Intake reversion at idle w/big cam

Discussion in 'Street/strip 400/430/455' started by 455monte, Mar 6, 2018.

  1. 455monte

    455monte Well-Known Member

    Ok guys i need some advice.
    My 67 400 has a nasty shake at idle.
    Cam specs are 227 241 @.050 lift
    Ground on a 107 lobe separation angle
    .500 lift.
    Obviously a pretty good size cam for a 10.25 to 1 400.
    It has the usual big cam lope at idle but sometimes it shakes violenly.
    I have figured out that if i set mixture screws to the fat side it will cover up the shake for the most part.
    This is where it starts getting interesting. The back four cylinders of the engine run lean and the front four cylinders of the engine show a little darker color plugs than I like for 93 pump gas.
    I am using a stock 1970 intake on my engine with a 4-hole adapter plate to put a 750 annular discharge Holley double pumper on there. Front blades have .090 holes drilled in them to help idle better with the low vacuum.
    I have noticed that the carburetor sets off center towards the rear of the intake probably 2 to 3 in. Does this have some effect on the engine not running all the cylinders equal at lower speeds?
    Somewhere along the line I remember reading a story on here about having a similar issue with the poston intake. Is there any of you Advanced engine builders out there that can shed some light on this concern?
    I have tried various idle speeds and timing settings sometimes it will smooth out but usually comes back.
    Would a single plane intake help me since the cylinders could pull from a common plenum?
    Sorry for the long write up guys but this ordeal is driving me crazy! I've got two drag cars with bigger meaner cams with less vacuum in them and neither one of them shake like this at idle.
     
    Last edited: Mar 6, 2018
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Probably has a lot of overlap due to the narrow LSA. It would help a lot if you ran constant timing with a locked out distributor. You would need a crank retard of course.
     
  3. 455monte

    455monte Well-Known Member

    Larry I had considered that. I kind of want to keep it simple though and not worry about having to hide boxes in the vehicle somewhere. I run a simple HEI distributor with lighter springs in it for quicker advance.
    Guy i would not think that the annular boosters would have any effect on the idle circuits of the carb.
     
  4. 455monte

    455monte Well-Known Member

    Just a quick thought while I'm sitting here brainstorming about it. That intake was designed to run a Quadrajet that ran only on primary idle circuits. I'm running a carburetor with four corner idle circuits. Is it possible that having fuel distributed through the back two Bores could be affecting the overall mixture signal on the stock dual plane?
     
    8ad-f85 likes this.
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    I am a big fan of the SP1 intake. It definitely helps fuel distribution. 67's are tough with hood clearance, that's the only thing.

    Chris, you could turn the timing up to 34* temporarily just to see how it idles. Just a thought.
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Also, really light springs that allow mechanical advance at idle speeds can make for a very unstable idle as the weights flutter around changing the timing constantly.
     
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  7. 8ad-f85

    8ad-f85 Well-Known Member

    I bet it's something simple or a combination of simple things.
    Intake positioning and 4 hole idle on top of the isolated hole adapter could be an issue.
    You're right, it isn't that much cam to have a problem.
    Shouldn't 'need' the 4 corner mix, have you tried just the front?
    Shouldn't be idling off the boosters.
    Advance springs and mechanism might not fully return.
    Power valve might be cutting in and out, throttle blades too far open (biggie right there)...
    Not enough initial timing (needed to reduce throttle blade opening).
    What is initial timing, how is advance hooked up?
    Is advance can cutting in from hooked direct and dealing with idle surge ot too much throttle opening?
     
  8. Robs455

    Robs455 Well-Known Member

    What timing do you run? With that cam you can run easy 30* at 900 rpm.
     
  9. 8ad-f85

    8ad-f85 Well-Known Member

    Start with much more initial and the idle backed way off.
    Work away from that after isolating all of the other anomalies mentioned.
     
  10. Mark Demko

    Mark Demko Well-Known Member

    You say the engine is shaking?
    To me that says a weak or dead cylinder.
    Have you done a compression test, or better yet a leak down test?
    I would start there, to make sure you have a solid foundation.
     
  11. 455monte

    455monte Well-Known Member

    Hey Mark.
    Compression average is 170 across the board give or take a few numbers.
    Leakdown is averge 17 pct.
    I did close off rear blades and rear idle screws last night. I got more uniform ext temps from all 8 cylinders by doin this.
    I pulled my innovate wideband out of the cabinet and calibrated it and shoved it up the tailpipe about 2 feet .
    Car was around 12.5 afr at idle in gear.
    So a lil tweaking got her to 14 to 1. At about 630 rpms. In park is 900 rpms.
    Also hooked vac advance to my hand pump. Brought initial timing up to 21 degrees. That made shaking worse in gear.

    Any higher rpms in gear and it is lunging against the converter . Im using a 2004r trans with a gn converter which isn't much more stall than a stock monte carlo unit.
    I believe the tight converter is my biggest issue now
     
    8ad-f85 likes this.
  12. Mark Demko

    Mark Demko Well-Known Member

    Does it idle better out of gear, meaning it lopes from the cam, but no shaking/rocking?
    I could see it lugging in gear at idle with the GN convertor, they were only about 2200 stall (the original D5's)
     
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  13. 455monte

    455monte Well-Known Member

    Mark it idles beautiful in park!
    I ordered a 2800 converter for it this am.
    I think it will help it tremendously.
    It still has lock up so should be highway cruiser friendly still
     
    Mark Demko likes this.
  14. Mark Demko

    Mark Demko Well-Known Member

    That's probably the majority of your issue, the convertor, especially if it idles fine out of gear.
    A too tight convertor will cause all sorts of idle in gear tuning issues, off idle, cruising, and wot, all across the spectrum;)
    You said you ordered a convertor, from who?
    Reason I ask is Jim Weise who owns the board here, and also owns Tri Shield Performance builds custom convertors specific TO YOUR COMBO:D
    I had a TCI convertor for years, my 350 GS WOULD NOT get outta the 14's
    After reading about his convertors, and him KNOWING BUICKS, he built me a 9.5" 2800 stall convertor, I went from 14.3 right down to 13.78, first run with his convertor:D:D:D
    I cant say enough about his custom convertors, worth EVERY penny!
     
    300sbb_overkill likes this.
  15. 455monte

    455monte Well-Known Member

    What bothers me about this converter deal is up until about a month ago i had the original sp400 in the car but had a sp300 1 inch smaller looser converter in it.
    It would shake at idle in hi stall but didnt buck against it. So obviously the 200 4r is tighter now.
    I ordered an off the shelf fti 2800 -3000 with lockup plate for it. Ive got to have the car road ready by next Thursday and it was in stock and had good reviews.
    I do agree that a good converter is worth its weight in gold! In my race car i run a bte custom nitrous converter and it was a great investment.
    With this ragtop its a cruiser with pep .
    It dont have to be perfect but i want it to be completely driveable for my wife and she doesn't like the shaking
     
  16. 8ad-f85

    8ad-f85 Well-Known Member

    Please look over comments regarding the throttle angle vs. initial timing amount.
    That can cause much of the problem you are having.
    I can see the shaking getting worse to add timing when it already has that much gear drop if the throttle is open too far.
     
  17. Fox's Den

    Fox's Den 355Xrs

    The problem is with the adapter and the stock intake. It just doesn't work. I tried this with my 350 and the carb was never right with the adapter, I used a open spacer. I would put a alum intake on there to get the carb in the right spot above the engine. Then it will run right. Or get a Q-Jet and get rid of the crappy adapter if you want to keep the stock intake.
     

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