"Cough" when the Secondaries open :(

Discussion in 'Holley' started by sriley531, Sep 21, 2015.

  1. sriley531

    sriley531 Buckle up, this could get rough....

    Hey all, looking for some feedback on an issue with my carb.

    Its' a 950cfm 4150hp (mechanical secondaries). Its been pretty rich for a while (even at idle you could tell), and always had a stumble when you matted it in lower rpms (2000-2500). You can tell its when the secondaries hit, there's a brief stumble and then its fine, pulls like mad. If you roll onto it, no stumble.

    I hooked up a vacuum gauge and started tweaking the 4 corner idle screws, which made a noticeable improvement. The idle smoothed some (and rose, but I turned it back down to ~800), and I gained about 4.5" of vacuum at idle. Took it for a test run, its not dumping fumes out the back like it was under WOT, but it still has that d#mn hesitation!!

    So I aske, what should I be investigating next? To be clear, I dont know what jets are in it, and have a feeling thats going to need to be addressed. Could that be an issue? Something else to check?

    DEADMANSCURVE my first word : truck

    if you have a carb part # myself or someone will be able to probably let ya know what the OEM spec jets etc should be . and did ya get that new or used ?
    with choke ( if one ) fully "off" flip carb over and make sure the primary transition slots are only showing enough to form a tiny square , only about maybe one full turn in on throttle set screw from "touch" . but if your idle adjustments made a dif that may be ok otherwise the idle screw adjustment won't make a dif because it is already bypassing the idle circuit and not allowing the idle screws to correct .
    a lot of times the hesitation when ya stab it is not too rich but too lean . secondary pump nozzle ( shooter ) may need to be bumped up a couple notch's ?
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Shawn, I'd be looking at that back accelerator pump shot. Easy to open the throttle and take a look at it. See if anything jumps out at ya, weak shot, clogged spray, that kind of thing.
  4. sriley531

    sriley531 Buckle up, this could get rough....

    Just the kinda feedback Im looking for gents, thanks!

    Del, I purchased the carb brand new from summit, however it was tweaked on the engine dyno (which makes these issues a bit more concerning to be honest), and I seem to have a vague memory that the shop may have changed the jets. I'll have to see if I can dig out the paperwork and see if its noted anywhere. If not, they are marked, correct? Also, this carb has NO choke.

    I would hope its not too lean, when it stumbled prior to me making some adjustments, it'd puff a bit of smoke out the back when it'd stumble. Maybe that doesnt indicate one way or the other about lean vs rich?

    With you and Del both suggesting that, I'll start there first. Ive been a little suspicious that the secondaries arent opening all the way. hope to check that as well. Just wanted to lay a little ground work to see where I should start looking.

    Its very frustrating because I really feel like, despite the fact the car does pull very hard and is a blast to drive, its noticeably not running up to its full potential. I dont really want to pay the shop to tune it, it's something I need to learn more about anyway (Im admittedly a novice with holley tuning). If it gets too annoying, those new terminator systems sure look nice though...... But thats a last resort.

    I'll look at the secondary shot and see what it looks like. Thanks again gents, I'll check back in when I get to play with it!

    DEADMANSCURVE my first word : truck

    get me that carb part# when ya got a sec .
    since ya called it a "4150" and no choke I'm guessing that it may be a street HP series , they list a 4 corner 950 that may be correct . chrome lookin' too ? holley part # 82951 ?
    hmmm - no spec info for that carb on holley web list ?
  6. sriley531

    sriley531 Buckle up, this could get rough....

    You nailed it Del, 82951
  7. sailbrd

    sailbrd Well-Known Member

    "Dyno tuned" is usually for most HP and not about anything else. So main jets were probably adjusted but nothing else. Main jets will not be the cause of a stumble. It is getting to the mains where the stumble will occur.

    Surprised that someone has not beat me to this but can you tell us about your timing? What is total WOT timing, at what rpm do you reach that and what is your initial timing? Do you run a vacuum advance?

    Cam specs? Makes a big difference on timing.

    On the other hand throw in a bigger squirter in the back and the problem my well disappear. :laugh:
  8. sriley531

    sriley531 Buckle up, this could get rough....

    Cam is a TA 288-92h, .525"-231/.525"-234 110 centerline, installed at 4. Timing is 36total, I'd have to dig up my notes as to the initial and when it's all in, want to say it was 2300, but that's off my (shaky) memory. No vacuum advance (msd billet/6al).
  9. sailbrd

    sailbrd Well-Known Member

    That all sounds in the ball park. You should have a lot of initial in that setup, say 20 to 24 degrees. With the MSD it is easy to set it up with 12* total. I suspect you are probably there so check the squirters. I was having a bit of a "moment" on mine when going to full from low rpms. Was fine from 2000 rpms. So I went to 37 primary, 41 secondary and never looked back.
  10. sriley531

    sriley531 Buckle up, this could get rough....

    Well, here was a quick visual check of the squirters. It seems the "streams" arent as defined with the secondaries, but that's to my highly untrained eye. Input?

  11. sailbrd

    sailbrd Well-Known Member

    Looks like two different size squirters, bigger in the secondary. You should be able to see the numbers just by looking on top of the squirters. When you take them out post a picture of the "bolt" that holds them in, there are two different kinds.

    This might help: https://www.youtube.com/watch?v=k-Nx5HEzvlY
  12. sriley531

    sriley531 Buckle up, this could get rough....

    That's a mighty good video, thanks for posting that. I'll try to pull them tomorrow and relay the info.
  13. shiftbyear

    shiftbyear Well-Known Member

    you might also want to check the float levels, a low float may delay your response, also check if the secondary accelerator pump arm is preloaded, you may have to adjust the spring/screw to make better contact or change cam position. I think holley has a troubleshooting section on their website, there are probably some good videos on utube also. good luck

    DEADMANSCURVE my first word : truck

    well - just was not getting anywhere on-line looking up oem specs on carb . I just like to have a starting out point and then you can compare to "after dyno" specs . did they give you a before n after list ?
    anyway , curious so broke down n called holley tech line .
    jets : primary 84 , secondary 84
    pump nozzles : primary 31 , secondary 31
    power valve : primary 4.5 , secondary - no PV
    air bleeds : 25's and 70's
    compared to standard 4781 850 DP that I'm used to that seems a little more aggressive ( jetting and PV mainly ) which makes sense . although it looks to have a standard 30cc rear pump while the 850 has a 50cc rear pump ?
    keep track of the plug readings also . good indicator .
  15. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    Adjusting your idle screws won't have much effect on WOT performance. If it stinks rich it is rich, and your leaving a lot of power on the table. Too much fuel will slow you down. how much tuning have you done to that carb? Tell me more than just slap it on and run it..
  16. sriley531

    sriley531 Buckle up, this could get rough....

    WOW Del, thanks for bird-dogging all that info!! You know more about my carb than I do!! Last time I pulled the plugs, they were thoroughly fuel-fouled. Went ahead and swapped them for a spare set I had on hand (NGK-FR5's). Hang with me, it'll be a day or 2 before I can get back out to play with it.

    The carb was put on and set up by the shop when the engine was dyno'd after being rebuilt. Thats part of the thing that surprises me a bit, these guys no doubt know what theyre doing, Im shocked its so rich. I guess the comment about them just tuning for peak power may explain it. Maybe. But as previously mentioned, Im a novice when it comes to these, so Im immersing myself now in "Holley tuning 101" via reading online, and more importantly all the great input from you guys on the board.

    Again, gonna be a day or 2 before I can play with it much, but Del, I'll be sure to check back in with what I find as far as the squirters and such, along with investigating the other suggestions here. I'll see if I have any paperwork on what was changed by the shop, I somehow managed to lose some of my dyno sheets/receipts from them... :(

    Thanks again for all the input, I'll be back with some more info as soon as I get a chance!
  17. 436'd Skylark

    436'd Skylark Sweet Fancy Moses!!!!!

    I would guess its wicked rich on the primary circuit and when you the secondary accelerator shot comes in it pukes and back fires.

    Tune your idle circuits, then your primary/main, then the primary accelerator shot, then mess with the secondary side. order the kit from summit and get ready..

  18. sriley531

    sriley531 Buckle up, this could get rough....

    That sounds like it could be a very logical possibility. What is this "kit" you speak of? Jet kit?
  19. sailbrd

    sailbrd Well-Known Member

    I am surprised that the stock setup is "Square" 84 primary 84 secondary and running a power valve. Usually you run 6 to 10 sizes less on the primary with a power valve. But we really don't know what the setup is on the carb.

    DEADMANSCURVE my first word : truck

    doug :
    yeah , that was a good little video .
    and now that you mention it - when I checked with quick fuel about their billet block kit , asked them about the rear block with no PV , they said they allow for that by bumping up the rear jets a few steps . but on this carb - same both ?
    I am curious as to the current ( dyno changes ) jets etc .
    Last edited: Sep 24, 2015

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