Misfires After 3500 RPM

[USAF]

First; sorry about the upside down pictures. They're right side up on my computer so I have no idea why they posted upside down.

Installed a new aluminum dual plane manifold and Edelbrock 500 CFM 4 barrel on the 195 hp GM crate motor 350 and I have misfires after 3500 RPM. Also installed a new MSD Digital 6-Plus ignition box, MSD distributor, and new coil. Runs great and revs quick from idle but misses bad above 3500 RPM. Timing is set at 12 degrees advanced with the vacuum advance removed. All vacuum lines are connected and the unused vacuum port on the carb is blocked. Don't have a vacuum gauge but just ordered one and I'll try that next. Do I hook the gauge to the vacuum advance port on the carb or would it be better to hook it to the larger vacuum port for the brake booster which is also new? Fuel pump is a new electric Holley with the pressure regulator set at 12 PSI. New fuel filter also.

I haven't adjusted anything on the carb except to set the idle screws. Haven't considered different jets or other adjustments on the carb.

Once I try the vacuum gauge I guess I'll see what to try next.

Having a custom exhaust installed next week and am running straight off the headers right now so could it be running too lean with the open exhaust and current jets?

Thanks for any hints and I'll post updates after I try the vacuum gauge.

Specs on the crate motor:

Part Number: 10067353
Engine Type: Chevy Small-Block V-8
Displacement (cu in): 350
Bore x Stroke (in): 4.000 x 3.480
Block : Cast-iron with 4-bolt main caps
Crankshaft (P/N 93426651): Nodular cast iron
Connecting Rods : Forged Steel
Pistons (P/N 93422884): Cast-aluminum
Camshaft Type (P/N 12364051): Hydraulic flat tappet
Camshaft Lift (in): .383 intake /.401 exhaust
Camshaft Duration (@.050 in): 194 intake / 202 exhaust
Cylinder Heads (P/N 93438649): Iron; 76cc chambers
Valve Size (in): 1.94 intake / 1.50 exhaust
Compression Ratio: 8.5:1
Rocker Arms (P/N 10089648): Stamped steel
Rocker Arm Ratio: 1.5:1
Recommended Fuel: Regular pump
Balanced: Internal

[rpmerf]

Ignition - what's your all in advance? What RPM?

At any point was it running fine past 3500 RPM? What has changed since then?

What kind of carb is it? Model number? Spread / square bore? Vacuum / mechanical secondaries?

[USAF]

Thanks for the questions.
I haven't determined the "all-in" advance. It misses so bad I'm hesitant to go above 3000 rpm to determine that.

I recently bought it and removed the EFI system the guy had on it and have yet to take it on the road except when I test drove it. It had lots of hesitation with the EFI system on it and I don't like fuel injection and why I removed it, to go back to "old school". So, I can't tell you that it was running fine before.

When I get my vacuum gauge what vacuum port should I connect to please?

Here's the carb specs:
Brand:Edelbrock
Manufacturer's Part Number:1404
Product Line: Edelbrock Performer Carburetors
CFM: 500
Fuel: Gasoline
Number of Barrels: 4
Carburetor Flange: Square bore
Choke: Manual
Secondary Type: Mechanical

[68 P.O.S.]

I agree, sounds like there's an ignition issue somewhere. Connect your vac gauge to the manifold vac port on the carb and readjust your idle mixture screws for best vac on the gauge.

[rpmerf]

Vacuum gauge would go to a manifold vacuum source. Not sure where exactly that is on your carb, usually lower, near the base. Size doesn't matter. Should read 15+" at idle, and drop to 0 at WOT. If it reads 0 at idle, it is a ported vacuum port.

Something that stands out to me off the bat is the fuel pressure of 12 PSI. Pretty sure that should be closer to 5.5 PSI. Not sure that is what is causing your issue.

https://www.edelbrock.com/pub/media/...ers-manual.pdf

I am suspicious of the MSD ignition, mainly the MSD Digital 6-Plus ignition box and how it is wired up. It transitions to single spark around 3300 RPM, so I am wondering if this is related. I'm wondering if there might be an issue with getting power, loose connection, etc. I would certainly double check everything related to this box. Makes me wonder if there is a special procedure to setting the timing with this box present. I would even be tempted to run without it until the issues are figured out. A stock HEI should perform sufficiently past 3500 RPM.

https://www.jegs.com/InstallationIns...1/121-6520.pdf

Back to basics a bit.
How are the plugs? Have you checked the gap? Verified the wires look good, no arching?

Have you checked the valve lash?

[D.PASSMORE]

Don't rule out lean condition at WOT. Look at ignition first, try to eliminate all possibilities. Then look a Air/Fuel mixture.

[USAF]

Thanks guys. I'll try the vacuum gauge first when I get that. The plugs, wires, and all connections are new and secure. I do have the wires in looms that are red anodized aluminum. I was thinking that they should be plastic to prevent cross-firing but these are high-end wires and brand new so hopefully that isn't the issue. I had the MSD Digital box from when I was building a 427 for my 67 Chevelle SS and I sold the Chevelle and just had the box laying around. The port for the brake booster is low on the carb base so I'll try that port when I get the gauge. Thanks again guys.

[tdangle]

Fuel pressure set to 12 psi? My understanding is that Edelbrock carbs don't like over 5 psi. That may be a problem

[68 P.O.S.]

Wow, I missed the 12 psi earlier....that is way too much pressure for an Edelbrock. They recommend regulating the pressure to 5.5 psi. Anything more and it pushes fuel past the needle and seats and causes flooding. Also, have you adjusted the floats to specs? If not, that will cause flooding as well. Looking at the pic of your engine bay, the carb also needs a heat insulating gasket between it and the intake because Edelbrocks also like to percolate fuel, causing hard starting issues. You want the Edelbrock 9266 with your intake & carb combo.

I agree the MSD box could be the issue. Definitely check it out thoroughly. Run the truck without it to see if it is the problem.