What are your plugs gapped at? Should not be more than .035 regardless of plug type, ignition type whether its points, HEI or other. Of course your plugs should be proper as far as seat, reach, heat range etc.

Depends on what it is actually doing. If in neutral it idles and accelerates with a snap of the throttle lever, it should be fine going down the road carburetion wise. If it fusses and falls flat on its face when you snap the throttle lever quick to half or more throttle, then you are either not getting ignition advance or you have a lean out from not enough accelerator pump enrichment to match the throttle opening. Check your primary side accelerator pump lever and make sure there is no gap between it and the plunger attached to the throttle lever on the primary side. Take out any slack and verify that the accelerator pump squirts
through the shooters the moment the throttle plates begin to open. Take care not to over adjust and cause the accelerator pump to overtravel and tear the diaphragm. To check for over-travel, fully open the throttle to its stop (ignition off of course) and press down on the accelerator pump arm. You should have a minimum .020" clearance between the plunger and the arm tip at the bottom of its travel at full open throttle. On the secondary side, verify you have a working accelerator pump the same way using the secondary opening as your reference point just like the primary side.

Throw a timing light on the car and verify your advance is working on both vacuum and mechanical advance. Your vacuum advance hose should be connected to the passenger side fitting on the carburetor metering block. That is your ported advance and will keep your timing consistent even if your motor has a fluctuating vacuum signal typical of long duration camshafts. Some cars like full manifold vacuum for the advance but I've found the ported vacuum to work best.

I had two Holley techs tell me conflicting information but we finally decided on the fact that the carburetor does need a regulator at the pressure my pump puts out. So I am putting on a regulator and after talking too them for a while I am sending this 750 back and getting a 770 with vacuum secondaries and its actually cheaper.

If you have more 7.0 psi delivery pressure your problem might have been flooding past the inlet needles. That said you, you didn't mention black smoke, starting issues and fuel overflowing from your vent tubes like a fountain that would result.

Assuming you were running an FI rated pump, do you not have a regulator to step down the delivery pressure to carburetor?

If using an in-tank pump, you'll need a Holley regulator with bypass. They come preset to 7psi for carbureted applications and can be plumbed into your supply and return lines for by pass so you don't overheat the pump. It's your only alternative. You can install them under your hood (more junk showing) or close to your tank with a short bypass if you don't want it under the hood. The longer line ahead of the regulator doesn't really make much difference in the pressure at carb but you can always tweak it if needed, but it amounts to next to nothing.

If you're using an inline pump, you could also swap it out for inline Holley street pump that is also preset for carbureted use and just run it unregulated.

I put in a Carter Competition series mechanical pump and use my electric Holley Street pump on a switch as a pusher. I've only used it when I had a touch of vapor lock in the heat of summer in traffic. A quick flick of the switch forces the vapor out through the carb vents. The mechanical pump keep up just fine on its own.

I am using the Holley mechanical pump which I was told initially was a 'matched item' and did not need a regulator. I was seeing up to 10 pounds on the gauge fluctuating

I always run the Carter street pump,no regulator.Tom

If you're not experiencing carb flooding, overflow out the vents or extended cranking time when starting, chances are you're OK though 10 psi is high. I've heard about Holley pumps being off spec and delivering higher than normal pressures. The Holley Red electric street pumps like I use as a pusher are limited to 7 psi max.

I have yet to see a mech pump run 100% on a performance Pontiac. Even the folks who swore by the RobbMc pumps ended up switching them out.

Said it before, will say it again, just build a performance fuel system and be done with it.


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x2 tom s

It’s not ignition related I don’t think. Car runs great all day, shut it off overnight and go back out and it bucks and shakes under load. Plus the whole garage smells like a gas station. Pull the plugs out, black as soot. I don’t want to just change parts around so I am gonna hold onto this 770 a couple days. I plan to take it off and inspect for obvious flaws. I am gonna replace base gaskets cause Holley said it may be a vacuum leak and then I am putting a regulator on it. Of course with fresh plugs. If that doesn’t work then we will know.

Been there dun that!

Sounds like a ultra rich condition from too high a fuel pressure or your fuel curve is causing a rich condition. I run 9 psi on my racecar with an electric pump and have no issue with excessive fuel delivery, so maybe check for fuel drip out of the nozzles while the car is running.

Electric or mechanical?

Mechanical.Tom

Is fuel tank vented? really vented? trapped pressure will overcome needles and seats no matter what fuel delivery you have

Verify that your choke is working properly. It's usually something simple. The choke plate should close fully on cold start and immediately snap open about 1/2" immediately after starting. The vacuum break that does this is internal. All you have to do is loosen the choke element cap retaining screws and rotate the cap clockwise for more cold enrichment (less gap) or counterclockwise for less (more gap). There is also a small adjustment screw that faces straight down in between the choke housing and carburetor body that adjusts the cold idle speed.

A leaking base gasket isn't going to foul your plugs and you already ruled out vacuum leaks with 11-1/2" of vacuum with a big cam like you're running. A vacuum leak will lean your carburetor out and would have to be massive to effect the power valve operation. You would have lean (gray) plugs at idle and momentary over-enrichment when going to full throttle all the time.

You don't have a (lack of) fuel delivery problem as others have suggested unless the car is bucking running out of fuel at full throttle. Part throttle blasts around town aren't going to tax all new components, even if they are stock and have 5/16" fuel lines. Your pickup, lines and pump are brand new, not 50 years old. The too-high pressure from the Holley mechanical pumps is pretty well documented and an inexpensive regulator is probably what is needed as the fuel pressure is lifting the inlet needles off the seats due to higher than normal fuel pressure. This will flood and load up your engine at low speeds where your fuel demand is low. On full throttle, engine demand compensates for the overflow.

I could go on and say your carb has a 6.5 power (enrichment) valve and a 5.5 would be more appropriate with your engine vacuum (and would probably clean up your idle and gassy smell), but it's not a big issue and is borderline. It's something you can do if you find no other cause. I run the 5.5 in my 750 with 11" vacuum. With the 6.5 power valve (factory setting), the car started and ran fine but just smelled gassy when driving..

Psssst, your pics show you have an inline fuel pressure gauge. What does it read?

Tend to agree - although one of my better "street" arrangements is a pusher electric pump, robbmc mechanical, cheap regulator and a cliff qjet. I put the pusher in to shorten the qjet evaporation and the long cranking.

I'll say this - I think a holley 4150 carb survives better on the street with a decent mechanical fpump than a qjet with mechanical fuel pump. Qjets starve easy, IMHO.

Its not liquid filled, it flies rapidly between 12 and 5