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440 still idles with screws turned in

Moparnocar

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Mild 440, 284/484 cam, new Edelbrock performer 650 and intake

Originally, it idled pretty rough 6-8 inches of vac at idle. Put in 3" metering springs and it improved, but still doesn't seem right, new plugs, wires, etc.

Went to adjust idle screws, and made no difference. Screwed them down, and it kept running. Off fast idle, probably around 700 rpm. As we were looking at idle, idle INCRESED probably close to 70-100 rpm. It was noticable.

It does not appear to be running off the main jets, I can see any fuel on the blades any way. Fuel pressure 5.5-6.5, bounces a bit. Went to change fuel filter, and alot of rust came out the intake side. Plugs are a little dark, but it was idling rich for awhile

Any ideas on the increasing idle, mixture screws?

Thanks
 
1. check that the throttle blades are fully closed at shut off. Then also check the idle circuit slots are covered at the idle position. Will require you to take off carb drain and turn upside down. Next check shafts are bent. Next look for vaccum leaks with a water sprayer and alcohol. If no luck check the power valve for blown seal.
 
1. check that the throttle blades are fully closed at shut off. Then also check the idle circuit slots are covered at the idle position. Will require you to take off carb drain and turn upside down. Next check shafts are bent. Next look for vaccum leaks with a water sprayer and alcohol. If no luck check the power valve for blown seal.
It definitely is on slow idle, from top, butterfly is not open. New carb, but it's possible it could be bent. I don't think performers have power valves??

I already checked for vac leaks, none found yet
 
Vacuum seems very low, too low. Was cam installed retarded? Before continuing tuning you need to find out why vac is so low. Check for vac leaks. I would expect that cam to have a minimum of 11" of vac.
Your problem is common with low vac. T/blades are open too far at idle & engine is idling off the t slots; mixture screws then have no control, do nothing.
You will also need a LOT of initial timing with this cam; probably around 30*. [ Yes, 30* BTDC initial!! ] .
Try this simple 5 min test, it will give you a taste of what you can have: Engine idling, loosen dist clamp & rotate dist SLOWLY to adv timing. RPM will increase considerably [ engine making more HP]. You may even be able to reset the carb idle & get adjustment on the mixture screws.
But I think you have a more advanced problem [ pardon the pun!! ] with vac too low to start with.
 
Mopar 284/484 Cam

Your doing good with 6"-8" of vacuum at idle with that cam and 700 RPMs at idle
And it’s probably bouncing around

You said new carb

Did you pop the top before installing and check float levels and float drop

Might bee on the lean side at idle and cruise with running that 650 cfm carb on a 440 with that camshaft also
 
Likely a vacuum leak, if it still idles with screws turned in. This would also explain poor idle with screws turned out to normal, and low vacuum readings. I use an unlit propane torch to check this. If engine speed increases where you are aiming the torch , there is your leak.
 
Are you familiar with the transfer or transition slot in the carb primary bores? They should have a square to barely rectangular section exposed below the throttle blades at idle position. That is really low vacuum but I’m not familiar with the cam. But that vacuum would indicate to me that the curb idle position of the primary blades is too far open, exposing most of the transition slot below the blades. That will bring in the transition circuit which is not controlled by the idle mixture screws - thus the motor will continue to run with the mixture screws closed. You will need to correct the low vacuum issue so the primary throttle blades can be closed down allowing the idle circuit to function - or is that is all the vacuum that cam can produce, you may have to drill small holes in the primary throttle blades to allow some air to pass and to allow closing the blades down to where they belong at idle.
 
You will also need a LOT of initial timing with this cam; probably around 30*. [ Yes, 30* BTDC initial!! ] .
unless we know exactly how much mechanical advance the op distributor is providing, having the initial at 30° may provide way too much all-in timing. Is it limited to 5°? Is there no limiter and has 30° mechanical? Only the owner knows for sure.
 
Mopar 284/484 Cam

Your doing good with 6"-8" of vacuum at idle with that cam and 700 RPMs at idle
And it’s probably bouncing around

You said new carb

Did you pop the top before installing and check float levels and float drop

Might bee on the lean side at idle and cruise with running that 650 cfm carb on a 440 with that camshaft also
I did not, new outta the box
 
Are you familiar with the transfer or transition slot in the carb primary bores? They should have a square to barely rectangular section exposed below the throttle blades at idle position. That is really low vacuum but I’m not familiar with the cam. But that vacuum would indicate to me that the curb idle position of the primary blades is too far open, exposing most of the transition slot below the blades. That will bring in the transition circuit which is not controlled by the idle mixture screws - thus the motor will continue to run with the mixture screws closed. You will need to correct the low vacuum issue so the primary throttle blades can be closed down allowing the idle circuit to function - or is that is all the vacuum that cam can produce, you may have to drill small holes in the primary throttle blades to allow some air to pass and to allow closing the blades down to where they belong at idle.
Drilling the hole was what I was thinking, looks like I'll have to look closer for vac leaks, I'm gonna smoke test it.
 
unless we know exactly how much mechanical advance the op distributor is providing, having the initial at 30° may provide way too much all-in timing. Is it limited to 5°? Is there no limiter and has 30° mechanical? Only the owner knows for sure.
It's a stock dist, no modifications that I know of.

I was wondering about the 30° when I have to have that limiter plate in there? I usually tune for 36° total so that would only give me about six degrees advance.
 
Just to confirm....does your carb have a provision for a vacuum fitting at the rear of the carb that gets a pipe plug installed if not used?
 
Post #9.
The idea of getting idle timing correct is so that the carb idle cct can be tuned. Once that is known, THEN a centri curve can be organised.
 
Post #9.
The idea of getting idle timing correct is so that the carb idle cct can be tuned. Once that is known, THEN a centri curve can be organised.
See post #12
 
I have a similar engine to the OP - mostly stock 440, 484 purpleshaft, headers, etc.
I've had more than a few of these in similar trim over the decades; I like 440's and
I really like the 484 cam on the street for goofing around.
That said -

IMO, a 650 isn't going to feed the bulldog here - surely not one out of the box.
These critters like the Holley 3310 750cfm with minimum tuning or similar old school
Edelbrock ones (I'm next headed for the AVS2 in 800 cfm for this one one day).

Timing? Mopar Electronic ignition conversion or similar. Set the thing to give you around
36 degrees "all in" (vacuum unhooked and plugged) and whatever the timing is at idle is
what it is, give or take (I "shadetree" set the timing using vacuum and RPM readings, going
for "best" at those two settings - usually works out to 35-36 all in).

Between getting jetting right on a proper carb and getting timing in the ballpark, these are
really lovely engines to live with and quite snappy, too. Very simple stuff.
 
If the idle screws are fully in there is no idle fuel except from an exposed transfer slot or some other fuel leak. If it also has a vacuum leak that means it would be really lean and I am not sure how it could idle. Once you turn off motor is there any fuel residue around the carb top and air valve pockets?
 
When you increase initial timing you also increase RPM which in turn allows you to turn the idle screw outward thereby closing the blades and cover the transfer slot back up to square which will increase vacuum. It may be necessary to drill very small holes in the blades , 3/16 ish but you may want to wait on that for now. Try around 18ish degrees of initial BUT keep in mind that you don't want too much total. Your car should also idle a lot better and not stink like fuel. What you need isa way to adjust the amount of timing " on the plate"( how much the dist. mechanically adds to your initial timing) So that your initial plus your mechanical doesn't add up to be more than the 34-36 that you want. I don't know what type of dist. you have as there are different ways to do this depending on what you have. Your'e really going to want at least a 750 carb as well.
 
I have a similar engine to the OP - mostly stock 440, 484 purpleshaft, headers, etc.
I've had more than a few of these in similar trim over the decades; I like 440's and
I really like the 484 cam on the street for goofing around.
That said -

IMO, a 650 isn't going to feed the bulldog here - surely not one out of the box.
These critters like the Holley 3310 750cfm with minimum tuning or similar old school
Edelbrock ones (I'm next headed for the AVS2 in 800 cfm for this one one day).

Timing? Mopar Electronic ignition conversion or similar. Set the thing to give you around
36 degrees "all in" (vacuum unhooked and plugged) and whatever the timing is at idle is
what it is, give or take (I "shadetree" set the timing using vacuum and RPM readings, going
for "best" at those two settings - usually works out to 35-36 all in).

Between getting jetting right on a proper carb and getting timing in the ballpark, these are
really lovely engines to live with and quite snappy, too. Very simple stuff.[/QUOTE
When you increase initial timing you also increase RPM which in turn allows you to turn the idle screw outward thereby closing the blades and cover the transfer slot back up to square which will increase vacuum. It may be necessary to drill very small holes in the blades , 3/16 ish but you may want to wait on that for now. Try around 18ish degrees of initial BUT keep in mind that you don't want too much total. Your car should also idle a lot better and not stink like fuel. What you need isa way to adjust the amount of timing " on the plate"( how much the dist. mechanically adds to your initial timing) So that your initial plus your mechanical doesn't add up to be more than the 34-36 that you want. I don't know what type of dist. you have as there are different ways to do this depending on what you have. Your'e really going to want at least a 750 carb as well.
I never messed with the idle screw, but I'm thinking more timing too. I want to put the plate and springs in the distributor, but the owner isn't sold on it yet
 
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