Carb Questions with High Vacuum Engine

[JoesEdge]

Lately, I've been taking some time to understand how a Holley works while driving. I've done some tuning to my Holley and I've seen improvement, but I had noticed that I had some enrichment (or lack thereof) problems.

In order to see what was going on with my engine as I drove, I plugged an Actron vacuum gauge to the manifold vacuum port on the base plate of the Holley.

First, is that the correct location to plug in the vacuum gauge?

I do not have any other source of vacuum on the engine that I can plug into, other than the capped off port on the metering block, but as far as I understand, that's the "ported" vacuum port.

Anyway, when fully warmed up my engine idles at 15 to 16 in/Hg. When I cruise (let's say at 2,000 RPMs steadily or even 3,000 RPM's steadily), I'm usually around a solid 22 in/Hg. Back off the throttle and the vacuum may jump up as much as to 26 in/Hg.

If I step on the throttle to accelerate and pass someone (not excessive speed), I can see the gauge go down to maybe 18 in/Hg. If I step on it hard and I have the pedal to the floor (I need a buddy to verify that it's really WOT), I do not see the gauge drop too far below 10 in/Hg. It's solidly at around 9.5 in/Hg to 10 in/Hg.

What I'm getting at is... I don't think my power valve ever opens. It's a 6.5 power valve that was recommended. To me, this explains why I do not see my WOT (or what I believe at this point to be WOT until I can verify) enriching; therefore, running leaner than I'd like at WOT.

This might also explain why the Edelbrock I tried was always too lean too. If the metering rods never raise, then it will not move into the power mode to add fuel to the in coming air.

Anyway... The largest Holley power valve is a 10.5 which I can try. But I'm wondering if my 750 is too small. I have read that the vacuum at WOT should be close to 0 in/Hg, and that anything more is a sign of the carb being too small. The next step up would be an 850 cfm carb, but would an 850 be too much for the street?

I guess I'm trying to get an understand how engine vacuum and carburetors work together. I'm hoping someone could explain it better so I can understand it further.

Much appreciated for any help and understanding.

 

[JoesEdge]

Here's my engine specs:

Block (CID): LA 360 (.030 over)
C/R: 9.6:1
Intake: Edelbrock Air-Gap Dual Plane with Moroso 1 inch 4 hole spacer
Ignition: MSD Pro Billet with MSD 6A ignition box and MSD plug wires
Camshaft: Comp Cams XE268H - 268/280 (224/230 @ .050”) - .508”/.512” lift *
Cylinder Heads: EngineQuest (EQ) Magnum Heads (2.02” / 1.60”) - Iron Heads
Rocker Arms: Comp Cams Magnum Rocker Arms - 1.6:1 Ratio
Exhaust: Dougs Headers 1 ⅝” primaries and 1.5” custom X-pipes into Flowmasters

NOTE: Valve lift numbers are corrected with the 1.6:1 ratio rocker arms rather than the stock LA 1.5:1 rocker arms.

• Timing set to 18* BTDC and 36* total before 2,500 RPM
• No vacuum advance for now, but probably in future
• About 15 to 16 in/Hg of vacuum at about 700 - 800 RPM
• Dyno’d at 403 HP at about 5,500 RPM (438 lbs @ 4,500 RPM)
• Dyno’d with 1 inch open spacer but switched to 1 inch 4 hole spacer for better idle

Here's some additional info to get an understanding of driving conditions:

Transmission: A833 4 Speed with Pistol Grip

Rear End:
Current: Chrysler 8 ¼ with 4.10:1 gears (open)
Coming-Soon: Chrysler 8 ¾ with 3.23 gears (open)

Wheels:
Front: 26”
Rear: 28” (65 MPH is about 3,200 RPM with current 4.10 gears)

 

[73Swinger18]

The ports at the carb base plate are full vacuum. The one on the metering block is ported and not where you want to take your measurement. I would have someone push the throttle to the floorboard and verify that your primaries are opening all the way. Do you have vacuum secondaries? Put a hairpin on the actuating rod to see if they are opening up too. Maybe the spring is too stiff.

 

[JoesEdge]

The ports at the carb base plate are full vacuum. The one on the metering block is ported and not where you want to take your measurement. I would have someone push the throttle to the floorboard and verify that your primaries are opening all the way. Do you have vacuum secondaries? Put a hairpin on the actuating rod to see if they are opening up too. Maybe the spring is too stiff.

Ok, cool. So that means I hooked it up at the right port.

I guess I just wanted to make sure that port was correct and that I didn't need to plug it into somewhere directly on the intake manifold.

It's a double pumper. This morning I put a pipe wedged on the gas pedal to hold it down. I found that I could open up the throttle even more. So I got rid of all the slack on my Lokar throttle cable. That will help it some, but I believe there's still more throttle opening to be had.

 

[73Swinger18]

That should help a lot. That carb isn't too big.