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Advanced 308 carb sync question -- throttle plates and progression holes

Discussion in 'Technical Q&A' started by Mike328, Jan 27, 2004.

  1. Mike328

    Mike328 F1 Rookie
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    Oct 19, 2002
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    I've been reading like a grad student, learning about sync procedures for our 308 carbs. I've read many different procedures, and understand the basic principles. I've identified the common points across everyone's carb sync procedure.

    I'm currently struggling with one thing, though, and was looking for some help.

    1. I know that the progression holes in the carbs are critical, since they help smoothen things out when transitioning from idle to off-idle.

    2. I know that the throttle plates should just cover the first progression hole, so that the moment the plates are opened the progression holes become active.

    3. I just had my carbs rebuilt by Pierce Manifolds. I checked, and I noticed the plates were configured to ALWAYS stop right where they were supposed to--at the first progression hole. A call to Mike Pierce at Pierce Manifolds confirmed this, this was done on purpose.

    4. What I'm struggling with is, there's all this talk about setting the "Idle Speed Screw," which basically translates to moving the throttle plates and MODIFYING their relationship to the PROGRESSION HOLES (bad bad) in order to set your idle. I don't want to set my idle this way. My forming opinion, based on things that I've read, is that there is only ONE good position for the throttle plates, and that position is covering the first progression hole.

    Point #4 was heavily influenced by a thread by Dr. Mike, located in the FerrariChat archives here:
    http://www.ferrarichat.com/discus/messages/256120/184093.html

    Thanks to Hans Hansen for pointing out the merits of this logic as well as a few other more subtle points.

    5. So it seems to me, therefore, that because my throttle plates are ALREADY set in their one perfect position in relation to the progression holes, I don't need to adjust the Idle Speed Screw aka Throttle Stop Screw on ANY of the carbs. For my purposes, they can be backed out or otherwise just touching the two carb throttle levers. Note that I'm also sensitive to the left/right linkage across the carbs, and have studied the mechanics of this and have found what I believe to be settings that leave all carbs throttle plates in their fully closed, "natural" positions.

    Point #5 is the big one. Workshop manual and lots of folks say to adjust idle speed here. This is going to affect the fundamental carb response, because I'm messing with the throttle plates in relation to the progression holes.

    Dr. Mike suggests that with this set, I can now leave the Idle Speed Screws ALONE and adjust the idle with the air balance / air bleed screws (which I'm not really proficient with at this point).

    So before I go too crazy and sync the carbs with some fundamentally false assumptions, what are your thoughts on this logic?

    If we can concur that this makes sense, I think we might be able to improve the carb response in many folks' 308s by syncing things differently than they have been synced in the past...

    And finally, I defer any success this method may have to Hans Hansen, who had these new sync methods in mind well before I laid a hand on a carburettor.

    Would be interested to hear comments on this, as well as ideas for how to set the idle speed with the air balance screws, and then also how the idle mix screws will play a role.

    --Mike
     
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  3. Peter

    Peter F1 Veteran
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    Yes, this is a different, interesting approach. I've done all my setting using the throttle stop screws for idle speed and all of my idle by-pass screws are shut tight (since airflow through the barrels were equal), up to now.

    Maybe you can be the guinea pig for the rest of us to see if this method works or not?

    ;-D

    For measuring air-flow through the barrels, I suppose you have a STE Syncrometer, which should read anywhere between 3.5 to 4 KgH of flow (whatever makes the car idle at a reasonable speed). If not, use a rubber tube crammed into your ear canal and shove the other end down the carb barrel (or a stethoscope). Listen for rushing air mixed with dull, muted popping. Hard to describe the sound, but as long as its the same sound for each barrel and the car idles at a reasonable speed.

    Idle mix screws can be turned in or out 1/2 turn at a time, listening for a rise or fall in RPM. Ideal setting is the fastest RPM.
     
  4. 4Webers

    4Webers Formula Junior

    Nov 12, 2003
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    Darrell
  5. 4Webers

    4Webers Formula Junior

    Nov 12, 2003
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    Darrell
    ... only after ridding your carbs of ALL nasty "sneezing" habits, right :)
     
  6. Mike Florio

    Mike Florio Formula Junior

    Jun 19, 2003
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    I just checked my Haynes Weber Workshop manual, which has a whjle chapter on the 40 DCNF, and the Dr. Mike posting. The only thing the Haynes Manual says is to "turn the throttle adjustment 1/2 turn in from the fully shut position." That's a really minor adjustment, and would probably just move the throttle plate a fraction off rest. Thinking about it, maybe the throttle adjustment screw is there just to take the spring load off the throttle plate and to prevent the throttle plates from jamming against the soft throat metal.

    If that's the case, then it seems setting your idle adjustment using the idle adjustment screws is the right way to go. The Haynes procedure is to:
    1) Set the Idle Adjustment screws to adjust the idle.
    2) Set the Air Bypass Screws to synchronize all the barrels to pass the same anount of air at idle.
    3) Re adjust the Idle Adjustment Screws by turning them equal amounts to gain the proper idle RPM.
    4) Set the mixtures
    5) If necessary, re adjust the Idle Adjustment Screws by turning them equal amounts to gain the proper idle RPM.

    I would be very interested in learning your results. Now that I've just about got my Distributors sorted out the carbs are next.
     
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  8. Mike328

    Mike328 F1 Rookie
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    Mike, I think you're on to something, BUT I'm confused. We have to be careful what we call the screws... There are three things we can adjust:

    1. Throttle Stop Screws / Idle Speed Screws / Throttle Adjustment Screws [our new method is to LEAVE THESE ALONE]

    2. Air Balance Screws / Air Bypass Screws

    3. Idle Mixture Screws

    What I was saying is, don't touch #1 once the throttle plate is set, only touch #2 and #3.

    What I think you're saying the Haynes manual says contradicts this new method [which is OK]. I think most folks before used the Haynes manual method.

    Can you clarify?
     
  9. Matt Morgan, "Kermit"

    Matt Morgan, "Kermit" Formula Junior

    Nov 12, 2003
    405
    Ferndale, WA
    There has been much great input here, as usual, but I don't see one detail, so I'll share my experience.
    I had to straighten out a set of Webers that had ben "overhauled" by a private party, all 4 in an hour and a half. Turned out to be a learning experience!
    On the topic of uncovering the progressive holes, a tip I found in a Weber Manual was if you have difficulty getting the idle speed right without the plate in it's proper orentation in relationship tothe holes, it is an old trick to drill a small hole in the throttle plate, .040 to start, but no more than .060. This allows a tad more air to get by, without leaning on the Air Bypass system too much. A very handy trick of you are running cams that aren't stock, or are, but not in their factory specs (dialed in for more top end for examle). Allowing the motor to draw from this circut too quickly can lead to some maddening "fat spots", or rich just off idle. This rich ness will also cause an increase in popping from the exhaust on deceleration, as it pulls even harder on that circut upon slow down.
    HTH
    Kermit
     
  10. Mike328

    Mike328 F1 Rookie
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    Mike, I think that this could in fact explain it. The Throttle Adjustment Screws are pretty finely threaded, such that 1/2 turn shouldn't make that much of a difference.

    The question is, is there really a "problem" by having the throttle plates against the throat. I know that that sounds "bad," BUT at the same time my carbs' throttle action seemed like there was some INTERNAL stop against the shaft itself, and that it was not necessarily the plates against the throat metal that arrested their motion... Because I could re-open again easily without feeling the plates "Sticking" or like they otherwise had jammed.
     
  11. Mike328

    Mike328 F1 Rookie
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    Don't worry, I'm *already* the guinea pig here. So is Hans Hansen. BUT, he said his idles were very high with this "neutral" throttle plate positioned (something that can be remedied with the air balance and idle mix screws, I hope).

    Yep, I just bought an SK1 Syncrometer. To use this, I should put the air horns back on. Should I also put the airbox gaskets inbetween the air horns and the carbs, as well? Or syncing will the "leaky" non-gasketed air horns do just fine?
     
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  13. Mike328

    Mike328 F1 Rookie
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    Yikes! Drilling my throttle plates? :) I'll hold off on that for now... I'm currently running stock cams w/ stock settings, though, so should be OK.

    I do see the logic in drilling the plate though, it seems like it flows more air in the very low RPMs but otherwise has no effect at mid to higher RPMS...
     
  14. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    As Mike alluded to, I had a bit of a problem using the "Dr. Mike" method -- With the throttle plates just covering the progression holes I get an idle of over 2000rpm. I had to back off the idle screws a bit, thus putting the throttle plates a little 'under' the progression holes. And, no, you can't correct this with the air bleed screws, as they just let even more air in!

    It's interesting that your carbs are set up with the throttle plates just at the progression holes when fully closed. Mine will go WAY past the holes at minimum throttle. I don't really see how this was done. I have to give the idle speed screws about 3/4 turn to put the plates in their "proper" position. However, as mentioned above, this gives me an idle speed that is too fast.

    Setting the RH carbs to their "proper" position was pure guess work on my part, because they don't have their own idle speed screws. I based my setting on comparing the LH and RH carb's air flows. As I then had to back down the idle speed, I'm reasoning that it kept the carbs in some sort of throttle plate agreement. (Doh! I guess that's why they call it 'syncronization'.)

    Anyway, it's running real sweet now, and all carb surfaces and fittings are dry as a bone for a change. No more weeping gaskets or vacuum leaks. I'm a happy camper.
     
  15. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    "...It's interesting that your carbs are set up with the throttle plates just at the progression holes when fully closed. Mine will go WAY past the holes at minimum throttle..."

    I don't know for sure , but I have a (very) vague recollection of prior posts (somewhere -- not even sure it was here) showing different progression hole patterns on two similar 40DCNFs. Anyway, just wanted to point out that that might contribute to the different physical descriptions...
     
  16. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Aha!!!

    My 1975 Webers are model #s 45,46,47,48.

    I think Mike's are numbered in the 70's. So his carbs are perhaps a little different. This is beginning to make sense.........
     
  17. Mike328

    Mike328 F1 Rookie
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    Mine are definitely 72, 73, 74, 75.

    What are these? I know they identify models of carbs. Is a set of four something like a "version" number? E.g. I've got Hans's carbs v2.0? :)

    I wouldn't be surprised if progression hole pattern changed across carb revisions... My understanding is that these are fairly variable.

    Mine positively close at exactly the 1st progression hole. Mike at Pierce said this was how it was supposed to be.
     
  18. ham308

    ham308 Formula Junior

    Nov 3, 2003
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    Just to make it clear, I know buggerall about webers, but I did get the Haynes for christmas and my eyes light up like Mr Bean's at the sight of all those screws to adjust.

    Seriously, my idle's crap when its hot so I want to adjust it sometime, but I'm puzzled on one thing. Maybe people could comment....

    It says to use the air by-pass screws to balance the flow between the chokes, either with the pipe in the ear or an air-flow meter. Trouble is the by-pass circuit takes air from outside the carb, not through the trumpet. So how does the meter register this then? How do you hear it when then is no more flow through the butterfly?

    I think the by-pass screws are there just because both butterflies are fixed on one shaft and there is no other way to balance both chokes. It would send more air through a choke but how to measure it?

    Isn't this also a fault in the idea of using only the by-passes to set the idle speed?

    Anyway, can't wait to get my hands on them :)
     
  19. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
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    IIRC, the different carb numbering series (4X, 7X etc) correspond to different sizing of the progression holes.

    Ham308, I seem to remember a comment in a Ferrari book indicating that Enzo thought owners "should keep their gold plated screw drivers away from the Webers". Having said that, I've got a set of 44 DCNFs awaiting my unplated screwdriver to be fitted to my 308 so you are not alone in the passion to understand these devices in detail.

    That said, balancing the carbs is all about equalizing airflow through the chokes so that each cylinder is receiving the same amount of flow [and subsequently weight of fuel/air mixture from the mixture adjustment]. Flow is measured, in most cases, by using a synchrometer above the throttle plates (as has been noted), or, in some cases using manometer gauges connected below the throttle plates (better - I'm going to modify my intakes to accept a bleed valve which can be opened to measure vacuum and then closed again) and balanced one choke to another using our little friends the idle speed control valves (and then one DC to another and so on).

    You can set them to be balanced at idle (most people do and like the feel of the car when it idles evenly). Tod, the master tech at Continental tells me you can also set them so that the chokes are drawing equal air at higher rpm levels, and this strikes me as a real trick but would require dyno time to place the engine under load.

    Now, where's Russ Turner or James Patterson to set us all straight!
    HTH
    Philip
     
  20. 4re gt4

    4re gt4 Formula 3

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    Richard: RE: air source for air bleed screws. I checked this carefully, as I was wondering if the syncrometer would pick up adjustment in the screws. The source is *not* external, but is from a hole roughly 2mm in size just above the progression holes. The air is then fed (after the screw) into the choke system. Thus the screws bleed air from just above the throttle plate to just below. Note that the choke system gets it's air from large holes in the top of the carb. These wouldn't be read by the syncrometer. But the air source for the screws is definitely separate.
     
  21. 4re gt4

    4re gt4 Formula 3

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    Philip: You can set the carbs to flow evenly both at idle and with the throttles partially open. Note that it's not super critical to have throttle plates completely sync'd at big throttle openings, as the % difference in airflow would be minimal.

    However, try this: (it's what I did) - TEMPORARILY turn the idle screws up so you're flowing a respectable amount of air and idling at 2K-3K. Measure the airflow and adjust the linkages accordingly to equalize. Note that the air bleed screws will have minimal effect at larger throttle openings (try it and see!), so airflow balance is determined by the linkage. Now, turn the idle screws back to where they are supposed to be and fine tune the balance at idle with the air bleeds.
     
  22. 4re gt4

    4re gt4 Formula 3

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    Mike: Do you remember what your progression holes looked like?

    Mine had 4 'main' holes arranged in a diamond pattern. The lowest one was somewhat smaller than the others, except for a very tiny 5th hole located just beside the top hole in the diamond pattern.

    I wonder how Luigi came up with this arrangement.
     
  23. Mike328

    Mike328 F1 Rookie
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    Opening/closing the air bleed screws had no effect on my syncrometer. I was *not* tightening down the nuts again after loosening the air bleed screws; I'm assuming that this wasn't necessary.

    Re: Progression holes, I don't recall exactly what mine looked like but I should be able to check with this fancy extendable mirror of mine... They weren't quite as simple as a "diamond" shape IIRC... Will look into this.

    By the way, so far I've not been able to get a decent idle without using the Throttle Adjustment Screws (Idle SPeed Screws) at least a *little*. (sigh).

    I didn't really notice ANY effect on the air bleed screws.

    I did notice the effect of the idle mix screws. I would get higher RPM / idle by opening them up more than I had them. I'd estimate I'm four turns open right now.
     
  24. Peter

    Peter F1 Veteran
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    You should be just fine with the air-horns only.
     
  25. 4re gt4

    4re gt4 Formula 3

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    Mike: The air bleed screws are, uh, not real sensitive. A 1/4 turn will be (probably) not registered on the syncrometer. Make BIG changes here. (Not necessarily so with the other adjustments.)

    Mix screws: There are a couple of "tapers" on these. Some of these screws (mine) give a big result with minimal turns; whereas the 'long taper' screws take more turns to accomplish the same thing. I have not a clue as to OEM application of fast .vs. slow screws, but many guys rebuilding carbs will put in the newer long screws. More sensitive.

    I've got the old ones, and about 2 turns out will get the car running. It's my impression that you need about 4 turns with the long taper screws to start the car.

    Anyway, don't be bashful about doing some serious turns on the air bleed screws.
     
  26. Peter

    Peter F1 Veteran
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    I noticed that too on my car using the STE (and I know the hole Hans was talking about before, where the air is drawn in for the by-pass circuit, so yes, it should register on the flow-meter, but I guess maybe it's too small of flow).
     
  27. 4re gt4

    4re gt4 Formula 3

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    Oh, BTW, you don't need the air horns to use the syncrometer. Although the airflow may not be the same as air horns w/air filter, the *relative* value hole-to-hole should be the same with everything "naked". (Hehe. Did he say 'naked'? Hehe. Insert Bevis and Butthead joke here.) Just stuff the big ass rubber funnel syncro thingy into the carb.
     
  28. Peter

    Peter F1 Veteran
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    Sorry Hans, I posted at the same time as you (but only mine showed up and therefore didn't see your "by-pass" comments...)
     

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