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308 carb sync attempt #1 and questions

Discussion in 'Technical Q&A' started by Birdman, May 24, 2004.

  1. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,687
    North shore, MA
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    Hi Carb Experts,
    OK, so I got my syncrometer, read up and went out to do a carb sync today on my 308. I pretty much followed the 308 GT4 shop manual on this.

    1. Closed all air bypass screws
    2. Opened all idle mixture screws 3 turns from closed.
    3. Started and warmed car.
    4. Increased engine RPM to 1500 (about) by using the throttle cable adjustmenter (not throttle stop screws on left side carbs)
    5. Backed the throttle stop screws way out so I knew they were doing nothing and not interfering with my linkage adjustment.

    6. Adjusted the linkage between the carbs to get them all the same, in my case, about 7 kg/hr. (Keep in mind this is at 1500 RPM not idle). I feel quite confident that these adjustments are good.

    7. Once everything was pretty close at 1500 RPM, I adjusted the throttle cable adjuster in again to drop the engine speed down to closer to idle. At this point, I had to adjust the throttle stop screws to get an idle at the right speed (I was shooting for around 1000 RPM). Once I had the proper slack back in the throttle cable adjuster, I knew that my idle speed was now being set by the throttle stop screws.

    One thing I noticed is that you can get a pretty serious imbalance between the front and back carbs at idle if you only use one throttle stop screw. (I was thinking that with the linkage all set right, just the rear throttle stop screw would be fine because the front would be tied to the rear through the linkage. Nope. Seems that there is enough play in the mechanism that things are really dicey with the throttles nearly closed). I played with both throttle stop screws to to match the flow between front and rear banks and also get the idle speed around 1000 RPM. OK, so I got the idle set. Or so I thought. I'll come back to that.

    There was only one carb that actually needed tweaks to the air bypass screws to balance the flow between the barrels. So I tweaked that one and they all match. However, the car is still spitting through the carbs a little. And the idle speed seems irratic. I get it set nicely at 1000 then a few minutes later it starts to get lower and lower and if I don't give it some throttle or turn the idle screws up a bit, it stalls. But if I turn the idle screws (throttle stop screws) up, it shortly goes way up to 1500 or so. I can't get it to stay put at 1000 for too long.

    I haven't even tried getting the idle mixture set yet. (They are all still 3 turns out from closed). I did notice that if I open them more than the three turns they are currently set for, the engine speeds up, and if I close them, it backfires and the engine slows. Can the idle screws be part of the issue? I decided to call it quits for the day because I was running out of time and had to go pick up my daughter.

    Thoughts from the experts? Any ideas? Will working on the idle mixtures help this idle weirdness? It was also raining like cats and dogs today, so I didn't drive the car to see how the throttle response is. Just to remind you all, the reason for the sync was:

    1. Poor idling/backfiring
    2. Hesitation just off idle in the "transition area" between the idle circuit and the main jets.

    Thanks again!

    Birdman
     
  2. Steve Magnusson

    Steve Magnusson F1 World Champ
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    Jan 11, 2001
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    "Can the idle screws be part of the issue?" = Yes

    On the 308 40DCNF carbs, the idle mixture screws should be out about 4~5 turns -- not the ~2 turns as stated in the Haynes Weber manual IME (don't know if this is due to a special thread pitch and/or end cone angle on the F 40 DCNF idle mixture screws, but other 40DCNF are ~2 turns).

    The other thing to check/verify is that when it is running at ~1000 RPM (even if it's running very poorly/weird) that the ignition timing is about where you want it.

    My guess is that at 3 turns open you've got some cylinders that aren't really firing so when you open them up a bit more those start to contribute and your idle speed goes up (which means that you should lower the air flow some). What airflow reading per barrel are you getting at 1000 RPM?
     
  3. yelcab

    yelcab F1 World Champ
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    Nov 29, 2001
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    I am sure there are more than one way to skin this cat, and your method may be just excellent. But, i never did it that way. This is how i did it, for me it has worked.

    First, assuming that your engine is in good condition, with good compression, and low leakage, carbs are clean, the needle valves are new, and the floats adjusted correctly, then go on to the synch. (I did start at 3 turns out, by the way, for me)

    Hook up your tachnometer, and start with the set up the book suggested.

    1. disconnect throttle cable, remove front to back linkage, leave side to side linkage attached.
    2. Adjust all four idle speed screws so that the RPM is roughly at 1000 RPM.
    3. pick one carb on the driver side, measure the airflow for both throats, if there is a mismatch of airflow between the two different throats, adjust the high one to the low one using the balance screws. repeat for the other 3
    4. find the carb with the lowest air flow, leave it alone.
    5. Turn the idle screw on the other three carbs until the air flow match the lowest one. If the RPM is too low, adjust all idle screw up the same amount, recheck with air flow meter.
    6. Turn each idle mixture screw IN 1/8 turn at a time until the RPM drops, back up to the previous 1/8 turn, and stop. Repeat for all other 7, you're done. (This is best done with a CO meter, but not all of us have one)
    7. check air flow for each throat, they should all match up. If not, adjust the idle screw to match.

    At this time, you reattach the front to rear linkage, attach the cable, and you are done with adjustments at idle. This process almost guarranties that your carbs are balanced at idle around 1000 RPM.

    If you find that you are having trouble with keeping the idle, then something is wrong with your carbs, maybe the float is bad, maybe the carbs various orings are leaking, maybe the needle valves are worn out, or the adjustment screws are not seating correctly.
     
  4. thecarreaper

    thecarreaper F1 World Champ
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    Sep 30, 2003
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    I NEED A JOB
    great thread! and timely too, i will have to re tune the carbs on my 308 when it arrives this week, it went from florida, ( sea level) to new mexico ( 5000 feet)! now back to GA. ( sea level) i am assuming its going to run really lean. former owner just had carbs completely done by Pierce Manifolds. first i have to put a radiator in it so i can test everything else. i will be watching this thread with great interest! michael
     
  5. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
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    Hans E. Hansen
    YelCab is right in that there are many ways to "skin this cat". His advise is excellent, as is your well thought out methodology.

    But a few comments:

    1. Idle mixture screws seem to vary from model to model. Also, some owners have replaced the screws with long taper ones. Mine are original, and I get proper (perhaps even a bit rich) at TWO turns out. The long "Euro" (as some call them) screws need more like 4 turns out.

    2. Ignition timing - Are you running the 'second' retarded points? Retarded timing makes setting a stable idle easier. I have Crane optical pickups, and therefore my idle timing is not retarded. This makes a stable idle a touch more difficult. Adding to this issue is the centrifugal advance in my distributors is just starting to move at about 1000rpm. This means that if I overshoot the idle speed to, say, 1200rpm, I get a bit more advance, which in turn wants to make the car idle even higher. My timing seems to be more stable around 800rpm.

    3. "2. Hesitation just off idle in the "transition area" between the idle circuit and the main jets." The 'main' circuit doesn't cut in until well after idle. This is a common misconception. Not counting accel pump and choke, there are 3 main paths for fuel:
    a. At idle, fuel flows thru the idle jet and then the mixture screw. As the mixture screw is much more restrictive, it is almost entirely responsible for idle mixture.
    b. Just off idle - and up to 70 or 80mph steady throttle cruise - fuel is coming from the 'progression' circuit. This is a series of holes drilled in the throttle bore. The idle jet totally controls fuel in progression. (However, the idle mixture screw will still have a small effect.) Note that the idle and progression circuits are dependent upon vacuum to 'suck' the fuel out.
    c. When there is sufficient air flow, fuel will flow out of the main circuit. This circuit is flow dependent, relying on the venturi principle. Considerable amount of air flow is needed for this circuit to start metering fuel. There is little manifold vacuum up high in the carb where the main venturi is.

    If you are experiencing a hesitation just off idle, it may be either idle mixture (not enough fuel present before progression takes over), or too small an idle jet (not enough fuel just after progression takes over).

    Throwing more mud on the issue is my recent discovery that different models of these carbs seem to have progression holes in distinctly different places. Mine are quite high in the bore, meaning that I have to have considerable throttle opening before they are uncovered - to expose them to manifold vacuum. Later cars seem to have them very low, where the slightest throttle opening will uncover the first one. It seems that I either have to set idle mixture a bit rich to provide fuel until the first progression hole is uncovered, or else I can 'snap' the throttle open just a bit which enables the accel pump to squirt some fuel in during this transition. Setting the throttle plates (via the idle speed screws) open a bit to put them near the first progression hole results in way too high idle speed (around 2000rpm).
     
  6. bill308

    bill308 Formula 3
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    Birdman,

    Your synch procedure is very good. The problem is likely, as Steve pointed out, your mixture is too lean to support reliable combustion. This hypothesis is supported by your observation that the engine is spitting back through the carbs and your experience of hesitation in the transition between the progression and main circuits. Assuming that the float levels are correct, open up (enrichen) your mixture screws to about 4.5 turns from lightly seated. This should put you on the rich side of where you want to be and was about right for my stock 40DCNF 72/73/74/75 (USA model years 78-79). With older carbs, there is always the danger that some time in their history, the mixture screws may have been tightened too much and distorted its mating seat. Any distortion of the seat will likely affect the needle to seat geometry and negate accurate metering when the screws are opened up to 3 turns. After I fitted a new set of 40 DCNF 45/46/47/48 carbs, with stock jetting and float level for this series, I found the idle mixture screws wanted to be about 3 turns open from lightly seated even though these carbs are spec'd for a 50 mm float level verses a 48 mm level for all other series. You can lean out the mixtures as necessaory after you establish a reliable idle at the proper rpm. Which series of 40DCNF are you using and what idle jets?

    Ball park air flow at 1000 rpm is about 3.5 kg/hr on each throat.

    Remember this is an iterative process. After you get the mixtures about right, recheck and adjust your balance and then the mixtures again as necessary.
     
  7. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Hans E. Hansen
    Again, there are different taper idle screws. The early cars work just fine at only *2* turns out - as specifiec in the shop manual. Fine tuning will often result in only about 1 3/4 turn. But, as I said, many cars have been retrofitted with long taper screws to make it easier to do very fine mixture adjustments. Also, it seems later manufactured replacement carbs have longer taper screws.
     
  8. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
    2,279
    Roseburg, OR
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    Hans E. Hansen
    I'll throw one more log on the fire.......

    What I'm about to describe is probably not related to your driveability problems, but I merely mention it to point out that unexpected things can make your day aggrevating.

    I was having a miss under certain circumstances. I chased it around, rebuilt carbs, sync'd them (several times), and changed plugs and wires. Nothing helped. It acted like the progression circuit(s) on one or more holes wasn't working right.

    I had managed via spark plug inspection to find the guilty cylinder. I was beginning to suspect that there was something mis-machined in one of the carbs. But first I inspected the idle jets. You could look thru them, blow thru them, and the holes even all gauged correctly. After a couple of six packs, I got the bright idea of installing a different idle jet in the lean hole. All I had was one a step smaller. It worked. It richened up the cylinder. I put the suspected bad jet into a different place and it leaned out that cylinder.

    Now, by all appearances, this jet is 'correct'. Measures to be the right size, undamaged, etc. But the damned thing doesn't flow the same as the others. I'm in the process of rigging up a flow device to see how much fluid actually flows thru all my jets, and use them accordingly.
     
  9. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
    6,687
    North shore, MA
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    Hi Guys,
    Thank you so much for all the thoughts. Man, I love this board. Some great info here and I'm learning a lot. I studied the Weber manuals for days before I started this task so I would understand everything. Seems less like "black magic" now! I knew that I should stop and get some advice today before proceding. Now I'm re-armed with more knowledge and I'll have at it again tomorrow. I'm going to tweak those idle mixtures a bit. To clarify some of the issues addressed:

    1. I pulled and cleaned the idle jets overnight in carb cleaner. They actually looked fine when I pulled them and I almost just put them back in. I could blow through them fine and I could see light through them. They seemed clean as a whistle, but I soaked them overnight anyway. So now they are super clean after a soak. They are the stock 55's that are supposed to be there according to my owner's manual. I replaced the o-rings on the jets too. The old ones were pretty squished.

    2. The idle mixtures were richer than 3 turns when I started. Most went in about 4 turns when I checked how they were previously set before I started. I was going by the factory service manual of starting with 3 turns out, but I realize this is not exactly a scientific measurement. Thing is, the plugs are dark, indicating a rich mixture, but the car seems to be exhibiting lean running characteristics (spitting, bad idle, hesitation.) What up with dat?

    3. Not sure what series the carbs are. Where do I find that info? On the carbs someplace I assume. I'll look tomorrow. It's a '77 and I'm 90% certain they are original.

    4. I do not have the equipment to test the timing but I had it checked last summer by a Ferrari place in the Boston area and they tweaked it to be "right" (don't ask me for numbers!) and said the advance was working great.

    5. No R2 points. Previous owner ditched them along with the air pump.

    Thanks again and if you think of anything else, let me know! I'll post results after tomorrow.

    Best,
    Birdman on a high-carb diet!
     
  10. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    About 3.5-4 kg/hr. Hard to be exact, as the idle speed is floating around a little. How does that sound?
     
  11. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Hans E. Hansen
    If the car has had any mods, such as K&N, aftermarket exhaust, etc., you may have idle jets that are too lean. Driveability probs with lean jets are often masked by backing out the mixture screws - this gives a mixture way too rich at idle, but makes off-idle work better. You might try bigger idle jets and lean down the mixture at idle. Worked wonders for me.
     
  12. 4re gt4

    4re gt4 Formula 3

    Apr 23, 2002
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    Hans E. Hansen
    That's what I get.
     
  13. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,358
    NY
    Good thread here. I need to turn out my idle screws to 5 1/2 to 6 turns to get it running like a clock. It is a little rich at idle but runs great all the way up to red line. Also the air bal is at 4" for all carbs except the rear drivers side where the vacuum hose is attached. This runs around 7" on one throat. Idle is 1000 rpm and dead on. Starts and runs very good.
     
  14. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    Hmmm. The car has an Ansa exhaust, stock airbox and filter. What do you think? Will the Ansa affect jetting? It came with that muffler so I never considered jetting issues, but it does have the stock 55 idle jets. Certainly worth a try. Where does one get idle jets? (I'll try a Google search but if you have a recommendation, I'll try it. How much can jets cost? (I may be sorry I just typed that).
     
  15. Birdman

    Birdman F1 Veteran

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    I almost forgot....which size idle jet would you try if I have 55's now?
     
  16. bill308

    bill308 Formula 3
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    Most of the usual Fcar sources can supply jets. Pierce Manifolds. in CA, is another source. A ball park price is about $5 each. The next standard size richer is a 60 (.6 mm) idle jet.
     
  17. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    Steve, what kind if exhaust do you have? From your bio pic it looks like Ansa. Just curious because we also have the same year car. Are you running stock 55 idle jets I assume?

    I'm going to order a set of 60's tomorrow. They are $3.95 a piece here:
    http://www.cbperformance.com/catalog.asp?ProductID=1162

    Birdman
     
  18. Doug

    Doug Formula 3

    Nov 13, 2003
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    Doubt you will have any luck with CB. They are a VW Bug shop and I called about DCNF jets a few months ago. They had not had any in years.
    Check the ferrariads in this forum for parts. One board member is selling various DCNF jets for $20/per set of 8
     
  19. Birdman

    Birdman F1 Veteran

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    FYI I just bought them from Weber Parts Direct. Same place I got the sync meter. Price was just under $5 each. It's a cheap experiment but I really think this might help my idle and hesitation issue.

    http://www.webercarbsdirect.com/idle_jets
     
  20. Birdman

    Birdman F1 Veteran

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    I didn't notice a vacuum line on mine but we have the same year. Where does that line go?
     
  21. Mike328

    Mike328 F1 Rookie
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    Here's the pic of the carbs on my 78.

    First thing you should do is identify the series of your carbs... e.g. mine on a 1978 were 72/73/74/75. They're stamped on the back face of the carb, basically on the rear wall of the float bowl. It's small print and you may have to clean to see.

    Second, on my 78 308, there was an additional vaccuum line on the driver's rear bank carburetor intake manifold... You can see a small hose in this picture coming from the manifold there (oriented vertically in the picture)... I think that's the line.

    --Mike
     
  22. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,358
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    Ansa and I assume they are stock 55's
     
  23. Verell

    Verell F1 Veteran
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    May 5, 2001
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    Jonathan,
    ANSA mufflers were stock on the earlier 308s, and up thru the Euro 308 QVs stock mufflers were made by ANSA also. The muffler input differed between the US spec & Euro mufflers, and I believe between the carb'd w/Thermal reactor & catalytic models.

    So the key question is do you have a stock ANSA, or a high-perf aftermarket or Euro one? The Ferrari brand & P/N is stamped on the factory mufflers, either on the pinch weld, or on a metal strip spot welded onto the muffler.
     
  24. pcelenta

    pcelenta Karting

    Nov 1, 2003
    216
    Hans makes a good point...the carbs require vaccuum to pull the fuel. I was going nuts trying to set my carbs up this past weekend..when a friend passing by said..."you should have these things tuned by now...something isn't right" (I had just spent the prior weekend replacing the exhaust donuts thinking I was sucking air in there)...turns out I had vaccuum leaks every where...the hoses coming from the charcoal canister that pump fuel vapour back into the rails under carbs were all loose and dry rotted...one was even charred..also, the vaccuum booster line going into the left hand carb was loose and it too had a ton of dry rot. bought all new hoses and am going to give it a shot today...bottom line, check out stupid little things like this before you start changing jets and things..'cause its impossible to set them right with leaks in the system.
     
  25. Birdman

    Birdman F1 Veteran

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    I haven't gone over the muffler really closely but I don't think it has a Ferrari name on it. It does say Ansa. I don't think it's a thermoreactor. What years did they use those? I find it unlikely that a 27 year old car still has the original muffler on it.

    I went and examined the vacuum line. I have only one, on the manifold (driver's side, rearmost carb, left barrel) going to the brake assist. Funny, I never noticed it until I looked. I wonder if that explains why that one carb barrel needed a lot more air bypass? To me that would mean that the vacuum line is leaking. The hose from that vacuum port on the manifold over to the one way valve (about 8" of hose) was really ratty. I'm going to replace it. But when I pulled the one way valve out of the hose that goes around to the brakes, it made a nice "pop" as the air sucked in when I pulled the valve out, meaning it was holding a vacuum since yesterday. My question is...if the vacuum is good (not leaking) then I shouldn't have to use so much air bypass. What gives? Maybe that one little ratty hose was leaking a bit.

    I decided not to diddle with it today. Still crappy weather for driving and I'll wait for the jets to come. My last order with those guys came in like 2 days. They said they had them in stock. Can't wait to try them out because I think they are going to help. All symptoms point to lean idle jets.
     

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