Potential Carb Bog Quesion
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Thread: Potential Carb Bog Quesion

  1. #1

    Potential Carb Bog Quesion

    Good morning. I promise I am not trying to be lazy - I've tried reading the numerous posts regarding different carburetors and am really stumped. First I'll admit I'm kind of a novice and have zero skills when it comes to tuning carbs. I think I could learn but it would be much easier if I had someone in person showing me as opposed to watching Holley youtube videos. I have a 73 XR-7 convertible that I love and have running 95% the way I would like. The engine is a 351C that I think is fair to describe as mildly modified. It was rebuilt in 1998 and the person who rebuilt it at that time said he brought it to more muscle car specs which it did not have coming out of the factory in 72/73. I know the cam is a 278 (pretty sure Crane Cams 278). When it was rebuilt in 1998 the builder put a Holley single plane intake with an Edelbrock carb. A couple of years ago I had another shop switch it to an Edelbrock performer intake with a Holley 83670 carb. Everything runs great EXCEPT from takeoff in gear. In drive at a stop the idle is about 650RPM which I think is appropriate (any suggestions though I welcome). When you try to gun it to take off the car hesitates dramatically, sometimes accompanied by a small backfire. I've tried living with it but enough is enough. I just had the C6 transmission rebuilt, a couple of years ago had the whole rear end done with 3.5 gears, the poor thing is ready to run and this is driving me crazy. Recently I've been reading about this possibly being an accelerator pump issue and that these nozzles can be changed out (watched a Holley video discussing it). My questions for the group are:

    1) Should I stick with this carb? It runs great other than the acceleration from a stop issue.

    2) If I should switch carbs what to go to? Sounds like Quickfuel is a fan favorite, but I also have questions about whether or not the 670CFM carb is the right size for a mildly tuned 351C? Some have told me that 670CFM might be too big but from reading other posts I find this odd.

    Thanks a million to anyone who can share their thoughts / experiences.

  2. #2

    Re: Potential Carb Bog Quesion

    I think your carb choice is fine, sizewise, for a mild 351C. 650 RPM sounds low to me for idle in gear on an auto. A stumble on transition from the idle circuit could also be incorrectly positioned primary butterflies. Since you have to yank the carb to do any of this, I would chase that first. Read up on butterfly positioning in the idle circuit slots. It is possible to be completely out of the slots and 'fix' it so it still idles fine with bandaids in idle mixture screw adjustment, but then it falls flat on its face transitioning off idle.
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  3. #3
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    Re: Potential Carb Bog Quesion

    I would definitely stick with your Holley. It has a lot of ways to fine tune the accelerator pump circuit so you can dial it in perfectly. The first two things I'd check are your float levels and accelerator pump linkage slack. Warm the car up to temperature and make sure the choke has completely disengaged (flap fully open).

    1) With the car on a level surface, check your float bowl sight windows to make sure the fuel levels in each bowl is right in the middle of the sight glass windows. If you do need to adjust the float level, never do it with the engine running. Fuel under pressure will spray all over the place. Place a box end wrench around the nut to hold it and unlock the locking screw with a large screwdriver. Once loose, rotate the nut 1/8" turn, tighten the locking screw, start the car and re-check the level in the sight glass. Counterclockwise will raise the fuel level and clockwise will lower it. http://www.quickfueltechnology.com.a...-fuel-pressure

    2) With the engine off, look down into the primaries and tap the throttle linkage. Is there a delay in the pump shot coming out of the shooters? If so, adjust the length of the accelerator pump linkage bolt/spring combo to take out any slack. Loosening up the nut will lengthen the bolt so the slack is eliminated. The reason you check/adjust this when the engine is warm (choke fully disengaged), is that you want the choke to be off the high idle detent. If you adjust the linkage with the throttle plates cracked open on the high idle detent, you'll end up with slack in the linkage once the choke disengages. You can also rotate the pump cam to a different position with the small slotted screw to change when and how much pump shot you get when your open the throttle. Each time you adjust or replace the pump cam, you must re-adjust the bolt/spring combo to take out the slack.


    If this doesn't solve the bog, you can adjust the pump shot further with this Holley Accelerator Pump Cam Kit:https://www.amazon.com/Holley-20-12-...s=20-12+holley


    or https://www.summitracing.com/parts/hly-20-12

    Here's tons more info, charts, etc. I've had good luck with the green and orange pump cams to solve my bog issues.
    https://www.google.com/search?q=holl...ih=692#imgrc=_

    If this doesn't fix the problem (which it should), you can play around with different diameter pump shooters.

    They also make a larger accelerator pump kit (the diaphragm and pump arm plate held on with 4 screws at the bottom of the primary bowl), but this is overkill for street use.
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  5. #4
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    Re: Potential Carb Bog Quesion

    Usually "carburetor problems" are electrical.

    The backfire when it hesitates tells me your timing is too retarded. This is common, as many mechanics have no idea how to use a timing light or how to perform a tune up.

    Try loosening the distributor clamp. Rotate it - the distributor - a few degrees clockwise. Tighten up the distributor clamp. Go drive the car. If it fixes the bog without pinging, you are done. If not, experiment with the timing until you are happy. If the car pings or becomes hard to start when warm you have too much advance.

    When you advance the timing it will increase the idle speed. Expect to adjust idle speed every time you change the timing. 650 RPM with the headlights on in drive is about right.
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