I'm running an Edelbrock 1406 on my 460. I achieve maximum RPM when the idle A/F mix screws are turned all the way in. I've tried searching and most suggestions do not seem to apply to my situation. I am running right at 5 PSI fuel pressure so it's not that. I've heard a lot of people suggest vacuum leaks messing with the mix, but I would think that would cause the opposite problem, where you would have to back the screws all the way out. Could it be an issue with the floats? This carb has about 100 miles on it. Thanks in advance...

 

You don't have the throttle blades closed enough to make the a/f screws work.
There is a slot in the venturi's that has to be closed off enough to make it draw fuel through the idle circuit. If it doesn't, then the screws don't make any difference.

If the screws are IN, then the motor should not be able to idle(should stall). Thus you likely have a vacuum leak somewhere making you turn the idle screw up far enough to make it idle. Doing this eliminates the idle circuit making the screws not work.

Check things like power brake hose and other vacuum sources for leaks. Plug them off to test.

What RPM are you set at?

 

Thanks for the input.


How do I adjust the throttle blades? Sorry if this is a dumb question, but is that the same screw used to set idle rpms?


Am I wrong in thinking that a vacuum leak would put more air in the system, which would cause the engine to run lean? In a lean condition, wouldn't the idle screws need to be turned OUT for best a/f ratio?


I have the idle set to 800 rpm. As I turn the idle screws in, there is a noticeable increase in rpm and smoothness, which tells me they are doing SOMETHING - does that still sound like a vacuum leak? I'll check for leaks regardless, but I'm still not sure I understand how a vacuum leak would cause me to get better performance by making the mixture even leaner. Any further input is greatly appreciated.

 

The idle screw controls the throttle blades(the two blades you see moving down inside the carb).

With a vacuum leak you need more fuel, thus you turn up the idle screw to allow more fuel to be pulled into the engine. Doing so, the idle circuit gets bypassed and becomes non functioning and the mixture screws will not do anything.

You need to get the "idle screw" turned down enough to get the "mixture screws" to work. Sounds like it might be close, but if the engine won't stall, then you don't have the idle turned down far enough.

Start checking for vacuum leaks.

 

Great info, thank you very much. I'll check for leaks tonight and update tomorrow.

 

Idle speed screw controls the amount of air.(when choke is off)
Idle mixture screws control the amount of fuel.


So far I read that it runs best with the mixture screws all the way in. This is a clear indicator that fuel is coming in somewhere else. Turning the mixture screws all the way in should stall the engine.


My first suggestion is to completely disconnect all throttle linkages to remove any chance they are effecting the carb. If that doesn't make a difference then look down in the carb while it's running looking to see if you can see any fuel.

 

Update: I checked for vacuum leaks and didn't find any. I am pulling 18" Hg vacuum when it is running as best as I can get it (screws all the way in). While in the shower (where I do my best thinking...) I thought it either has to be a float or stuck needle that is letting fuel in. Disconnecting the PCV hose (lots of extra air flow) made the engine run much faster/smoother, which goes along with my theory that it is running too rich and needs to be leaned down. No matter what I change, the optimum running conditions are always with the idle mix screw completely turned in. Aside from setting float height, what else should I be checking?

Again, thanks for the info so far - I'm learning quite a bit.

To Freightrain: I turned the idle screw down to the point of nearly stalling and that did not help bring the idle mix system into play. Regardless of where I set the idle speed, the best running conditions were still with the idle mix screws all the way in.

 

The choke is open, right? Like someone said, look down the carb throat and see if fuel is dribbling out. Use a flashlight and you should be ablento see where its getting fuel.

 

Yes, the choke is open. I will look down the carb and see what I can see.

 

The fact the throttle blades are open too far is why the mixture screws are not working. The fact it won't run with the idle turned down means it is too lean. It has to have the blades open to allow the engine to draw enough fuel through the main metering to run.

Try this, open the mixture screws out 1 1/2 turns. Then turn down the idle screw. If you leave the mixture screws closed tight and turn down idle, it should quit.


Here is a good diagram showing how it works. The blades have to be below the transistion slot so that it will draw fuel from the idle circuit.


Where is the timing set?

 

I'll give it a try....


Last night I turned the idle down until it was hardly running at all - at the point of stalling. As I turned the idle mix screws out, it died completely. With the idle mix screws out 1 1/2 turns, I could not get the truck to start unless the idle speed was cranked WAY up.


Again, as the idle mix screws are turned in the final 1/2 turn, the rpms go up, the idle gets smoother at it seems to run fairly well. Anything to make the mix richer (open idle mix, close throttle, engage choke, pinch off PCV hose, etc) causes the engine to start stumbling.


My gut feeling at this point is that it's leaking fuel, but I don't want to jump to conclusions. I'm open to any/all advice. Thanks!

 

Question: should a properly tuned carb be able to run with the throttle plate completely closed?

 

Good points all. Always make sure fuel pump is serviceable and able to provide 1 pint in 30 seconds, and carb float drop is adjusted correctly. If not, it will never run right.

Next, find the mechanical "zero" on the linkage, have an assistant (engine off) floor the pedal and make sure there is no binding and, that the throttle butterflies are perfectly vertical. Very common mistake.

Turn idle mixture screws 1 and 1/2 turns out. Start engine and set RPM to spec (maybe 600 tops?) and turn each screw in the same amount. Adjust the idle RPM back down after each adjustment, for maximum RPM vacuum/smooth idle etc. This adjustment has to be done at spec RPM.

This is the "lean drop" adjustment, the idea is that ONLY the idle circuit is in play at idle, and just as lean as possible, though no more. The throttle butterflies will be completely closed or nearly so.

 

Pinching the PCV should cause an increase in RPM. That would cause a richening due to the lack of extra air coming in under the carb.

 

Only if it's running lean.