'76 carb idle adjustment trouble

[dengerin]

Hey, folks

So I just rebuilt the carb in m '76. I am having a hell of a time getting the thing to idle correctly. I am oh, 99% certain the thing is put together properly, and I just can't seem to get the damn thing to idle well. It is rough and the best I can get out of it is a rough idle at around 1000 rpm and 8-9 inHg vacum. Anything below that and the whole truck shakes and dies after 10-15 seconds. Timing is 7deg BTDC. It runs great in the hegher rpm's.

Is there 1) a good starting position for the idle mixture and throttle speed screws, and 2) a best procedure for tuning after those initial spots?

Tips on things I might check beforehand would be nice, too. Happy to answer any questions not explained above.

Thanks in advance.

 

[dengerin]

Update

So I found this:

http://www.ih8mud.com/tech/carbinfo.php

which, though it refers to the '73-4 carb, was some help. My '76 is idling better now.

Once the thing was warmed up, I turned the idle mixture screw out 4 turns as it suggests, and turned the idle speed adjustment screw out until it was not in contact with the tab. Then I adjusted the idle speed screw until ~650 rpm, then adjusted the idle mixture screw to max vacuum. I repeated this 2-3 times and that was a lot better.

A couple of problems still:

-It idles rough.

-If I idle the thing, then turn it off, then start it up again right away, it is difficult to keep it running at idle speed. Same goes for driving a little ways and then stopping and trying to idle.

-The most vacuum I can get is around 11 inHg at idle. Max seems to be ~20 inHg at 3500rpm. Acording to the book, this is low.


Any thoughts on the above?

 

[Degnol]

So I found this:

http://www.ih8mud.com/tech/carbinfo.php

which, though it refers to the '73-4 carb, was some help. My '76 is idling better now.

Once the thing was warmed up, I turned the idle mixture screw out 4 turns as it suggests, and turned the idle speed adjustment screw out until it was not in contact with the tab. Then I adjusted the idle speed screw until ~650 rpm, then adjusted the idle mixture screw to max vacuum. I repeated this 2-3 times and that was a lot better.

A couple of problems still:

-It idles rough.

-If I idle the thing, then turn it off, then start it up again right away, it is difficult to keep it running at idle speed. Same goes for driving a little ways and then stopping and trying to idle.

-The most vacuum I can get is around 11 inHg at idle. Max seems to be ~20 inHg at 3500rpm. Acording to the book, this is low.


Any thoughts on the above?

Sounds like you have a vacuum leak. Common spots: base of carb, booster and manifold. Spray carb cleaner at idle and when you get to the leak, the idle will jump up. GL

Ed

 

[Pin_Head]

Sounds like you have a vacuum leak. Common spots: base of carb, booster and manifold. Spray carb cleaner at idle and when you get to the leak, the idle will jump up. GL

Ed

X2 vacuum leak

 

[Lupdog]

I've had a couple of 2f's, both stock and they always seem to have a "lumpy" idle sound even after tuning. (which I happen to think is really cool!)

Q. Do 2f's every really idle smooth?

 

[bsevans]

My 8/78 purrs like a kitten at 650. Punch the throttle and I've got a handful of RPMs with no hesitation. I would check all the vacuum lines. How long has it been since they were replaced? How hard are they? You might try pulling the booster line from the manifold and put your finger over the port. I would systematically pull vacuum lines and cover the port and hose end to see if you’re idle improves. You might want to look at how loose the butterfly shafts are in the carburetor casting.