1975 gmc sierra (305 v8) won't run past an idle until warmed up?

[Brandon]

I have to let my truck warm up for 15 minutes in the winter time before I can drive it. If I don't then it will die when I give it gas. Not as bad in the summer. A few seconds and I can start putting along. I am thinking vacuum leak but I haven't found any obdvious ones. It also shifts very quickly through the gears when you aren't pressing down on the pedal harder, and sometimes doesn't down shift to first a stop, and then starts out in second. this only happens sometimes. I replaced the vacuum line that feeds the transmission and it didn't help. I haven't done a vacuum check but it sure seems like it's just enough to make everything work. I did notice that the very top flap on the carberuter is held open by a wire. Not sure if this will cause my problem or why it is not hooked up that way. Can someone give me some ideas to what I should be looking at?
transmission fluid and filter were changed 6 months ago.

 

[oldragracer]

The top flap on the carburetor is the choke. It closes on cold starts to provide a richer mixture and gradually opens as the engine warms up. The wire you described IS the cause of your problem- your engine is running too lean when cold, and if you remove the wire, you will run too rich when hot. Your carburetor needs to be rebuilt or replaced. Based upon the vehicle you described, you most likely have a Rochester Quadrajet (aka. Quadrapuke, Quadrabomb, Quadrajunk). These carbs are a royal pain in the a$$. Personally, I remove these from the engine, toss them in the nearest trashcan, and replace it with something better, such as a Holley or a Carter AFB (spreadbore design).

 

[scrubbag]

Your choke is not working right. It needs to go almost all the way closed, and set the cam to fast idle, so that the engine can get the gas it needs to run, while cold. Once it warms up, the choke slowly opens, until it is full open on hot engine. Trying to run the car with choke open and engine cold will do exactly like you are saying. Cold gas doesn't burn good.

When you say the choke is held open by a wire, do you mean the rod that opens it, that is connected to the choke mechanism, or are you saying someone wired the choke open.??

Get a book, maybe at the library, that will show you how to adjust the choke, and set it so that it is closed when cold. Setting it wrong will cause problems also, if it doesn't open like it should, it will flood the carb and run very rich. Or if it opens too soon, you will experience hesitation when stepping on the gas, like you do now.

 

[niko45]

Hello everyone.i am a new member here.Looking forward to learn from this forum.I do agree with scrubbag,that you should get hold of book from the library and you'll get the detail instructions of how to adjust the choke.