Do I Need to Rebuild my Carburetor? Holley 94 on a Ford Flathead V8

[Angelfish]

It's a 1949 8BA Flathead in a truck.  It currently has a Pertronix ignition in the original distributor and an electric fuel pump (I know, it was a long time ago and I've never switched it back).

 

Truck has been running rich with poor fuel economy.  Spark plugs were nice and black. 

 

I reset the timing with the vacuum advance disconnected, just this and the truck did run smoother.

 

Fuel pressure was 3.5 psi so I lowered it to 2.5.

 

Turns out my vacuum gauge is stuck so I was unable to do a vacuum test.

 

But it runs best with the mixture screws all the way in and bottomed out, if I turn them out even slightly it will gets a little bit of a stumble.  Turning them out further seems to make no difference, just the same slight stumble.  Turn them all the way back in to bottomed out, and it smooths out.  

 

It idles pretty well and sounds good when it's run up, although there's always been a lag before the engine will accelerate. 

Edited September 7, 2023 by Angelfish (see edit history)

[carbking]

MANY possibilities.

 

(1) Low compression

(2) Too much fuel pressure (are your 3.5 and 2.5 ACTUAL numbers from a pressure gauge, or numbers on a dial-type regulator?).

(3) A pertronix without upgrading to 12 volts AND an alternator.

(4) Other ignition issues.

(5) Bad, or misadjusted, carburetor float

(6) Other

 

Even though I would bet (if I were a betting person) on ignition, I would start with a compression test, followed with a fuel pressure test, by inserting a fuel pressure gauge (if you haven't already) right at the carburetor. If the carburetor has the correct orifice fuel valve, it is rated at 4 1/2 psi. If compression is good and measured fuel pressure is below this figure, next would be a complete ignition test (a good starting point would be firing voltages at the plugs).

 

If the results of the compression test, fuel pressure test, and the ignition test are all satisfactory, then time to open up the carburetor.

 

Jon

 

 

[Angelfish]

The truck is 12 volt with alternator, I was a little scared to admit that on this forum, and I did use a fuel pressure gauge at the carburetor.  Regulator is a Holley low pressure with screw adjust. 

 

The compression test was on a cold engine with throttle closed, no oil added.  95-100 with one at 103.   I found my paperwork from 20+ years ago and it indicated 65-80, so things have improved. 

 

Ignition voltage test.  I was not able to find much information on how to do this so any information would be thankfully accepted.   Spark plug gap was good but some of them did seem a little loose.

 

On 9/6/2023 at 6:32 AM, carbking said:

...If the results of the compression test, fuel pressure test, and the ignition test are all satisfactory, then time to open up the carburetor.

 

Jon

 

 

 

[carbking]

Looks like you have already done most of the testing.

 

Firing voltages may be tested by the use of an oscilloscope (which most hobbyists do not own, but it is a terrific tool!);

 

OR

 

By the use of a clamp-on voltmeter. The probe of the voltmeter clamps over a spark plug wire.

 

It may be carburetor time.

 

But first, maybe someone more schooled in Ford ignition systems than I could comment on if that particular distributor requires a specific carburetor with a spark valve; and if so, does that effect the action of the pertronix?

 

And you did not mention, but what carburetor are you using?

 

Jon

[Angelfish]

3 hours ago, carbking said:

And you did not mention, but what carburetor are you using?

 

Jon

It's the original Holley 94 (named in the thread title but not in the text).

 

Thank you for your help. 

Edited September 7, 2023 by Angelfish (see edit history)

[Bloo]

On anything with a 94 running too rich, the power valve is suspect.

[Pfeil]

On 9/6/2023 at 5:32 AM, carbking said:

MANY possibilities.

 

(1) Low compression

(2) Too much fuel pressure (are your 3.5 and 2.5 ACTUAL numbers from a pressure gauge, or numbers on a dial-type regulator?).

(3) A pertronix without upgrading to 12 volts AND an alternator.

(4) Other ignition issues.

(5) Bad, or misadjusted, carburetor float

(6) Other

 

Even though I would bet (if I were a betting person) on ignition, I would start with a compression test, followed with a fuel pressure test, by inserting a fuel pressure gauge (if you haven't already) right at the carburetor. If the carburetor has the correct orifice fuel valve, it is rated at 4 1/2 psi. If compression is good and measured fuel pressure is below this figure, next would be a complete ignition test (a good starting point would be firing voltages at the plugs).

 

If the results of the compression test, fuel pressure test, and the ignition test are all satisfactory, then time to open up the carburetor.

 

Jon

 

 

Jon, last summer a friend with a 65 GTO Tri-Power come up and ask if I knew what the problem was with his GTO idle. Well, it was lack of idle or to drive or if you would call it that, he had to keep the revs way up to keep it going and blowing smoke rings all along. I had never seen such black smoke except from an old diesel. Taking the center carb off and apart and looking down into the float chamber were the two main jets rolling around at the bottom. All the years playing and working on the 2GC I had never seen that happen. To be honest I don't know how it ran at all. 

[Angelfish]

What do you think of these mixture screws?

 

[Angelfish]

I received a kit from Jon - CarbKing, and rebuilt the carburetor.  The kit included mixture screws and power valve.  I set the float to about 1 11/32 right side up.   Reinstalled and the truck started right up.  I adjusted the mixture to about 17 on the vacuum gauge, that's has high as it would go and had a slight amount of flutter in the gauge needle and a warm engine.   The truck starts, idles and runs well.

 

It's too early to tell if it's still rich, but it does seem better.  It still has the lag when I stomp on the gas.  The lag is minimal if the choke is out a little bit.  This has existed on this truck for 30 years and I've never solved it, but I haven't spent much time trying.   A couple weeks ago I set the timing to Zero with the vacuum advance disconnected.  The vacuum advance mechanism on the distributor has never been rebuilt since new, so that seems like a good next thing to check.