Setting idle mixture screws on edelbrock..... Help!!

[vanrj1]

Ok, so I posted in another post that I have low vacuum and later ran across a thread talking about timing. there was a link to Diamondback engines page and I found this white page about tuning dual carbs. After following the instructions for this, I got my vacuum steady at 18hg by adjusting idle mixture screws (all about 1.5 turns out) and running smooth. That being said my idle is now 1100 rpm's with "idle set" screws backed off all the way. Timing set a 8° BTDC runs real well but idle seems high. here is link. it says to adjust to get highest steady vacuum. look at number 3. , 1. vac gauge method

This is a poly 318 with 264/450 cam

http://www.diamondbackengines.com/technical-white-papers/setting-up-dual-quads/

(This is a repost with new title stating "Idle mixture screws" instead of Idle screws)

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to confirm Timing, here is a picture of my timing marks. The Green is TDC and the blue is 10° BTDC. Just want to confirm Counter clockwise form zero is BTDC?

 

[67 coronet]

if your idle speed screws are backed all the way out and you are still at 1100 rpms then something is causing your idle to stick.how about the choke?is it adjusted correctally?is the cable free to move farther down?nothing is hitting at the base of the carb to interfear with throttle plates or shafts?

 

[benno440]

67 i had the same problem, my edelbrocks were wound all the way out and still sitting at 1300rpm, i found out that my timing was too low and my air fuel mixtures were wrong, fixed these now she idles at a healthy 900. not sure if OP is having same problem,

 

[vanrj1]

if your idle speed screws are backed all the way out and you are still at 1100 rpms then something is causing your idle to stick.how about the choke?is it adjusted correctally?is the cable free to move farther down?nothing is hitting at the base of the carb to interfear with throttle plates or shafts?

I un-hooked cable and nothing is sticking or hitting, choke is not making contact as it is warm.

67 i had the same problem, my edelbrocks were wound all the way out and still sitting at 1300rpm, i found out that my timing was too low and my air fuel mixtures were wrong, fixed these now she idles at a healthy 900. not sure if OP is having same problem,

Ok, if I advance timing to 10-12° that increases the rpm's. I had it idling at 900 rpm at 12° BTDC and idle mixture screws turned in about 3/4 turn but then the vacuum is 11hg. Do I compromise at some point. The white paper says to set idle mixture screws to obtain maximum steady vacuum. Which is more important of all the settings?

 

[coloradodave]

If your throttle blades are actually completely closed (idle speed screw not making contact with its stop) on both primary and secondary sides of both your carburetors, your motor wouldn't run. The exception to this would be a giant vacuum leak such as leaving out the carb base gasket. I would recommend you pull the carburetors and varify that the throttle blades are fully closed. You found out, correctly, that advancing the timing will increase your idle speed. The idle MIXTURE screws should land somewhere between 1 and 2 turns OUT from being seated. Neither the mixture or timing, at the settings you stated, will cause the motor to fast idle with the throttle blades COMPLETELY closed. First things first. You'll find it, keep digging...

 

[Meep-Meep]

If your zero mark on the vibration damper lines up with the blue 10 deg mark on the tab you are 10 BTDC and is probably a good starting point for initial timing.

To set the carbs you want to start by disconnecting the link between them so each throttle lever can move independently. Back off both idle speed screws (throttle stop) until the blades are lightly seated in the bores then run the screws up until they just make contact then go one full turn. This will set both throttle blades in about the correct position relative to the transfer slot, or slightly low, which in this case is fine with dual carbs. Idle mixture screws are about 2 full turns out from lightly seated.

This might be a good time to make linkage adjustments and to make sure the cable is not hanging things up. If you have a 1:1 linkage you want to make sure both carbs are completely in sync by making adjustments requires so both levers move exactly at the same time when actuated from the exact same place as the cable hooks to (very important). Also at this time check accelerator pump settings and make both the same. Once the linkage is properly adjusted and springs are back on, etc.. start it up and see where you are. Make precise idle speed adjustments to each carb as required. If you still have a high idle then either your timing is too far advanced (not likely at 10 BTC) or your secondaries are not seating, or something else is not quite right.

 

[vanrj1]

If your throttle blades are actually completely closed (idle speed screw not making contact with its stop) on both primary and secondary sides of both your carburetors, your motor wouldn't run. The exception to this would be a giant vacuum leak such as leaving out the carb base gasket. I would recommend you pull the carburetors and varify that the throttle blades are fully closed. You found out, correctly, that advancing the timing will increase your idle speed. The idle MIXTURE screws should land somewhere between 1 and 2 turns OUT from being seated. Neither the mixture or timing, at the settings you stated, will cause the motor to fast idle with the throttle blades COMPLETELY closed. First things first. You'll find it, keep digging...

Thanks Dave, I will check the blades can't understand why it idles high with current settings.

If your zero mark on the vibration damper lines up with the blue 10 deg mark on the tab you are 10 BTDC and is probably a good starting point for initial timing.

To set the carbs you want to start by disconnecting the link between them so each throttle lever can move independently. Back off both idle speed screws (throttle stop) until the blades are lightly seated in the bores then run the screws up until they just make contact then go one full turn. This will set both throttle blades in about the correct position relative to the transfer slot, or slightly low, which in this case is fine with dual carbs. Idle mixture screws are about 2 full turns out from lightly seated.

This might be a good time to make linkage adjustments and to make sure the cable is not hanging things up. If you have a 1:1 linkage you want to make sure both carbs are completely in sync by making adjustments requires so both levers move exactly at the same time when actuated from the exact same place as the cable hooks to (very important). Also at this time check accelerator pump settings and make both the same. Once the linkage is properly adjusted and springs are back on, etc.. start it up and see where you are. Make precise idle speed adjustments to each carb as required. If you still have a high idle then either your timing is too far advanced (not likely at 10 BTC) or your secondaries are not seating, or something else is not quite right.

Thanks Meep, I will go through the process again and re-check everything. Based on nice steady reading on vacuum gauge, that eliminates a lot of other symptoms. Will check good seated blades.

 

[Jonnyuma]

I'm not running duals, but I have the exact same problem with my single 4 bbl. Although I haven't been able to actually FIND it, I'm positive enough (99 44/100%) that I've got a massive vacuum leak at the rear rail of my intake manifold that I'll be resealing it this weekend.

"When all else is well, a vacuum leak is your villain"
-Sherlock Holmes*

*not really, i just made that up

 

[vanrj1]

I'm not running duals, but I have the exact same problem with my single 4 bbl. Although I haven't been able to actually FIND it, I'm positive enough (99 44/100%) that I've got a massive vacuum leak at the rear rail of my intake manifold that I'll be resealing it this weekend.

"When all else is well, a vacuum leak is your villain"
-Sherlock Holmes*

*not really, i just made that up

I will check that as well but based on the smooth vacuum with my gauge it would seem there is no leak. A vacuum leak would have a slow waving needle back and forth 2-5hg and mine does not do that. (this according to diagnostic white paper from Diamondback engines page)

 

[Jonnyuma]

My needle is low and steady also. Haven't read Diamondback's write up yet, but generally speaking a low but steady vacuum reading is a symptom of a leak.

 

[Moparsmitty]

Question for everyone. If his engine is idling with the idle speed screw backed all the way out isn't it a simple question of where is the air coming from? Like Coloradodave says, if the throttle blades are closed it has to be a vacuum leak somewhere in the intake system. The engine has to be getting air fron somewhere.
I'd be curious as to what would happen if you closed your hand over the carb air horn while it's idling. I'm thinking that if the engine dies, it's the throttle blades, if it continues to run at all it's a big vacuum leak. Am I correct of full of BS. (I could be both).

 

[krowbar]

I believe in you smitty

 

[vanrj1]

My needle is low and steady also. Haven't read Diamondback's write up yet, but generally speaking a low but steady vacuum reading is a symptom of a leak.

It's not low and steady, it's normal and steady with mixture screws adjusted 1.5 turns out it is at 18-19hg of vac.
when I got it from the place that broke in the cam, it was like they used the mixture screws to decrease idle because they were only out about 3/4 turn form soft seat and 12 hg vac. I then adjusted to get max steady vacuum per white sheet and that ended up being 1.5 turns out which is where Edelbrock says is normal and got it to 19hg. so ????

Question for everyone. If his engine is idling with the idle speed screw backed all the way out isn't it a simple question of where is the air coming from? Like Coloradodave says, if the throttle blades are closed it has to be a vacuum leak somewhere in the intake system. The engine has to be getting air fron somewhere.
I'd be curious as to what would happen if you closed your hand over the carb air horn while it's idling. I'm thinking that if the engine dies, it's the throttle blades, if it continues to run at all it's a big vacuum leak. Am I correct of full of BS. (I could be both).

I will try that since it is quick and see what happens. I am 99.8% sure it's not a big leak at intake carb base or hose but will definitely not rule it out without a definitive test. Thanks

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I believe in you smitty

Me too! :headbang:

 

[Jonnyuma]

When I cover the air horn w my hand theres literally no change in idle speed. It sucks, and its sucking from somewhere I can't get to under normal conditions.
I thought we were having the same symptoms, but now I think not so much.

 

[vanrj1]

When I cover the air horn w my hand theres literally no change in idle speed. It sucks, and its sucking from somewhere I can't get to under normal conditions.
I thought we were having the same symptoms, but now I think not so much.

Well, based on a lot of information above, you should have some ideas on where to look for the problem I hope.

 

[Moparsmitty]

When I cover the air horn w my hand theres literally no change in idle speed. It sucks, and its sucking from somewhere I can't get to under normal conditions.
I thought we were having the same symptoms, but now I think not so much.

Check all the normal vacuum leak possibilities first. Does the engine have a pcv valve, power brakes, etc. If so temporarily disconnect and block off those lines. If not there may be a failure of an intake gasket on the inside that you can't see. That will be a little tougher to diagnose. Good luck Jon.

 

[Meep-Meep]

A rough idle usually accompanies a vacuum leak so I guess it depends on the quality of idle.

 

[62 Dart Convertible]

It would be great if you could update this thread with an outcome vanrj1.

It's like I read the first few chapters of an interesting whodunnit, and I want to know the outcome....

(I'm guessing it was eithera vacuum leak - like a leaky booster or a maladjusted linkage...)

 

[vanrj1]

It would be great if you could update this thread with an outcome vanrj1.

It's like I read the first few chapters of an interesting whodunnit, and I want to know the outcome....

(I'm guessing it was eithera vacuum leak - like a leaky booster or a maladjusted linkage...)

Sorry for the delay, didn't do anything to the car over the winter. So The car is set at Idle mixture 1 3/4 turns out, Idle screws out all the way, (no contact) timing at 8° BTC vacuum 16hg and idle 1000 rpms. The car drives nice from soft start to WOT so I am leaving well enough alone right now. Might take to a shop if I can find one that knows how to work with the set up I have.

 

[AMX364]

I had thought that there was a hidden screw from the bottom of the carb, but when I checked both my Carter and Edelbrock carbs they do not have one.