Carb synch tutorial for 308 | Page 2 | FerrariChat
Buy

Carb synch tutorial for 308

Discussion in 'Technical Q&A' started by Birdman, Jun 3, 2004.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, Skimlinks, and others.

Page 2 of 3
  1. pcelenta

    pcelenta Karting

    Nov 1, 2003
    216
    #26 pcelenta, Jun 4, 2004
    excellent write up...birdman...one question, what about the lower threaded linkage from front to back? You state to adjust the upper to get the two banks in synch..just wondering where the lower should be set. I've been fussing with my carbs for the last two weeks...found vacuum leaks from old hoses, a pinched throttle cable and now when I think I am almost there I can't seem to get the two banks to synch at idle (5 flow on rear and 3.5 on forward bank throttle stops not touching). I was overlooking the upper threaded linkage that you mentioned and plan on making adjustments there this weekend...but what about that lower linkage?

    regards,
    Paul
     
  2. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    #27 4re gt4, Jun 4, 2004
    This is a great write-up. Birdman mentions a couple of things that often are done incorrectly:

    1) Many will adjust the front-rear balance with only the idle screws, ignoring the threaded linkage. This will make for a terrific idle, but can cause real problems just off idle. Adjusting these linkages at a fast idle is the only way (that I know of) to make sure they're syncronized.

    2) As he mentions, there are at least two - and probably more - different idle mixture screws. Mine are properly adjusted at about 2 turns out, but those with a longer taper usually are at 4 turns, some apparently even more.

    As a way of simplifying holding a fast idle, you can put a folded up matchbook cover in the cold fast idle mechanism. This will 'pull' on the linkages evenly and is easier than messing with the throttle cable adjuster.
     
  3. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
    Full Name:
    Hans E. Hansen
    #28 4re gt4, Jun 4, 2004
    I thought this deserved to be in a separate post from that above:

    IMPORTANT, WILL ROBINSON, IMPORTANT:

    You need to check the linkage at full throttle. The full throttle stop is at the gas pedal inside the car. Two incorrect scenarios:

    1) Carbs throttles aren't opening all the way with pedal to the floor. This isn't the end of the world, but you're not getting 100% power. If it's reasonably close, don't mess with it, tho.

    2) Carb linkages are hitting their stops. My car had this problem. What happens is that there is room under the gas pedal stop after the throttles are all the way open. When you pull out to pass that P-Car in front of you, you will force the pedal down hard. This will bend the linkage at the carbs. Big problem, as it can bind or even break. So be sure to check for this.
     
  4. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #29 Birdman, Jun 4, 2004
    Hi Mike, a good point and I considered that. I decided that since you are just adjusting the carbs relative to each other, the lack of trumpets, as long as it is the same for all the carbs, will not change the sync. The only thing it would do is cause a slightly different flow but since the adjustments are relative more than absolute, I decided to not worry about it. Several issues come up if you want to do it with the trumpets on. You need to have gaskets under them all and they all need to be tightened down so the gaskets don't leak. That represents a huge place for error in the reading if they leak. Since the trumpets make almost no difference in the flow at low RPM where you are doing the sync, they actually play far less of a role in the flow than the lack of an entire airbox! In theory, we should be doing this not just with the trumpets, but the airbox and air filter. Obviously, you can't reach anything with all that stuff in the way, so some error must be tolerated. But again, since the "error" is the same with all the carbs, the adjustment of one carb relative to another will remain once the airbox is put back on and the trumpets too.

    Birdman
     
  5. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #30 Birdman, Jun 4, 2004
    Hi Paul,
    This is from memory, but the throttle cable pulls on a bar that is mounted on a bolt in the middle so it can rotate. One end of the bar has the "upper" adjuster (connected to the rear carbs) and the other end of the bar has the "lower" adjuster (connected to the front carbs). When the throttle cable pulls, the bar turns counterclockwise as viewed from the left (driver's side) and pushes on the upper bar and lower bars to open the butterflies. Within a reasonable range of adjustment (i.e. if the lower and upper are in the ballpark) they actually do the same thing...reduce or increase the length of the total linkage between the carbs, so you can adjust either one and get the same result. Unless of course one of the bars is so far out of whack that the butterfly connected to it won't open or close all the way. It would be pretty hard to get them that out of whack. So the short answer is that in general, you can just adjust one and that does what you need it to do (increase or decrease the opening of the rear butterflies versus those of the front). Since the top one is far easier to reach, especially when the engine is hot and sticking your hand in there will get you burned, that's the one to use. (This is a place where a diagram would be helpful.)

    Birdman
     
  6. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #31 Birdman, Jun 4, 2004
    Man, you aren't kidding about that! The linkage adjustment is absolutely critical to the engine running smoothly in the "just off idle" range, which is 90% of your around-town driving. Once that is dialed in, getting the idle set is a cinch.

    My fast idle mechanism has either been removed or was never there. I have what appears to be a little cam near the throttle assembly that might have been part of it, but nothing else.

    I actually found using the throttle adjuster quite easy. Once the lock nut is loose, you can dial in the exact RPM you want very easily just by turning the adjuster by hand (no wrench required). Just be careful not to touch the valve cover...it's hot.
     
  7. pcelenta

    pcelenta Karting

    Nov 1, 2003
    216
    #32 pcelenta, Jun 5, 2004
    thanks birdman..re: the rear bank adjustment screw...I use a "sidewinder" box wrench...picked up a set at sears..these are the box end wrenches with a built in ranchet...makes that adjustment a bit easier.
     
  8. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #33 Birdman, Jun 5, 2004
    Great minds think alike...that's what I used!
     
  9. PeterS

    PeterS Five Time F1 World Champ
    Silver Subscribed

    Jan 24, 2003
    52,649
    Goodyear, AZ
    Full Name:
    PeterS
    #34 PeterS, Jun 5, 2004
    Excellent post! I think you convinced me to pay the $400.00 (If I can watch and learn the job). It still looks like I could screw it up!
     
  10. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    2,934
    Outside Detroit
    Full Name:
    Don the 16th
    #35 don_xvi, Jun 8, 2004
  11. 208 GT4

    208 GT4 Formula 3

    Dec 27, 2003
    1,769
    Brighton (UK)
    Full Name:
    Dan
    #36 208 GT4, Jun 9, 2004
    An excellent tutorial, so big thanks to Birdman.

    P.S. any chance of an Ignition timing tutorial of similar high quality?
     
  12. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #37 Birdman, Jun 10, 2004
    Don_XVI:
    Looks like the same tool I have. Funny, I bought mine for $39 at Weber Carbs Direct and right after that they raised the price to $60. I must have gotten the last one and when they re-ordered, the price had gone up.

    208-GT4:
    When I get around to learning how to check timing, you can be sure I'll write it up. Right now the timing isn't doing anything wrong, so I'm sticking with my mantra: "If it ain't broke, don't f*ck with it!"

    Birdman
     
  13. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    13,673
    San Carlos, CA
    Full Name:
    Mitchell Le
    #38 yelcab, Jun 10, 2004
    Birdy

    I have done it already and did not take pictures. But, if you convert your dizzy to a Crane Cam system, it would be a great article to write up. In the end, you also have to do a cam timing thing, so it will include that too.
     
  14. thecarreaper

    thecarreaper F1 World Champ
    Silver Subscribed

    Sep 30, 2003
    18,128
    Savannah
    #39 thecarreaper, Jul 10, 2004
    bump. so i can find the thread, tired of using the archives every time!!! :)
     
  15. don_xvi

    don_xvi F1 Rookie

    Nov 1, 2003
    2,934
    Outside Detroit
    Full Name:
    Don the 16th
    #40 don_xvi, Jul 10, 2004
  16. ham308

    ham308 Formula Junior

    Nov 3, 2003
    358
    NE Switzerland
    Full Name:
    Richard Ham
    #41 ham308, Jul 11, 2004
    don't know if anyone has answered this, but what you can do is...
    get the synchronisation set up on the throttle stop screws (presuming the idle mixtures are OK) with the lower linkage loose. Then put some thin card or whatever between the cold start high idle cam thingy and measure the clearance with a feeler gauge that you have now produced between the rear throttle stop screw and the carb. Now adjust the lower linkage so that you produce the same clearance at the front bank. You can try it with different thicknesses of card and the clearance should be the same front and back in each case. The carbs are then synchronised front to back just off idle.
     
  17. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #42 Birdman, Jul 11, 2004
    Richard,
    That method would set the same physical position of the butterflies between the front and rear bank, but would not in any way take into account the actual FLOW through the carbs. Unless I am misunderstanding you, this doesn't sound like it would be very accurate.
    Birdman
     
  18. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #43 Birdman, Dec 20, 2004
    NEWS!
    I have written this tutorial up for inclusion in the resources part of Fchat so it won't just be buried here. Rob promises to have it up soon.

    Birdman
     
  19. ar74spider

    ar74spider Rookie

    Nov 4, 2004
    23
    #44 ar74spider, Dec 22, 2004
    why can't I see the pictures?
     
  20. Jdubbya

    Jdubbya The $10 Trillion Man
    Silver Subscribed

    Dec 28, 2003
    43,665
    Hell's waiting room
    Full Name:
    John
    #45 Jdubbya, Dec 22, 2004
  21. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #46 Birdman, Apr 20, 2005
    Last edited by a moderator: Sep 7, 2017
  22. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #47 Birdman, Apr 20, 2005
    Last edited by a moderator: Sep 7, 2017
  23. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #48 Birdman, Apr 20, 2005
    Last edited by a moderator: Sep 7, 2017
  24. 208 GT4

    208 GT4 Formula 3

    Dec 27, 2003
    1,769
    Brighton (UK)
    Full Name:
    Dan
    #49 208 GT4, Nov 2, 2005
    Last edited by a moderator: Sep 7, 2017
  25. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,689
    North shore, MA
    Full Name:
    THE Birdman
    #50 Birdman, Nov 2, 2005
Page 2 of 3

Share This Page