Where is the mixture screw on a stock 305 carburetor for a 1985 gmc sierra?

Answer 1

You should have a "Rochester" carburetor on your 305 engine.

The "idle" mixture screws are hidden behind 2 caps (plugs) on the lower "front"

and left and right of the carburetor base. They are hidden (plugged) by the factory

and unless you are modifying the carb should not be tampered with.

But you can carefully grind or drill off the plugs with the carburetor off the engine.

(that is NOT RECOMENDED).

The screws require a special flexible carburetor mixture screw , screw driver to turn them. Usualy $10 to $15 dollars at parts store.

Spray with W-D 40 first, then count the number off turns and partial turns as you turn them in till they seat. DO NOT TIGHTEN DOWN HARD. gently do this because you can damage the "point" on the other end of the screw and ruin the mixture ability of the screw / needle.

You probably have another problem somewhere else that leads you to believe adjusting those screws will cure a poor running condition.

Search, "Rochester four barrel,/ tuning, tips, tricks, idle mixture / Chevy 305 carb,

etc etc.

You can also get the part number of the carb itself located on the drivers side of the carb youll need to clean the small area and probably use a flashlight.

Dont use sand paper, soft wire brush at the most, and carb cleaner is best.

The number looks something like this,

ex: 17085227

3385 HLZ

this is off a "rochester" on a 1985 vehicle with a 454 engine.

Take the number, go to a knowledgable parts store like NAPA, not the big chain places (they sually don't have the technical expertise to do what im telling you) and have them check the carbs part number located on the drivers side of the carb housing verses the rebuild kit number verses the engine size the number applies to.

To make sure the right carb is on there. This is a relliable way to do it.

I bet yours is the wrong carburetor if its been replaced.

Also check thoroughly all vacum hoses connections tune up stuff etc.