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Fuel Starvation

G Guest
A few months ago I replaced my autolite carb with a brand new Holley 670 Avenger. After installation, I noticed that during hard runs the engine would stumble and stutter for about 30 sec then return to normal once the fuel quantity (pressure) would return to normal as the fuel pump caught up with the increased demand. I attributed this to a bad (electric) fuel pump which is fairly old.

Yesterday I replaced the fuel pump with one of Holley's high flow units (160gpm) and the problem seems to have gotten worse. It didn't call for a pressure regulator so I didn't install one.

I'm pretty sure the carb is still starving for fuel during these spirited runs and it's not an ignition problem...therefore what else am I missing? I'm not carb savvy at all..but now I think the problem is with the carb. Do I need to adjust something in the carb?

Thanks for the help!
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bosswrenchbosswrenchMember
I've had very bad luck with glass/chrome fuel filters. They have small holes for fuel flow, and over half the attaching thread is cut away for more flow, making them weak. The center rod is made of brass so is none too strong to begin with. Mine snapped in half while idling in the driveway, saving me from another engine fire! I suggest pitching the filter & running direct to see if it makes any difference. If yes, get a large sized filter from Holley or other vendor. Stock 4bbl carbs nearly always have a sintered brass or paper fuel filter built into the inlets (2 on most carbs) that will work fine for a while, then plug up and again restrict flow.
marlinjackmarlinjackThe Machinist #5723 AMARGOSA MACHINE 'Running on Empty- Middle of Nowhere'
ABSOLUTELY!!!!GET RID OF THAT GLASS FILTER RIGHT NOW AND KNOW THAT YOU SAVED YOURSELF FROM A GASOLINE FIRE! AS WITH JACK I TOO HAD THE THREADS STRIP (MINE WAS A KIND OF POT METAL) I MAY HAVE ONLY HAND TIGHTENED THE D**N THING A TEENSY BIT TOO FAR, FOUND IT OPEN AND POURING FUEL OUT JUST A MOMENT BEFORE I CRANKED IT UP. "BELIEVE US NOW OR BELIEVE US LATER". MARLIN.
TB Tom B. (Guest)
Kevin,

I also own the 670, and have exactly the same problem! Thought I was alone. My problem occurs at full throttle, starts around 5000 RPM, bogs for a moment,
then continues accelerating. I have written Holley...no reply. I have checked my float level on the secondaries, and all seems normal. Next, I will try a different vacuum spring for the secondaries. There is an extensive trouble shooting guide on the holley website. They suggest the secondary springs. Let me know if you fix it

Thanks, Tom
marlinjackmarlinjackThe Machinist #5723 AMARGOSA MACHINE 'Running on Empty- Middle of Nowhere'
IF YOU INSTALLED A HIGH FLOW (?HIGH PRESSURE?) PUMP BUT NO REGULATER, MAYBE YOUR GETTING TO MUCH FUEL? I KNOW FOR A FACT THAT HOLLEY NEEDLE/SEATS WILL ONLY HOLD BACK 7 PSI AT THE VERY MOST. ABOUT THE SECONDARY SRINGS: YES IT IS POSSIBLE THAT TOO LIGHT A SRING WOULD ALLOW YOUR SECONDARIES TO OPEN WAY TO SOON, STUMBLING YOUR ENGINE (UNTIL, AS YOU SAID, IT CAUGHT UP).TOO HEAVY OF A SPRING WOULD JUST LEAVE YOU ON 2 BARRELS LONGER AND FEEL LIKE, WELL, 2 BARRELS, BUT PROBABLY WOULD NOT HAVE A STUMBLING EFFECT. JUST MY 2 CENTS! GOOD-LUCK, MARLIN.
PR paul ritzen (Guest)
quote:
Originally posted by Kevin Perry:
Forgot to add that the glass fuel filter is brand new as are the fuel lines. I haven't checked the small mesh filter inside the carb fuel inlet but the carb's been doing this since almost new so that shouldn't be the problem. Thanks again.

I've noticed that you changed your fuel lines.
I may be wrong, but isen't it nessecary to
put a restrictor in your return fuel line?
Otherwise the fuel wont't hold enough pressure at high fuel demands.

Greetings Paul.
J Jim (Guest)
Return line?? With a carbureter?? You must be thinking of an injection system.
Electric pumps usually run higher than 7 PSI. You will probably need a regulator.
Have you checked float levels?
I had a problem with weak spark, but that resulted in backfiring when the back two barrells opened.
TB Tom B. (Guest)
Kevin,

Setting the floats is easy on the 670. You should have clear sight plugs that came with the Avenger. There is one on both the primary and secondary fuel bowl. The fuel level at idle should come to the very bottom of clear sight plug. You should be able to see the proper fuel level through the plug. (use a flashlight) I could not see the fuel level because the clear "see through" plug was not so clear. So, simply unscrew the plug, while the car is idling. Fuel should ever so gently trickle from the bottom of the hole indicating the float level is OK. If it does not, adjust the level by first loosening the large float level set screw on top the carb, then turn the adjusting nut counter clockwise until the fuel begins
to seep out the sight hole. Lock down the set screw and you are set. Do the same on the secondaries.

Tom
PR P.H.C. Ritzen (Guest)
quote:
Originally posted by Jim:
Return line?? With a carbureter?? You must be thinking of an injection system.
Electric pumps usually run higher than 7 PSI. You will probably need a regulator.
Have you checked float levels?
I had a problem with weak spark, but that resulted in backfiring when the back two barrells opened.

Yes, A return line with a carburator!
Otherwise you get a maximum pump fuel pessure
with low fuel demands.
And if there is no restriction in the return line, there is less resistance in the return fuel line than in the carb. fuel input line, so most of the fuel goes back to the tank, and there may not be enough left for the engine.
Another problem that can occur when there is no return fuel line, is that at very low speeds on a hot summer day, when you are cruising in the city, the fuel consumption is so low that the fuel heats up too much, because it takes up much heat in the long period that it travels too your carb, you could get your fuel boiling in your float chamber.
With a return line just before your carb. there always is enough fresh cold fuel with the right pressure for your carb. fuel intake.

Greetings Paul.
TB Tom B. (Guest)
The vacuum secondaries are controlled by a diaphram and a color coded spring. Holley makes a number of different springs with different tension on the springs. You can change the springs and change the opening moment. The color coded springs run from light tension to heavy:

White - Lightest
Yellow (Short Spring)
Yellow
Purple
Plain (Steel grey)
Brown
Black - Heaviest
If there is a bog or hesitation when the secondaries open, the spring tension is too light, go to the next heavier spring. Holley offers a kit (PN-20-13) which contains one each of the above color springs.

Tom
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