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Thread: Vacuum Leaks

  1. #1

    Default Vacuum Leaks

    I've been wrestling around with every video, article, forum, and bugging my friends about Holley noggin/lean backfire/hesitation for almost two months. I have had the carburetor off and apart numerous times. In the process I have learned about the subsystems and adjustments. Nothing I did had any positive effect, until I pulled the carb the first time. You can see in this photo that the previous owner, an experienced hot rodder, had put a lot of thought in the detailing as he mounted the carb.

    We could not get the car to run right and the first thing I did, wrongly was yank the distributor and start messing with the timing. Then I began to replace secondary springs and idle cams, ultimately replacing the nozzle. One point there was a lean backfire that was very powerful and which I'm sure killed my power valve. Everyone wanted to start yanking at the jets up front, but I had noticed changing the springs and the idle cams seemed to help. I finally decided to replace the existing nozzle, going up from .31 to .35. When I pulled the carb this is what I found.

    I went out and bought a stud kit for the manifold and bought a set of gaskets and a spacer and retorqued the carb back to the manifold. I now have to go back through the process of selecting the right cam and springs again. I replaced the power valve with a 6.5. The one in the carb did not have a number in it. The car responds well now. Here are a few things that show the difference.

  2. #2


    So, I have now moved to the blue accelerator cam and the white secondary spring. I'm running the blue cam in the two position. Things have improved a lot, but I keep slipping between hesitation in idle transition and hesitation in mid range. I suspect I still have flooding or vacuum leak issue. I reset the fuel levels in the bowls. I have ordered two new needle sets with washers & set screws. Beyond that I'm running out of ideas.

  3. #3


    Keep in mind of the circuits you are dealing with at these areas. Naturally this assumes that there is nothing else going on (i.e. vacuum leak). Double check your timing. Timing is a big factor here. You should be running 15-18 degrees of initial timing and be all in of 34-36 degrees of timing by 2600-2800 RPM.

    I'm attaching a couple of pics that shows accelerator pump cam profiles, and which circuit is being used when driving in normal to progressive modes. Transition fueling really comes from two areas. IDLE CIRCUIT and TRANSITION CIRCUIT. Not a lot can be done to the transition circuit without major changes to the carb. (Altering the trans slot or drilling and tapping the main body to allow more/less fuel. You should not have to make these kind of changes.) So, you need to make sure your idle mixture screw are setup properly. Get a vacuum gauge and adjust your idle mixture screws for maximum vacuum. Then back off 1/8th of turn just so you know your a tad rich. Also, make sure your primary throttle plates are adjust properly over the transition slot in the primary bores. If too much slot is exposed, you will get hesitation for lack of transition fuel. If after you adjust your primary throttle plates to the correct amount of transition slot exposed, you find that your idle is too high or too low, try adjusting your secondaries first to get closer to your target curb idle. That way you only have to adjust your primary idle adjustment in very small amounts. Also, if your idle mixture screws are to lean you will get a bad hesitation regardless of accelerator pump cam selected.

    You can see in the pump cam picture that you are pretty much maxed out with the blue cam in the 2 hole position. So essentially you have been putting a band-aid over the real problem. Like I said before, I try to advise people to separate the carb into 2 sections. Primary & Secondary. TIE OFF THE secondary, so it can't open and then get the vehicle to run good in idle, cruise & slight acceleration. Once you get this done, then you can bring in the secondaries. Keep in mind your main jets won't start flowing until the 2000-2400 RPM area. The boosters that feed the throttle bore work from a vacuum effect. Until a good amount air gets moving over those boosters will they start flowing fuel. Now with that being said, your idle well and transition slot are fed by the MAIN WELL. The main well is fed directly by fuel bowl which is metered by the main jet size. See pic attached.
    Click image for larger version. 

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    Last edited by 69cam; 01-04-2017 at 10:19 AM.

  4. #4


    Quote Originally Posted by 69cam View Post
    Keep in mind of the circuits you are dealing with at these areas. Naturally this assumes that there is nothing else going on (i.e. vacuum leak). Double check your timing. Timing is a big factor here. You should be running 15-18 degrees of initial timing and be all in of 34-36 degrees of timing by 2600-2800 RPM.
    So timing is 13° at base and advances to 34° at 2500 RPM. Mechanical advance using two blue springs on the MSD. I'm changing the needles & o-rings today, to see if I have a deterioration problem due to the carburetor sitting. I have used the vacuum gauge to check the pressure, I'm getting around 15 psi max. I have noticed the needles do directly influence the AFR significantly. What components constitute the transition circuit? Car idles great.

  5. #5


    Need to set your idle mixture screws for highest vacuum. Not just hook up the vacuum gauge to "CHECK" for vacuum. YouTube it. there some good video explaining exactly how to do it.

    Needle and Seat have NO direct affect on AFR. Needle and seat is just there to stop fuel from entering bowl at preset float height. If the needle and/or seat/gasket are leaking, it will allow the fuel bowl/s to fill up with fuel and it will start to drip out of the boosters on the carb. If that is happening, it will idle or die within 5 min after starting the car. Initially it will be fine and depending on how much is leaking in and if the car is consuming fuel faster then the leak.

    Naturally your accelerator pump, transfer slot, and if you hit accelerator fast enough to cause a low enough vacuum drop the power valve. The idle mixture screw can have an affect on it as well. If you look at the last picture I posted, you will noticed the the transfer slot gets its fuel from the IDLE WELL. How fast the idle well replenishes with fuel is determined by the "IDLE MIXTURE SCREW". If your to lean on the idle mixture screw it will drain the idle well and not replenish fast enough and cause a bog or lean pop.
    Last edited by 69cam; 01-06-2017 at 10:23 AM.

  6. #6


    So, I replaced the float needles with 6-504s new o-rings and readjusted the fuel levels in the sight window. Took her out & wow, made a ton of difference. After about an hour of driving though it seems the hesitation/miss cam back. Question, could the carburetor be heating up affecting the fuel in the fuel bowls or floats? What other heat problems might I try to check for. I did cover the from air horn with the engine idling, and it killed the engine so I'm reasonably sure I do not have a vacuum leak. I did notice that the fuel is about halfway in the sight windows. Should I lower it further?

  7. #7


    I wouldn't lower it any further. If anything, I might raise it to be about 3/4 of the way up the glass window. Always better to be a little rich, then a little lean when troubleshooting fuel issues.

    Depending on how hot it's getting under the hood, you could be vapor locking. Vapor lock happens when the fuel somewhere in the line is getting hot enough to turn into vapor causing an air pocket in the line.
    It would have to be getting pretty dam hot though and vapor lock usually stalls the engine.

    The floats only determine how much fuel is in the bowl. Nothing more nothing less.

    I would go drive it again until the problem comes back. As soon as it come back pull over quickly, turn the car off quickly. Look at the sight glass. Is the fuel level at the same level you adjusted it to?

    If so, and there is still a slight hesitation adjust the idle mixture screws. ALL 4 OF THEM. 1/4 turn counter clockwise. Start it up & see what happens.
    Last edited by 69cam; 01-11-2017 at 09:27 AM.

  8. #8


    Thanks. Car is an open bay roadster. Question, could I have the advance curve mismatched with the fuel settings? I.E. I'm using MSD blue springs. Baseline is 13° coming to 34° at 2500 RPM. I'm using the blue cam position #2, and the white (lightest) secondary spring. Car has a 3.73:1 gear and weighs 2100 lbs. So I went out and did what you said. Turned the screws 1/4 CCW drove it a while then looked at the indicator window. Fuel level was right where I had it set. Noticed at idle though the fuel pressure at the regulator was a little over 7 psi, backed that down to 5.5 psi and the car seem to respond well now. I can cruise at 40 in 5th and accelerate without a hesitation.
    Last edited by wrp; 01-11-2017 at 02:06 PM.

  9. #9


    At 2100 lbs, that's super light. You want that timing all in by 2500 RPM like you have it. Especially for a light car. It can handle it. Also, I think your fine with the lightest secondary spring as well. Normally when a secondary opens to soon it will cause a slight hesitation or bog for heavier cars. So if your happy with what you have, then drive it till the wheels fall off. If you want to take it a little further, it's time to get an AFR gauge so you can fine tune your fuel curve to get great economy while cruising, and max performance while WOT. Yes, you can achieve both with carb. Especially with your car being so light. Sean
    Last edited by 69cam; 01-12-2017 at 02:06 PM.

  10. #10


    Actually I do have an AFR gauge, as well as a vacuum Gauge. I'm studying how to tune the AFR. I have adjusted the needles with the vacuum gauge. There's an interesting article in Hot Rod this month talking about how valve springs & heads can cause some of the same problems I was wrestling with. Here's the AFR on my best pull to date. I'd like to raise it one point, but I've been unable to do so.

    Attachment 2000

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