Register now to get rid of these ads!

Technical tuning 3x2 carb setup on a Y-block

Discussion in 'The Hokey Ass Message Board' started by 1LIFE2LIVE, May 1, 2015.

  1. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    I have recently finished my 27 roadster pickup with a 292 ford y block and a 3x2 manifold with holley 94 carbs. I have the motor running and have driven it some but I know its not running the way it should. The motor has a comp cams thumper cam shaft, i measured vacuum at idle and its at 8.5. Im running a stock distributor with mechanical and vacuum advance, the timing is at 15 base and about 34 total timing. I have a progressive linkage and the main carb has jetting at 51 and the secondaries at 47. The power valves are 6.5. I have a 3 speed with over drive manual trans and 4.11 rear gears.

    The car starts fine and idles well, no black smoke at startup. When driving and going through the rev range in each gear the motor starts to break up but does continue to climb in rpm. The motor seems lazy in response and never really clears up when climbing in revs.

    The motor acts as though maybe its rich in the upper rpm or that it needs more timing. I have purchased two power valve block offs and plan on installing them in the secondary carbs. The only thing I have not done is a wide open throttle chop to get a plug read. Any advice or info in helping me tune the carbs is greatly appreciated
     
  2. 8.5 vacuum at idle? Thumper cam, aren't they only supposed to sound radical while maintaining drivability?

    What condition are your heads in? Got compression? What about your secondary carbs? Are you running idle circuits, idle delete?

    Your idle vacuum bothers me and I think that should be addressed before anything.
     
    1927graham likes this.
  3. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Heads and motor are in good shape. It was a running engine when I purchased it and only added the thumper cam. I am running stock bases on all three carbs that have the longer throttle shafts. I am not able to adjust the idle mixture screws and get any change in idle.

    Why does the vacuum bother you? Not sure what it should be. I have the idle set around 800 rpm
     
  4. You should be in the 14-18" range. if fiddling with the idle trim screws does not make any difference and given that you are maintaining low vac I would think that you have a major vac leak. before you go overboard look down the outboard carbs and see if the throttle blades are open too far. next get yourself a can of starting fluid or WD-40 and with it idling start spraying it around. When it revs up you found your leak.

    Running rich in the upper RPM range is an oddity with a carb by the way, they have a tendency to lean out in the upper RPM range if they are good at idle. At least that is the most common complaint about carbs.
     

  5. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Ok I will give it a look and see what's what.
     
  6. Blue One
    Joined: Feb 6, 2010
    Posts: 10,964

    Blue One
    ALLIANCE MEMBER
    from Alberta

    Let us know what you do or discover.
     
  7. Easy way to do this, is to get it running perfect with a single carb, the tune the three two's back to running perfect. That way you know any issues you have are carb related. Doesn't need to be anything complicated, just unhook the two outboard carbs, and make a solid block off gasket for each out of gasket material.
     
  8. alchemy
    Joined: Sep 27, 2002
    Posts: 16,575

    alchemy
    ALLIANCE MEMBER

    What year is the distributor? Pre-57 Ford distributors have a poor reputation on modified setups due to the way the vacuum works/doesn't work with multiple carbs.

    Do the throttle plates in the secondary carbs close tightly? So no light can be seen?
     
  9. PackardV8
    Joined: Jun 7, 2007
    Posts: 890

    PackardV8
    Member

    After fifty years of spending too much time guessing about WIGO with carbs, I got a direct reading O2 meter and learned how much I didn't know. Now, I won't touch a carb, not even think about multiple carbs, without the O2 meter. Anyone can see in real time, on the road, what the engine is seeing for air/fuel ratio.

    The question you didn't ask is what happens when one installs a Thumpr cam in an otherwise stock low compression 292" Y-block. Answer - no more compression. Most 292"s were 8.0-8.5 compression. The increased duration (231/245@.050"), the added overlap and late intake closing is what is giving the low vacuum reading and the lazy performance. Bottom line - not enough dynamic compression and it will never run really strong.

    If it's one of the later engines with the small valve heads, it's not going to be able to use much of the 3x2 when the end carbs open.

    You can tune the carbs and ignition to make the most of what you've got, but the underlying compromises will still be there.

    jack vines
     
    HemiDeuce and loudbang like this.
  10. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Thanks for the input. The distributor is a reman distributor I bought at napa. It was ordered for a 64 model year car.

    I only got the cam for more of what it might create in noise vs. performance but I didn't want it to create poorer performance. I did a block off of the secondaries when I was first playing with it but never drove it on one carb. I may go home today and block off the secondary carbs check my tuning and try to drive it and see what's what.
     
  11. nickleone
    Joined: Jun 14, 2007
    Posts: 330

    nickleone
    Member

    Your secondary carbs should not have an active idle circuit.
    Speedway has a kit http://tinyurl.com/o67vjh6
    They have install instructions you can use.
    GEARZ had an episode on tri power setup but I cant find it.
    In that episode they replaced the base plates with no idle circuit.

    Nick
     
  12. nickleone
    Joined: Jun 14, 2007
    Posts: 330

    nickleone
    Member

    I found the tri power episode on You tube:

    Nick
     
  13. nickleone
    Joined: Jun 14, 2007
    Posts: 330

    nickleone
    Member

    It is a two part video:
     
  14. They'll run fine with or without you just tune them different.
     
  15. clem
    Joined: Dec 20, 2006
    Posts: 3,222

    clem
    Member

    With my little knowledge, sounds like it is starving of fuel. I'm experimenting with a 283 at the moment. main jet should be 57. Block off secondary power valves and go larger jets. See vintakes thread. He has lot of info.
     
  16. clem
    Joined: Dec 20, 2006
    Posts: 3,222

    clem
    Member

    Does that mean that he would be better to run fixed linkage and smaller jets?
     
  17. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Played with it a little this afternoon. Blocked off the secondaries and fired it up. Still only have around 8.5 vacuum. I was able to see and hear a change in idle and vacuum playing with the mixture screws but it was hard to get a good reading because the idle seems to jump around. Didn't get a chance to drive it I will do that in the morning
     
  18. Change your power valve to a 3.5, check the gasket for leakage. You should have light springs in the distributor to bring in the total advance by 2500 RPM. My 272 liked a lot of initial timing 10 to12.
     
  19. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Got it running pretty good. I unhooked the secondaries and drove it. Seemed ok and cruised fine. Got back and hooked up the other two carbs. Once I got the power valve block offs tight enough that they didn't leak I got it running pretty good at idle but rich. Took it for a drive and holy crap what a difference. When the secondaries open it really wakes up. My only issue is now when coming to a stop it wants to die out. I have adjusted the idle a couple times but I don't think that's it. It is puffing a little black smoke at idle and I'm wondering if that's not the problem. I shut the idle adjustment screws off on the secondary carbs and the center carb is at 1 1/2 turns out.
     
  20. start at 1 1/2 turns out turn in one at at time till she runs bad then back off 1/2 turn then do other side thas it......
     
  21. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Should I do this to all three carbs or just the main center carb? My thought is if it's only idling on the center carb then the other carb idle mixture screws should be shut off. Am I wrong?
     
  22. Yes, only the center carb. BUT, the throttle blades on the secondary carbs must be completely shut so you don't get extra idle air past the blades (resulting in a lean condition), and / or fuel being drawn out of the idle transfer slot (D on the diagram) resulting in a rich condition. Did you happen to get the number off the power valve on the center carb? It should be several inches below your 8.5" vacuum idle (I believe Rick said it should be a 3.5, which means it opens when vacuum drops to 3.5". if it's to high, 6-7" or above, it could be opening (or leaking, if it's bad) at idle and adding extra fuel, resulting in your rich idle. Is there any fuel dripping or visible in the carb. throat when it's running? Holley 94 Idle circuit..gif
     
  23. Oh yeah, the stumble when stopping could also be high float level in one or more of the carbs.
     
  24. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    The power valve is a 6.5. Guess I will need to get a 3.5. I didn't have a rich idle condition until after I put the other carbs back into the system from running it on one carb. Even prior to removing those carbs it didn't run rich at idle.
     
  25. 8
    Yeah, a 6.5 is gonna come in awful fast. Are the throttle blades on the secondary carbs all the way shut? It would probably be worth while to pull them off and ensure that they transition slot is completely blocked. That would also be a good time to ensure the float levels are correct. This could affect the idle richness 2 ways, the blades uncovering the slot could cause excess fuel to be drawn in, and the blades being open could also low the vacuum signal the power valve in the center carb is seeing (think of the throttle blades as vacuum leaks, since their no longer being used for idle control). Not sure if it's safe to back the idle speed screw completely out (could cause a throttle sticking incident), but you want to be as close to completely backed out as you can.

    Posted using the Full Custom H.A.M.B. App!
     
  26. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Drove the roadster today for its first long drive. 50 miles to a show and no real issues. Runs pretty good still doesn't want to idle. I ordered 3.5 power valves for all three carbs. The throttle blades are closed on secondaries I had them off and shined light through them and they are closed. Once I get the smaller valves I will mess with it some more
     
  27. When you change power valves, make sure you are using the flat ones or the small gasket on the new tapered style (nylon ones don't leak)
     
  28. 1LIFE2LIVE
    Joined: Oct 20, 2006
    Posts: 422

    1LIFE2LIVE
    Member

    Still don't have my power valves but I did adjust the secondary carbs idle screws to 1 1/2 turns out and it seems to be idling now
     
  29. clem
    Joined: Dec 20, 2006
    Posts: 3,222

    clem
    Member

    Yep! They will drive you nuts, but it's all fun learning.
     
  30. Joseph Cumbey
    Joined: May 10, 2018
    Posts: 1

    Joseph Cumbey

    Similar issues. Vac only 10" Hg, Idles poorly, runs rich. 12deg BTDC timing. Pertronix. Vac advance disconnected. Cannot find a vac leak to save my life. Power valves blocked on secondaries. Running a 6.5 PV on Primary, which I'm sure now is way too high. Idle screws on secondaries also blocked, and primary idle screws seem to have no impact on idle. I suspect low vac is causing 6.5 power valve to dump fuel, causing rich condition. Also, this would seem to be confirmed by the fact that you have to choke the primary to get the engine (292) to start. Lastly, have adjustable jets on primary. Figuring out now that they're just an added variable that I don't need. Will probably go to 51's.
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.