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Hot Rods Carb question

Discussion in 'The Hokey Ass Message Board' started by bob8823, Feb 7, 2015.

  1. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    I have a holley 600 cfm carb on a sbc 283, with a single four barrel tunnel ram and a 2" spacer. Seems to run ok off idle, but as soon as I let it idle it starts to fluctuate. Any idea what causes that to happen. Appreciate any help. Thanks.
     
  2. 325w
    Joined: Feb 18, 2008
    Posts: 5,509

    325w
    ALLIANCE MEMBER

    Have any idea of the fuel pressure
     
  3. saltflats
    Joined: Aug 14, 2007
    Posts: 10,623

    saltflats
    Member
    from Missouri

    Have you checked all your tune up stuff like plugs , points , timing ,
     
  4. Need more info! Fuel psi, timing, etc. Check for vac. leaks.
     

  5. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    Fuel pressure is around 8-9 psi. 194 heads with all new valves, springs, guides. New plugs and Mallory
    Unilite distributor, timing is at 8 btdc
     
  6. saltflats
    Joined: Aug 14, 2007
    Posts: 10,623

    saltflats
    Member
    from Missouri

    What does it have for a cam
     
  7. caton462
    Joined: Jul 17, 2013
    Posts: 176

    caton462
    Member

    Spray flammable brake clean around the gaskets to check a vacuum leak. A small leak will affect idle and not seem to affect faster running.
     
  8. DrJ
    Joined: Mar 3, 2001
    Posts: 9,418

    DrJ
    Member

    If turning the idle mixture screws in or out has little or no effect you may have the idle speed screw screwed in way too high and it's trying to run off the off-idle middle circuits. If so, set your mixture screws where they should be for a starting point for that carb ( 2-3 turns?) and lower the idle speed screw until the idle speed is closer to what it should be, (650 ± 50 RPM) then fine tune the mixture screws for best idle. Turning them now should make a difference!
     
  9. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    Saltflats - Stock cam.

    caton 462 - I need to check for vacuum leaks, haven't done that yet.

    DrJ - Idle speed screw is barely touching the linkage, even at that the idle is fairly high. Does that mean I
    have to fine tune the idle mixtures screws.

    Thanks for all the help guys.

    Bob
     
  10. Budget36
    Joined: Nov 29, 2014
    Posts: 5,857

    Budget36
    Member

    Hows the intake vacuum at idle?
     
  11. Tony
    Joined: Dec 3, 2002
    Posts: 7,350

    Tony
    Member

    The fact that it's idling high with the screw barely touching tells me vacuum leak..because the throttle blades are just about closed yet it's still getting enough air to pull in fuel keeping the idle high...
    So definitely check for vacuum leaks like you said you're going to do.
    With a stock cam (and assuming not too many heavy mods to the motor) that tunnel ram isnt doing you any favors for low rpm drivability...
    Yes, they can and have been run for years..not knocking it, just stating it isnt the best set up unless you have the right stuff inside to use the long runners of a tunnel ram.

    Tony
     
  12. DRJ
    2-3 turns is the correct starting place.

    Here is a question is it an eye burner? The 600 holley has a little screw that you pretty much need to remove the carb to get at, it is an adjustment for a starting place on the secondaries. Some times they are open too far causing an overly rich situation at idle, for most street applications they want to be just barley cracked.

    If I was playing with the carb tune anyway I would also make sure that I was using the proper power valve. Chek your vacuum at idle, then divide that by 2. That will be the value or at least the best starting place for your power valve. I.E. If your idle vacuum is 14 then your power valve will be a 7.0 (actually in a nearly stock 283 a 6.5 with 14 inches of vacuum is a better match).
     
  13. jimpopper
    Joined: Feb 3, 2013
    Posts: 115

    jimpopper
    Member

    Spacer or tunnel ram on a smaller displacement V8 with 600cfm creates a weak or diluted signal to the power valve. Having Both is definitely a problem. You can compensate by installing a power valve rated at 1 to 1.5 Inches of vacuum less than actual idle vacuum. If the power valve is sized for more vacuum than is present, the enrichment circuits dribble and load up the idle. Ex. #65 power valve (6.5" vacuum opening) on a engine that gauges at 8" vacuum at idle would be ideal. If the engine pulls only 6" of vacuum the enrichment circuit is engaged by the power valve and she runs rich. Source Disclosure: Haynes Holley Carb Manual chapter 8.
     
  14. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    Had no time today, so tomorrow need to check the idle vacuum and also verify if there are any leaks.

    Tony - The tunnel ram is really for the looks, I know its not the ideal setup but hoping to make it work.

    Porknbeaner, jimpopper - Next I'll verify if there are any leaks and get the carb tuned in and if that
    doesn't help then I'll go to checking the power valve.

    Again, thanks to all for the input.

    Bob
     
  15. Bruskie
    Joined: Apr 9, 2014
    Posts: 50

    Bruskie
    Member

    make sure your secondary butterflies are closed as this will bring your idle up and not allow idle circuit to operate
     
  16. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    Did a vacuum test today and it wasn't too bad, although the idle is still fairly high, right
    around 1000 rpm. Tested two ports off the carb, only options I have. One read 11 and the
    other 16-17. Checked for leaks and sure enough where the two sections of the manifold
    bolt together, I got a mild increase in rpm when I sprayed there. So I took the two sections
    apart and found that the gasket on the front part was soaked with gas, and the rear gasket
    had some also, just not as much. There were no visible gas leaks. Is there anything special
    I should do or just replace the gaskets? Appreciate any comments. Thanks.

    Bob
     
  17. The 11 inches was ported vacuum the bigger number is what you are looking for. 16-17 inches of vacuum should out you in the number 8 power valve range to 8.5. Seems odd that you are pulling that much vacuum with a tunnel on a small inch motor. Could be that you are not idling yet at 1000 RPM. ;)

    Anyway most holleys come from the factory with a 6.5, you should work on getting your idle down to the 600-700 RPM range if you can then trim your carb, use your vac gauge to check while you are trimming, work back and forth and play until you get the largest vacuum at idle speed.
     
  18. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    I really don't think it was idling, but that's as low as I could get it and still have it run on its own. If I turn in the
    idle mixture screws any more or back off the idle speed adjustment it just stalls out. Have to manually blip the
    throttle to keep it running. How about the gaskets being wet, should I just replace them with new ones or is
    that a sign of another problem. Thanks for the help.
     
  19. dave lewis
    Joined: Dec 12, 2006
    Posts: 1,364

    dave lewis
    Member

    Nobody has commented on your fuel pressure..
    Holley says 5 to 7 psi...
    In 45 years of messing with holley carbs.....
    I have found they really run better at 5 psi for idle..and low end..
    At 3500 plus 7 is fine. .
    Most holley needle and seats will not seal at 9 psi..dribble..and spill over = crappy idle. .
     
  20. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    Ok, update finally. Replaced the gaskets in the tunnel ram, ditched the spacer and put a new gasket under the carb.
    Still can't get the idle down. It actually runs better when I unplug all the vacuum ports. I don't remember before changing gaskets, but now I can hear it constantly sucking air. Is that normal. I'm no carb expert but I've never had
    this much trouble before. This is a rebuilt carb I bought, so possibly its no good. Thoughts??
     
  21. If your engine takes a deep enough breath you can hear the air passing through the air horn. I never asked you haven't milled the air horn off correct?

    if it runs better with the vac ports open then it is running too fat, you should take it off and check the secondary butterflies, if they are open too far it will run fat, there is a little screw on the bottom of the car that allows you to adjust how far open the secondary blades are open, you want them to be just barley cracked.

    next make sure that you don't have crap in the needle valve and seat area, they unscrew and you can blow them out, then replace them and make sure that your floats are adjusted properly, you should be able to pull the site plug and just have fuel spritzing out at idle.

    Someone mentioned fuel pressure, make sure that your fuel pressure is within the 5-7 psi range, maybe until you get it sorted out keep your fuel pressure closer to 5 psi.

    Once you have gone through all that try closing your throttle blades and fiddling with the idle trim screws, what you want is the idle around 700 on a nearly stock motor and making as much vacuum as possible.

    Don't give up you'll get it sorted eventually.
    Luck
    Benno
     
    mrconcdid likes this.
  22. davy g.
    Joined: Feb 16, 2015
    Posts: 4

    davy g.

    I have a 600 Holley 4150 carb that needed a 1/8" pipe plug on the right side of the primary metering block to plug up a vacuum leak. Perhaps your carb needs it too?
     
  23. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    Ok, back again, thought I had the problem solved, but.... Been a while since you guys helped me get this
    thing running, been working on the interior so haven't been able to drive it. So a couple days ago I took it
    out for a ride and it ran like crap. Was ok driving along, but would not idle. Had to keep giving it gas to keep
    it running.

    Today I started trying to find the problem, so first I checked the fuel levels in the bowls and the bowl for the secondary's was low, but I am unable to get it raised. When I tried to adjust it, fuel came out around the
    adjustment screw but fuel level in the bowl never changed.

    When I run the motor it runs fine at idle until I put it in gear and then you have to keep giving it gas to keep
    it running. Does the fact that there is no fuel coming from the secondary's have any effect on the idle, or no.

    When the motor cools down I'm going to remove the carb and take that back bowl off and see if the float is
    stuck or what. Also, at idle the fuel pressure is just a little over 4 psi.

    Any help would be appreciated. Thanks.
     
  24. You don't have to remove the carb to remove the bowl, you can and its no big deal but you don't have to. Unless you have opened the secondaries the fuel level in the rear bowl won't change much and you may just have something blocking the needle or inlet it happens.

    Rough idle is usually one of two things idling too slow ( the idle drops when you stuff it in gear with an auto) or vacuum leak. It sounds like a vacuum leak to me and believe it or not there are lots of places for a simple Holly to leak. I had one leak one time @ the choke/high speed idle there was a hole that that has a Farrell fitting and a little screen on some of them and I had to block it off. Only had that problem on one carb so I don't think it is a common problem. First fix your problem with the rear bowl then start looking for a vacuum leak. that is what I would do.

    While you got the carb off make sure that your secondaries are closing off. There is a little screw that you can find when you turn the carb over on the pass side of the carb that adjusts the secondaries. You should just be able to see light through them when they are closed looking from the bottom of the carb. Look at it really good and you'll find what I am talking about.

    4 psi isn't a lot for a holley it should handle 6 pounds easy and I have one hear that will handle 7. That said it is not likely that you are starving at idle.

    More then you need for the moment try that and then let us know what you have figured out.

    .
     
  25. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    I knew I didn't have to remove the carb, but didn't want to make a big mess if it had fuel in it. Anyway, I just took
    off the bowl and it was bone dry. So, I cleaned the needle and seat and readjusted the float where I thought it
    should be. Ran the pump and saw the float still needed to be lowered, I now have it right but that needle and seat assembly is in as far at it can go. If I needed to lower it any further, I wouldn't be able to, at least not with that
    adjustment screw.

    Anyway, I ran it again and tested for vacuum leaks and found a small leak at the base of the carb. The gaskets
    are all brand new so I'm guessing that maybe that old tunnel ram is the problem. I think at this point I am
    going to take it off and replace it with a regular 4 barrel intake and see if that doesn't cure my problem.

    Thoughts??

    Thanks for any input.

    Bob
     
  26. saltflats
    Joined: Aug 14, 2007
    Posts: 10,623

    saltflats
    Member
    from Missouri

    How level is your carb
     
  27. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    saltflats. The carb is perfectly level.
     
  28. saltflats
    Joined: Aug 14, 2007
    Posts: 10,623

    saltflats
    Member
    from Missouri

    If you are spraying carb cleaner around the base to check for leaks make sure you are not picking up a leak around the throttle shafts it could be coming from there and not the base gasket.

    What kind of spacer are you running under the carb you said 2 inch is it an open spacer or a 4 hole one
    With an open one it could be hard to get the carb to see a good vacuum signal and a four hole may work better or try it without the spacer.
     
  29. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,480

    Rusty O'Toole
    Member

    Worn out carb will leak air around the throttle shaft causing fast uneven idle. Uneven, because it will leak different amounts of air depending where the shaft falls.

    Do you also have the idle mixture screws out more than 2 turns? If it takes 3 or 4 turns to get an idle mixture you definitely have a leak someplace.

    Shake hands with the throttle shaft, any up down or side to side movement is trouble.
     
  30. bob8823
    Joined: Apr 16, 2011
    Posts: 39

    bob8823
    Member

    I took the spacer out a while ago. Throttle shaft is fine, nice and tight, no movement. Idle mixture screws are only
    out 1 1/2 to 1 3/4 turns. Tired of screwing with it, probably going to try a regular manifold and see if that doesn't
    fix the problem. Appreciate the help. Any other suggestions are welcome. Thanks.
     

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