomputerized version of the BBD? here is some info on one particular BBD issue

To answer the choke question. Gasoline does not evaporate/atomize or explode well when very cold. one solution is dump a lot extra in to get it going. In a fuel injected system, its easy, squirt more in. in a carb, it has to suck it in, so you need a strong vacuum. the easiest way is plug that big vacuum leak at the top of the carb with a plate, but once its started, it needs to open up to run. From there its a bit of a balancing act to let just enough air to run good, but not so much that the still-cold motor won't suck enough gas. In the old days, you'd slowly push that choke lever in as it warms up, or forget about it and get -really- bad gas milage and have black smoke pouring out the tailpipe.

One easy way to open up the choke slowly is hook it to bimetal spring(thermostat) that moves it slowly as it gets warmer. Ok, well how to warm up the spring? some older ways involve the actual heat from the motor, but it has been found that setting it up to heat itself by running 12 volts through it is sufficient to provide a smooth opening and it can be adjusted to match the warm up rate of the motor. it aslo usually moves a "fast idle cam" with holds the throttle part way open while the motor is cold to allow more air/fuel in because it is not warm enough to combust efficiently yet.

I kinda skipped around and its a very broad description, but hopefully it comes across.

-keith[addsig]