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Troubleshooting lean idle

Napoleon1991

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I’m having an issue with an extremely lean idle on my recently rebuilt 440. Its so bad that the engine will not idle on its own without the initial timing being set at 40 degrees. My A/F gauge shows between 16 & 19 when the engine is idling. It runs very rough and surges, but clears up instantly when I hit the gas. I have checked all over for vacuum leaks and even replaced the intake gasket with no change. The engine ran great earlier this year until I spun a couple bearings due to sucking the oil pan dry. The machine shop I took it to cleaned the engine, replaced the crankshaft and rods and rebalanced everything. The cylinders were honed and new rings were installed. I’m running the same pistons, carb, cam, heads and intake as before so nothing else should have changed. Before the rebuild my idle A/F ratio was around 14 and it ran great. I have tried swapping the carb and distributor with spares that are known to be good with no change. I did a compression test and all cylinders were in the 160s except cylinder 6 which was 148. Not sure why its lower, but I wouldn’t think that alone would cause the issue that Im having. Im running out of ideas of what to check for next. Any help would be much appreciated.
 
Still sounds like a vacuum leak, maybe a bad brake booster? Spray either around intake see if it changes speed.

Old dampners can move and go bad. Double check TDC w your timing mark.
 
Still sounds like a vacuum leak, maybe a bad brake booster? Spray either around intake see if it changes speed.

Old dampners can move and go bad. Double check TDC w your timing mark.
I disconnected the brake booster and plugged the port on the intake to rule that out. I sprayed starting fluid around the intake and carb, but it made no difference. The damper is only 2 years old and TDC has been verified. This problem really has me stumped.
 
I've never had much luck spraying solvents looking for leaks. Might have to use a smoke machine.
 
Did the cam get degreed in...? Timing sets sometimes are way off.
 
How many turns out are your idle mixture screws ? Try going out 1 full turn on each and see what happens. If it idles turn back in 1/8 turn at a time until you hit that lean spot again then go back out 1/8-1/4 turn ..... this is assuming you have no vacuum leaks !
The 340 in my dart is very sensitive to mixture screw adjustment. 1/8 turn in and it won't idle, I blame the cam!
 
How many turns out are your idle mixture screws ? Try going out 1 full turn on each and see what happens. If it idles turn back in 1/8 turn at a time until you hit that lean spot again then go back out 1/8-1/4 turn ..... this is assuming you have no vacuum leaks !
The 340 in my dart is very sensitive to mixture screw adjustment. 1/8 turn in and it won't idle, I blame the cam!

Mixture screws are 4.5 turns out, the same as they were before. Moving the screws seems to have little to no effect on the idle. I also tried swapping carbs, but it didnt help.
 
Mixture screws are 4.5 turns out, the same as they were before. Moving the screws seems to have little to no effect on the idle. I also tried swapping carbs, but it didnt help.
What were the different carbs you tried so far?
 
4.5 turns out is too many. The screws should function in the 2.5-3.5 range in a Quadrajet. Are the transfer slots exposed at curb idle? They shouldn't be visible. That would explain why there is little effect turning the mixture screws. Perhaps some dirt is blocking the idle pickup tubes? Those tubes do fall out of the casting quite often. Have you had the carb apart yet?
Mike
 
4.5 turns out is too many. The screws should function in the 2.5-3.5 range in a Quadrajet. Are the transfer slots exposed at curb idle? They shouldn't be visible. That would explain why there is little effect turning the mixture screws. Perhaps some dirt is blocking the idle pickup tubes? Those tubes do fall out of the casting quite often. Have you had the carb apart yet?
Mike
I dont know much about carbs but im assuming the transfer slots are the horizontal ones? If so, they appear to just barely be visible. I havent had the carb apart yet, but i still find it strange that the problem doesnt go away when swapping carbs.

IMG_4569.JPG
 
those butterfly's are open to much. The slot should not be exposes that much , if at all.

As mentioned that's why your idle screws are having no effect .
If you can't get the car to idle by adjusting the main butterfly, you can adjust the secondaries slightly to get the air you need .
 
I dont know much about carbs but im assuming the transfer slots are the horizontal ones? If so, they appear to just barely be visible. I havent had the carb apart yet, but i still find it strange that the problem doesnt go away when swapping carbs.

View attachment 1027198

Open too much! Add in some timing which will raise rpm's, then lower rpm's by adjusting throttle screw on carb. Repeat. Hook a vacuum gauge up to the motor and advance timing until you plateau at near max vacuum, bump it back a bit for wiggle room.

Be advised if this works, you will need to limit your mechanical advance as your new all in timing will be too high.
 
Open too much! Add in some timing which will raise rpm's, then lower rpm's by adjusting throttle screw on carb. Repeat. Hook a vacuum gauge up to the motor and advance timing until you plateau at near max vacuum, bump it back a bit for wiggle room.

Be advised if this works, you will need to limit your mechanical advance as your new all in timing will be too high.
Here's an easy solution for the timing situation.
https://www.ebay.com/itm/323796558921
 
I realize that my current idle setting is way too high. I just recently turned it up because that was the only way I could get it to idle at all. The carb & timing had been set the way it was before I had to rebuild the motor, but for some reason it's running way too lean now. I have already tried messing around with the timing, but it will not seem to idle unless I give it 30-40 degrees initial. Yesterday I tried yet another known good qjet and I am still having the same problem. My guess is I have a large vacuum leak somewhere, but I have been unable to find it so far. My A/F ratio is still in the 16-18 range when idleing.
 
I couldn’t find my vacuum leak, so did this on mine to confirm there wasn’t one, but is a little unconventional.

Loosen the rocker shaft bolts to close all the valves, and remove the carb and spark plugs.

Install a plate where the carb was and pressure test the intake manifold thru the manifold vacuum port.

Kind weird but hey it was piece of mind for me...
 
I couldn’t find my vacuum leak, so did this on mine to confirm there wasn’t one, but is a little unconventional.

Loosen the rocker shaft bolts to close all the valves, and remove the carb and spark plugs.

Install a plate where the carb was and pressure test the intake manifold thru the manifold vacuum port.

Kind weird but hey it was piece of mind for me...
Good outside the box thinking! We will keep thing on this problem.
Mike
 
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