Carb Help

Discussion in 'Fox Engine Swaparoo' started by samk, Jun 11, 2012.

  1. i have a 90 lx with a 306 302 .30 over comp cam with .613 lift, forged pistons and rods, windsor sr heads milled .30s, msd box and distibutor when i got the car it was fuel injected missing alot, went carb, heres my question i decided with the weiand excelerator intake, holley 750, it wont idle had it to the shop it will sit and idle but when u drive it wont idle at a stop sign carb has stock jets, but u smell alot fuel, if your not over 2500 rpm it bogs out, any suggestions (smaller carb maybe)
  2. Is it a 750 vacuum or mechanical?
    The carb is not too big, just out of tune.
    The jets don't matter at idle.
    You need to get the idle screws (in the side of the primary block) set first.
    It's a bit of a balancing act, because those idle screws will also move your idle rpm, so it takes a while to get the screws tuned properly along with the idle speed screw tuned properly. If you adjust one, and it throws the other out of tune, you will have mixed idle stability like you describe. I like to think I am good with carbs, and this is one area of tuning that gives me fits.

    Your "over 2500 rpm bog" issue could be jets, but I suspect you have a vacuum secondary and the secondaries are opening too soon.
    If you have mechanical secondaries, then your bog is an issue with secondary pump circuit tuning.
    You pump circuit tuning is where you get to take advantage of a carb's awesome throttle response.
    The Holley accel pump tuning involves pump cam, lever arm adjustment, and shooter size.
    If you have these out of whack on primary or secondary pump circuit, you'll have tip in issues (primary) or mid-rpm range bog (secondary).

    Give us a little more info on what carb you have, and we can narrow it down.
  3. Hey, that cam sounds pretty radical.
    What is the duration at .050"?
    Does it lope hard?
    What kind of vacuum does it have at idle.

    I just realized that your idle might not clean up if you have a radical cam and don't address the power valve first.
    If the cam is really low vacuum, you need to sort out your power valve before worrying about anything else.
    New Holleys come stock with what is called a '65' power valve.
    What that means is the power valve opens to allow more fuel into the engine after the vacuum drops below 16.5".
    If you have a lumpy cam with low vacuum, you can actually have the power valve opening at times during idle.
    This will cause the surge and fuel smell.
    So will a damaged power valve, which happens if you get a 'spit' back, or backfire through the carb.
    I don't know how old your carb is, Holley started putting power valve protection on the carbs a few years back.
    Carbs before that mod will blow a power valve in a NY minute. There are PV protection mods you can do to an old model.

    You might need to go to a 45 PV to compensate for low vacuum caused by a lumpy cam.

    Make sure you have the PV sorted out, then tune the idle screws and idle rpm, and then finally tune the accel pump circuit(s) and/or vacuum secondary. After that, you worry about jets.
  4. I misread your 2500 rpm issue.
    I first read it as a bog at 2500.
    Now I read it and it seems to be you are saying it is doggy below 2500.
    Do I have it right now?

    The same thing goes... PV first, idle mix and rpm second, accel pump/vac secondary third, then jets.
    The only comment that changes is the idea that your secondary tuning is WAY off.
  5. Now, I don't want to be guy to take away ones opportunity to get some tech help, but this thread is in the wrong place I think...probably should be under 5.0 tuning...

    edit: that being said, I found something in one of your replies (power valve) that may help me out with some fine tuning on the carb for my '72 Plymouth :D...runs a little fatter than I care for.
  6. Yeah, this really isn't focused on "Engine Swaps".
    You would get more responses if this were in the "Tuning" forum.
  7. Sorry about the wrong spot im new here i know nothing about carbs reallyim gonna guess its an older carb with vacum secondairies ive drove it a little bit when i hit the brakes it wants to die the idle is not steady turn the screw for secondarys it helped some