Carb Wont Adjust

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Apr 21, 2005
Charleston, SC
Just desmogged my 87, and at first the carb was adjusting great and then nothing. The RPMs would fluctuate by turning the mixture screw and now I cant get it to do anything unless I screw it all the way in. Why would the mixture screw not do anything? Could I have damaged the end of the mixture screw? Any thing you guys could do to help would be......helpful. Thanks.
My first guess is there's a big air leak, rendering the mixture screw useless.
How does the vehicle drive.. Technically if you do the LEAN DROP METHOD you should have the fuel mixture screw at a point where no matter what you do with the Idle Mixture screw the idle should not increase (or do I have that backwards again). Also, remember that this is an analog system so the carb will take a moment to figure out what you just did. Try making small adjustments and then thumbing the throttle a big to revv the engine.

If the truck drives fine then you have found that magical "ideal" spot for the fuel mixture and idle mixture screws where you are getting "optimal" mixing.
idle speed screw in too far will let fuel past the idle circuit...screw idle mix screw in gently till it just bottoms, then back it out about 2 turns. fire up rig and back the idle speed screw out till you get about 450-500 revs going- make sure the fast idle screw isn't getting in the way, adjust idle mix screw bit by bit for max vac then turn the mix screw in about a half a turn or so till the vac just drops off, then adjust idle speed screw to about 625-650. then check timing is at BB with the vac advance hose removed and plugged. then reset the idle speed to 625-650 revs... if the idle speed is set too high, the idle circuit can be/is bypassed...
I have the air cleaner off when I do mine and it doesn't seem to affect thing too badly...I fine tune the speed with the cleaner in place...
if it won't stay light dialed down to 450-500 revs, give it more gas(mix screw out) or there is a vac leak as suggested earlier...
Thanks for the input. I will adjust and report back. When I first adjusted it I could adjust mixture then idle speed and kept that up til it adjusted no more. Now the idle wont change with the mix screw. I will start from the beginning and give it a go. Thanks again.

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