The Stovebolt Site Forums The Shop Area Engine Shop Adjusting Rochester carb

What's the proper way to adjust the idle and mixture screws? I've attached a vacuum gauge to where the vacuum hose for the wipers. The needle is bouncing between 15-20. My idle screw is almost fully screwed in. Didn't used to be and if I adjust out too far, it starts knocking. Have tried adjusting the mixture screw in and out, but now it's not starting. I've been able to start until this morning. My choke cable won't move the choke plate open or closed so had to unhook and now move by hand. It's on a 235 in a 1951 GMC 3/4 ton pickup. Need to start over but not sure where. Anyone have suggestions?

1) turn in the mixture screw in until it just bottoms out. Be easy and don't over tighten. Turn it out 1 and 1/2 turns. The engine should start and idle at this setting. Once it warms up you can fine tune it.

Why does the choke not work? Will the cable move freely when not connected to the choke? You'll need full choke to get started.
PB Blaster or similar can unstick the problem.

You may have flooded the engine. Reset the carb and let it sit overnight. You can also check the plugs to see if they're gas soaked.

Good luck

A bouncing needle is seldom corrected by carburetor adjustment.
Sticking valves, and vacuum leaks are more likely.
You need to have a functioning choke to make your life easier.
Check to make sure that your flapper valve in the manifold is functioning properly.
Have you checked to see where your timing is?

Thank you! I'll try that tomorrow. I had this carb rebuilt and when I installed it, I put the cable through and tightened the screw. But it would not move. I unhooked it and the cable moves freely. Choke seems a bit hard to move manually. Wasn't sure if maybe one of the springs is stretched? Worked fine before I pulled the carb out.

Yeah, I need to check the timing but I don't see where or what to loosen to move the distributor. Where it's attached to the vacuum advance doesn't look like anything can be loosened. Or am I looking in the wrong place? Thanks for your help.

Thanks Tim, but mine doesn't look like that.

Do you have a stock distributor? The screw (takes a flat blade screw driver) which you need to loosen to move the distributor is behind the distributor and is a bear to get to. The head of the screw faces the front of the engine.

Not knowing what yours looks like, I'm going to add to Tim's post anyway. Unless your engine has a special distributor, all 216/235/261 engines have the same set up as shown in Tim's attachment.

What is not shown clearly in Tim's attachment is the distributor clamp. It can hide in plain sight if you don't know where to look...and it is not easy to work on. The bolt and nut of the clamp can get covered in grease and be hard to find if you don't know what you are looking for.

In Figure 32 of Tim's attachment, see the square nut that is just above and to the right of the octane selector? That is the clamping nut for the distributor clamp. The clamp itself is the band that goes around the distributor shaft...that square nut is a part of the assembly. Not shown is a view from the other side of the distributor. On that side, there is a slotted head screw that attaches to the square nut. To loosen the distributor, loosen that slotted screw. If your assembly is covered with grease, consider squirting it with brake or carb cleaner so that you can see what you are working with.

I use a slotted head screwdriver bit in a socket wrench to turn that screw because only a stubby screwdriver will fit into the available space and that does not always allow enough torque to loosen or tighten the screw, at least with the rheumatism in my hands.

Many people set the timing closely using the distributor clamp, then fine tune it by using the octane selector. I find I can fine tune it using the vacuum gauge technique with just the distributor clamp as long as I am using the screwdriver bit in a socket wrench.

The indicator line for the octane selector that shows up so clearly in Tim's attachment can also be hard to find. It is etched into the flat part of the block itself. Cleaning the area well will allow it to miraculously appear. Once you find it, consider marking it with some paint. Knowing where it is will allow you to set the octane selector at zero before you begin timing.

Matt

Thanks Matt! You explained to me in laymen's terms that I now understand. I thought the screw in Tim's photo (in front of the vacuum advance) was what needed to be loosened and that's how it was adjusted. Mine doesn't have that screw. I do see the nut and screw. I had run out of gas and my filter was filled with rust and gunk. Changed the filter and it still won't start. Pulled the carb and squirted cleaner through the needle valve. Got a few specks of gunk out. Put it back on and still nothing. But will keep tinkering. Thanks for the help....to everyone!

Thanks Carl. I'm understanding this better with all the help. Does it take much to loosen?

The only issue is that there isn't enough room for a screw driver in a fat hand to get on the screw head. I have thought of changing that screw for an allenhead so you can use a ball-head allen wrench on it.
Carl