Low idle vacuum on a freshly built engine

[superfragl]

I have a rebuilt engine that has zero miles after rebuild and about 1 hour of run time.
The motor has 9.5 static compression, Lunati cam with following specs

• Advertised Duration (Int/Exh): 268/276
• Duration @ .050 (Int/Exh): 226/234
• Gross Valve Lift (Int/Exh): .494/.513
• LSA/ICL: 110/106

Regular iron heads, Edelbrock intake and carb.
I broke the cam in following manufacturer procedure. Ignition timing is set to 12* initial, 36* total mech. by 3000 RPM, with vacuum advance total timing is about 54*.

It idles fine, revs fine, has good oil pressure, starts right up but develops only 10 inches of vacuum at idle. When I rev the engine I can see the vacuum goes up to 25 or more.
Manufacturer says I should have at least 14 inches with this cam.
I did degree the cam during assembly, but I did one intake and one exhaust lobe only to verify cam timing.

So far I have checked following:
- Verified the gauge reading with another vacuum gauge.
- Checked the lifters by pulling the covers and making sure rocker arms are not loose.
- Tested brake booster for leaks and it holds vacuum at 10 inches (could not develop more vacuum with small hand pump for bleeding brakes), it did not make a difference with booster port plugged anyway.
- Tested distributor vacuum canister for leaks.
- Did and compression test and got all cylinders within 146 and 150 - engine cold.
- Checked the plugs and they look good - tan color - no black soot and not white (I think if there was an intake leak plugs would look white because of the lean mixture)
- Tried advancing the timing to 15*, 20* and that did not make any difference
- Tried adjusting carb. idle screws and throttle screws with no effect.

When I installed the intake I used standard steel gasket and silver colored sealer that came with it.
I have not looked for vacuum leaks yet, but how do I test?? If there is a leak under the intake - above the valley pan - how to check this?
Maybe I need to put some miles on this engine in order to see better vacuum readings?
Any other thoughts?
I would not bother with low vacuum, but I am concerned about power brakes being inefficient...

 

[commando1]

The most common remedy for low idle vacuum with Edelbrocks is to change the step up springs on the metering rods.

 

[superfragl]

Do you mean lighter springs so the rods block the main jets at idle?
How would that affect the vacuum?

 

[66/440]

Do you mean lighter springs so the rods block the main jets at idle?
How would that affect the vacuum?

From the Edelbrock carb owners manual:
Long Duration Camshaft If the engine has a fairly radical camshaft it may require an excessive amount of throttle opening for idle and/or have low idle vacuum levels. Either condition can lead to poor levels of adjustability and erratic idles. • Another fix for the above condition is to run as much spark advance as possible at idle. If the distributor is fitted with a vacuum advance unit, connect it directly to manifold vacuum. If you are not able to employ vacuum advance for some reason, then the mechanical curve should have a low limit, which will allow you to use plenty of initial spark advance. • Measure the manifold vacuum at idle. If it is below 7" Hg, there is a good chance that the Metering Rods are in the up (rich) position. When combined with a high idle air rate this can cause the Nozzles to discharge fuel at idle. Use a weaker Step-Up Spring (see section on Step-Up calibration) to keep the Rods down at idle. With some cams, a stiffer spring (pink or silver) is necessary. Experimentation is the best way to determine which is best for your application.

http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf

 

[66/440]

Home made smoker to check for vacuum leaks.
Engine Test Stand

 

[66/440]

What size engine and cfm of your carb?

 

[commando1]

Do you mean lighter springs so the rods block the main jets at idle?
How would that affect the vacuum?

Yes, you want a lighter spring. I was taught this trick by an old racer. Has to do with enrichment of the A/F ratio. I'm not too clear on the theory but it worked for me in a couple of situations.

 

[superfragl]

I did read the manual and will try changing the springs.
The engine is 440 and carb is 1411 (750cfm)

 

[66/440]

I did read the manual and will try changing the springs.
The engine is 440 and carb is 1411 (750cfm)
I get the idea of the smoker to test for leaks and I have done this on fuel injection engines, but how do I use it on my 440? There will be quite a few valves open and the pressure will not build...

You should be able to generate enough volume of smoke to identify any leaks even with the valves open. You will get your primary saturation in the top end of the engine before you get smoke out the valves.
But I think Stan has you headed in the right direction.
Is your vacuum advance pot one of the adjustable ones?

 

[superfragl]

I just realized that )) Thanks

 

[superfragl]

Yes, it is adjustable

 

[66/440]

Just a thought...are you using a spacer/thick insulator under the carb? I was using an Edelbrock 9625 insulator (fully open) and changed to a 9266 and my @ idle vacuum increased by 3 inches. Not sure what caused it but changed back to the 9625 and vacuum dropped again.

 

[superfragl]

The carb is installed with a regular gasket over the manifold. With a spacer I will more than likely have hood clearance issues.

 

[1Fury1]

From the Edelbrock carb owners manual:
Long Duration Camshaft If the engine has a fairly radical camshaft it may require an excessive amount of throttle opening for idle and/or have low idle vacuum levels. Either condition can lead to poor levels of adjustability and erratic idles. • Another fix for the above condition is to run as much spark advance as possible at idle. If the distributor is fitted with a vacuum advance unit, connect it directly to manifold vacuum. If you are not able to employ vacuum advance for some reason, then the mechanical curve should have a low limit, which will allow you to use plenty of initial spark advance. • Measure the manifold vacuum at idle. If it is below 7" Hg, there is a good chance that the Metering Rods are in the up (rich) position. When combined with a high idle air rate this can cause the Nozzles to discharge fuel at idle. Use a weaker Step-Up Spring (see section on Step-Up calibration) to keep the Rods down at idle. With some cams, a stiffer spring (pink or silver) is necessary. Experimentation is the best way to determine which is best for your application.

http://www.edelbrock.com/automotive/misc/tech-center/dl/carb-owners-manual.pdf

I am going through this right now with the same 1411 carb which most say is too big for a small block but the head work etc my engine builder claimed 750 cfm was needed. Experimenting with timing and found 13 deg to be optimum. Also jetted it down a notch and changed metering rods accordingly. I have 9-10" vacuum at an idle. Still wasn't performing 100% so I started on the step up springs. It came with orange and I tried 1 weaker which is yellow but it ran worse so I went right to silver which is the stiffest one in the 1489 calibration kit. Big difference it runs really good all the way through the gears from just above idle to 5000 rpm.

It idles really nice however there is an issue in the transition from idle. When I let out the clutch it stumbles. I need to stay on the clutch and keep the revs up to get moving. Once it gets past that point it runs great. Have played around with the idle mixture screws but can't seem to make a big difference. It's really annoying almost wants to stall if you're not careful.

 

[commando1]

I started on the step up springs. It came with orange and I tried 1 weaker which is yellow but it ran worse so I went right to silver which is the stiffest one in the 1489 calibration kit. Big difference it runs really good all the way through the gears from just above idle to 5000 rpm

That is interesting to note.
Seems like the smartest way to change springs is to have an assortment on hand, from softer to stiffer when experimenting. Thanks for your input in this.

 

[1Fury1]

That is interesting to note.
Seems like the smartest way to change springs is to have an assortment on hand, from softer to stiffer when experimenting. Thanks for your input in this.

Yes as a carb tuning novice the kit came in handy. It includes a few different jets, rods, and springs. The Edelbrock manual is straight forward for part and wide open throttle setup, but when it comes to the springs it's guess n by golly.

 

[65sporty]

When I installed the intake I used standard steel gasket and silver colored sealer that came with it.
I have not looked for vacuum leaks yet, but how do I test?? If there is a leak under the intake - above the valley pan - how to check this?

External vacuum leaks can be found with some carb cleaner and listen for rpm change, internal leaks can be found by removing the pcv valve and blocking the other side off if you have a fresh air breather. Feel for vacuum if you have vacuum you have a leak, no vacuum or some pressure your ok. I just realized you have a BB ignore the internal leak it would only apply to a engine without a separate valley pan like a SB

 

[superfragl]

Step-up springs should be delivered toady, so I will be playing with those tonight if I have enough time.
I did the smoke test last night. I got no intake manifold or carb base leaks.
I have small leaks around the throttle shafts (I don`t think this is critical) and one bigger leak from choke unit.

This is the hole on carb where it connects

The leak only happens when the choke is fully open and the vacuum piston is all the way down. When the choke is closed or partially closed the piston travels up and there is no leak...
Is it supposed to work like that?

 

[70bigblockdodge]

Make sure you are not exposing too much of the transfer slot with curb idle screw, the transfer slots are directly effected by the power pistons/metering rods not being pulled down to lean at idle and low speed.

 

[superfragl]

I looked at the transfer slots last night and the throttle is open enough to affect the slots. I guess I will try advancing initial ignition to 20 - 25 * so I can close the throttle enough and not expose transfer slots.
I also called Edelbrock and they confirmed there is supposed to be an opening on the choke unit - the vacuum pulls air and cools down the heater element.
They also said this carb comes with orange springs which are rated at 5" of vacuum so the rods should be pulled down in my case and there is no need to replace them.