Register now to get rid of these ads!

Tuning Dual Rochester Carb's

Discussion in 'The Hokey Ass Message Board' started by 55ChevyArmyKat, Feb 19, 2009.

  1. 55ChevyArmyKat
    Joined: Jan 16, 2009
    Posts: 13

    55ChevyArmyKat
    Member

    Hello all,
    Well, I am now the proud owner of a beautiful Dual Carb set up Offenhauser intake, already pre assembled, with Linkages and all, that i picked up from Ebay for a great price for my 235. Just out curiosity if i have problems, what's the easiest way to tune the bad boys to run smooth at idle and high speed? The guy i bought it from had built this set up with two Rochesters from different years, but installed the same jets in both, and he's never mounted it, cause he decieded to go with Carter carbs instead. What's a good baseline for starting? I know that it can be a pain sometimes to get these running good, so that's why i'm picking at the brains of all of you for knowledge. I've also attached a few pic's of my buy, let me know whatcha think. Thanks.
     

    Attached Files:

  2. old1946truck
    Joined: Apr 9, 2008
    Posts: 685

    old1946truck
    Member

    I would also like to know how to do this does anybody? Does anybody have any info?
     
  3. xderelict
    Joined: Jul 30, 2006
    Posts: 2,477

    xderelict
    Member Emeritus

    Go to inliners,http://www.inliners.org/ find tech tips,find multiple rochesters,that should help.Then PM dickster27.
     
    Last edited: Feb 19, 2009
  4. 55ChevyArmyKat
    Joined: Jan 16, 2009
    Posts: 13

    55ChevyArmyKat
    Member

    Thanks for the info, i took a look at the tech info provieded, but it more talked about rebuilding, not any real tips on tuning, multiple carbs. But, i've got some time before i redeploy and get to tinkering with my car, so i'll keep on searching the great wide web.
     

  5. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    two Rochesters from different years

    The internal parts may be quite different. Changing the jets is 10% of it.
    Look up the internal bits from the carb numbers.
     
  6. Abomination
    Joined: Oct 5, 2006
    Posts: 6,702

    Abomination
    Member

  7. GassersGarage
    Joined: Jul 1, 2007
    Posts: 4,728

    GassersGarage
    Member

    Edelbrock use to sell a "Unisync" for tuning mutiple carbs. Don't know if they still sell them. Basically it measured vacuum at the mouth of the carbs I believe.
     
  8. carbking
    Joined: Dec 20, 2008
    Posts: 3,126

    carbking
    Member

  9. parksquijada
    Joined: Aug 6, 2008
    Posts: 316

    parksquijada
    Member
    from norcal

    Doesnt matter if carbs are from different years as long as they are same carbs. Loosen idle speed screws till butterflies completely closed then open ea. Approx. 1/2 turn. Bottom out ea. Airfuel mix screw gently and open 1 1/2 turns. Dont connect carbs together. Tune idle mix on ea. Carb. Screw in till stumble, back out till just smooth then 1/8 turn more. Get 2 ft. Vacuum hose or 5/16 fuel line, put in ear and other end hold around choke area and adjust ea. Carb till same vacuum sound. If idle to hi/lo adjust accordingly. Reapeat. Hook carbs together. Drive. If you have a "unisyn" use that instead of hose. Put unisyn on carb top, adjusy little cone thing till red float floats in middle of tube and use to adjust each carb so floaty thing is at same ht. On both cards. Simple :eek:

    DRIVE AND TUNE NORMALLY. UP A HILL, 2ND GEAR 3/4-FULL THROTTLE, PULL IN CLUTCH & KILL MOTOR SIMULTANEOUSLY, PULL OVER, CHECK PLUGS, BLACK-RICH, WHITE-LEAN, TANNISH-GOOD. YOU CAN GET DEEPER INTO PLUG READING SUCH AS GREYISH TO TANNISH RING AT BASE OF INSULATER IF YOU WANT. IF HOTROD HIGH COMPRESSION RUN 1-2 STEP COLDER PLUG. GOOD LUCK
     
    Last edited: Feb 20, 2009
  10. gas pumper
    Joined: Aug 13, 2007
    Posts: 2,956

    gas pumper
    Member

    +1 on ^^^

    Mine ran good OK right out the box. With maybe 57's for jets.

    Did have leaking out of the accelerator pump slots. Finnally wound up LOWERING the float to have a lower fuel level in the carb.
    I did 1/8 or 3/16 don't really remember, but anyway it cured the splashing and did not hurt the driveability.

    Mine ran fine with the 57's as far as street driving. I did eventually increase the jet sizes to get a darker plug reading at the track.

    The dual setup straight linkage, not progressive, drives nice. and is not requiring constant adjusting and tinkering. .

    Frank

    I forgot to add, the most important thing is not to have any vacumn leaks. You can't try to tune around leaks and they won't go away. Do everything right mounting the manifold and the carbs. And run Vac adv to the dist.
    And I run mine without heat to the manifold and never had an icing problem.
    I have the same manifold and "B" carbs, too.
     
    Last edited: Feb 20, 2009
  11. 55ChevyArmyKat
    Joined: Jan 16, 2009
    Posts: 13

    55ChevyArmyKat
    Member

    Well, you kats are a great wealth of knowledge, thank you for letting me pick you heads for it. Now one more thing, that was brought up at the last posting by Gas Pumper....for the vaccum advance, which carb would i put this on???? right now it's to the one and only carb, but if i'm adding an additional carb, would that lower the vaccum on each carb a little bit? and if so would this mess with the way the distributer advances? if not then i won't have a problem, just shorten the hose a bit and hook it up to the front carb and it should be fine.
     
  12. carbking
    Joined: Dec 20, 2008
    Posts: 3,126

    carbking
    Member

    Manifold vacuum is below the throttle plate(s). If you have multiple carbs which are properly adjusted; manifold vacuum should be the same as, or very near the same as vacuum with a single carb, except as WOT. Assuming you are running solid linkage, take the manifold vacuum from the carb that makes the installation the easiest.

    What will change is air velocity in each carburetor; as each carburetor will now be supplying approximately one-half (depending on the manifold design) of the air and fuel for the engine.

    Since the air velocity is what determines the amount of fuel pulled from the fuel wells in the carburetor; lots of folks are surprised when LARGER FUEL JETS are required in the dual carbs. This is not always true, but often is the case.

    Jon.
     
  13. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    Biggest single cause-o-misery in dual carbs is non-matching carbs ("well, they look the same", "well, the guy said they were the same", "well, they were in the same box").
    Next is non-matching adjustment, especially power system and float level.
    Next is vacuum leak due to loose throttle shafts - if any leak, idle quality is poor, etc. but if only 1 leaks the leak "drops out" as speed goes up.
    Sometimes you can find it with spray at the shafts - if ether, propane makes it speed up or CO2, nitrogen makes it go down it's leaking. No change: it's fine.
    Another test: get both to idle well, then raise the idle speed on both to 1,000.
    Now kill 1 carb (back the speed screw all the way down), see how low the speed drops. It should drop about the same amount if you kill the other carb.
     
  14. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    The amount of manifold vacuum at a constant speed is determined by the engine load, not the carburetor size.
    However, any any point of throttle opening, by % or angle, vacuum is "split" between all of them, which means your metering rod, power valve etc. will open.
    Full throttle vacuum is reduced = bigger main jets.
     
  15. gas pumper
    Joined: Aug 13, 2007
    Posts: 2,956

    gas pumper
    Member

    55CAK, I have my vac line to the dist off the 1/8 pipe hole in the side of the manifold between the two carbs.

    You'd be OK off the front carb, too. I think.
     
  16. budd
    Joined: Oct 31, 2006
    Posts: 3,478

    budd
    Member

    your type of carbs come in two different sizes that i know of, ones from a 292 are larger then ones from a 250. has anyone used merc. sticks to synchronize duel carbs on a single manifold?
     
  17. panic
    Joined: Jan 3, 2004
    Posts: 1,450

    panic

    What's recommended for the 235 is 2 from the 216, which have smaller venturis (always) and smaller throats and bolt patterns (sometimes).
    AFAIK the 216 is 1-1/2" throttle disc, 1-7/32" venturi (1.219") and has a 2-11/16” bolt center.
    If it's bigger, there are at least 50 different models in at least 4 sizes.

    The 235 is 1-9/16", 1-11/32" venturi (1.344"), 2-15/16" bolts.
    If it's bigger than that you may not be happy with it. 2 from a 292 is not recommended.
    Are there tags on them?
     
  18. HEATHEN
    Joined: Nov 22, 2005
    Posts: 7,591

    HEATHEN
    Member
    from SIDNEY, NY

    They also came on 216s, with a smaller throttle bore and bolt pattern. Also, they came with several different power pistons, so it's still best to start with two identical carbs.
     
  19. 55ChevyArmyKat
    Joined: Jan 16, 2009
    Posts: 13

    55ChevyArmyKat
    Member

    i'll have to check all this out once i get home from Iraq. I"m working on getting tips to make my transition from one to two carbs easy. Thanks all.
     

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.