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Stock rebuild of my 67 330 HC with original Quadrajet (professionally rebuilt carb). Runs very well but doesn't idle smooth. I've read this is common with this carb. Any tricks? Thanks
Need a better description of not idling smooth. Is the engine shaking and/or missing. How does it run above idle? If you look down into the cab while the engine is running do you see fuel dribbling? Does the exhaust smell rich and has visible smoke?
What ignition and tuneup settings are you running, timing/dwell if applicable. Is your advance connected to manifold or ported vacuum? Have you looked at your spark plugs to see what they are telling you?
I'll have to look at some of that you suggested. What I can tell now is idles a little rough, shakes some, but not missing. Set the timing and rpm to specs, but would have to look that up again. Changed to an accel pointless ignition. No smoke, but tailpipes are black with soot from initial running, have since dialed back the screws a little a few times to lean it out. Planned to look at the plugs after I put a few more miles on it, which may just help it out anyway. Will post again when I have more info. Thanks.
Triple check the firing order. Seen a few engines run very poorly due to this and some just a little less than perfect, it varies with how the order is off.
Ok, I've got answers to all questions. Runs very well above idle, slight shake at idle. At 800 rpm timing is set to 7-8 deg BTDC. I do see fuel dribbling in the carb. Since first running months ago, I have turned the carb screws in half a turn at a time as it was too rich and blackened the tailpipes. The dist. vac advance is on the carb port on the right. 15# vacuum at idle, down to 5 or so on acceleration. I did find a small vac hose off at the a/c, but it didn't change anything. Made sure the carb bolts are tight. Firing order is ok, plugs show normal condition. One issue is I cannot get it to idle less than 800, linkage just can't be adjusted that low. As I said it is only a slight shake at idle. The problem could be me, I tend to be too much of a perfectionist. And o yes, I am running premium fuel 93 octane. Thanks.
A slight shake typically indicates ONE cylinder is misfiring. There is no carb failure or misadjustment that affects only one cylinder. Look to plugs, plug wires, distributor cap. Also try a leakdown test to verify the condition of individual cylinders. If you have access to an ignition scope, that's the easiest way to pinpoint the problem.
If fuel is dribbling when your looking down your carb at idle, every big fuel drop will cause a fluctuation in idle quality and speed. Not being able to turn the idle down and a rich exhaust smell could accompany this issue. It may be caused by an internal carb, gasket, or float issue. It could also be caused by a high fuel pressure problem. If you disconnect the accelerator linkage rod from your carb, can you lower the idle with the adjustment screw?
If fuel is dribbling when your looking down your carb at idle, every big fuel drop will cause a fluctuation in idle quality and speed. Not being able to turn the idle down and a rich exhaust smell could accompany this issue. It may be caused by an internal carb, gasket, or float issue. It could also be caused by a high fuel pressure problem. If you disconnect the accelerator linkage rod from your carb, can you lower the idle with the adjustment screw?
Actually, this is a good point. You can check float level through the vent tube with the engine running. If the float is low (due to improper setting or ethanol-impregnated plastic float), you will have problems.
Here's a way to make a duplicate of that gauge. Use thin plastic. Note that the zero mark starts at 39mm up from the tip. Mark off the correct float level from zero.
Roger, if you have fuel dribbling and have to remove the carb, check the throttle blade position from underneath (pictured). Usually with this problem you end up with the idle speed screw adjusted incorrectly. Also make sure the secondary throttle blades are closing completely, the fast idle cam is disengaged completely, and the linkage return damper is not affecting anything. Good luck, nicely detailed engine.
Did a few tweaks, set it up with a vacuum gauge, changed out the accelerator linkage on the firewall, I think it may have been binding a little. Mostly, I think just driving it some will take care of it. I can live with the current condition for now. Thanks again and Merry Christmas to all.
Did you verify the linkage was not holding your idle up? Also a good idea to ensure the throttle is opening all the way. The rod is adjustable, although I had to bend the pedal lever up a bit to get it in the ball park with a carb change.
No, I've adjusted the linkage, and run it with it unhooked. Also had to cut away a small piece of carpet as the gas pedal was hanging up a bit. Like I said, it gets better the more I run it.
We must remember that the carburetors were designed and engineered to be used with pure gasoline, not that which has been diluted with alcohol. The alcohol will mess up the idle circuit air/fuel mixture. I found out from a professional carburetor rebuilder that to fix this problem with a 10% alcohol mixture you need to adjust the idle air bleed circuit. You will need to find the idle air bleeds in your carburetor, measure the diameter of them with a numbered pin drill bit, and open them up .002" with a larger drill bit.
The first car I did this on was a 58 Cadillac with an original Rochester 4GC carburetor that I could not get to idle smoothly. (I consider myself a carburetor expert) I measured the idle air bleeds at .060" and opened them up to .062" and the idle mixture screws worked correctly and I got it to idle really smooth. I have since done this to my other carburetor equipped cars with the same result. My 67 442 with the original Quadrajet idles like it was 1967 again with 100 octane, leaded, no alcohol fuel, even though I only have 93 octane, lead added, octane added, alcohol diluted fuel.
Try this fix, it really works.........................
Roger, here's a youtube video which show one way of removing the idle tubes. I buy my replacement tubes from Cliff Ruggles, but there are other vendors. Basically fuel is pulled up through the idle tube, then mixed with air(to emulsify the fuel), then routed back down through a idle/channel restriction to the idle mixture discharge screws and transfer slot. Good luck
Here are pictures of a 67 Olds Quadrajet showing the Idle Air Bleeds. Different years and engine uses had different configurations of the idle circuits. You can drill them out with the carb on the car, you can put a rag to catch any filings, but any little filings that get into the engine will be burnt up.
Remember, open them up .002" from where they are to compensate for the alcohol in the fuel.
Attached is a photo from the 67 Shop Manual and photos of a 67 Carburetor where you can see the idle air bleeds next to the air horn screw. I don't know why my photos turn sideways when uploading...
Actually, this is a good point. You can check float level through the vent tube with the engine running. If the float is low (due to improper setting or ethanol-impregnated plastic float), you will have problems.
Here's a way to make a duplicate of that gauge. Use thin plastic. Note that the zero mark starts at 39mm up from the tip. Mark off the correct float level from zero.
December 16th, 2017, 07:31 AM
Terrific!
