DCNF 40 carb questions | FerrariChat

DCNF 40 carb questions

Discussion in '308/328' started by bl10, Oct 13, 2024.

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  1. bl10

    bl10 Formula Junior

    Jun 8, 2011
    440
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    Quick question on DCNF 40 carbs. Have to get it through CA smog and no I'm not leaving the state.
    1978 GTS US with all emission gear.
    Pertronix Ign, coils and Iridium plugs.
    Stock plug wires and low resistance plug extenders.
    Initial ign timing set at 7 degrees BTC at 1K Rpm with 34 degrees at 5K Rpm
    Stock cams set at 111 degrees lobe separation angle and 2 degrees retarded.
    Insulation removed from air box.
    Vacuum lines per Steve M's recommendation.
    Carbs completely closed with 2 thou gap between stop and idle speed adj screw which insures the progression passages are covered or above the throttle blades..
    Carb Air Bypass screws closed.
    Carbs flow between 3.5 and 4 kG/H
    Jets idle 60, main 130, emulsion F36, accelerator 40, air correction 200
    Stock 55 125 F36 40 200
    Sealed throttle shaft bearings with careful alignment of throttle blades it insure as complete close as possible..
    Idle mixture screws 4 to 4 1/2 turns out. (1/4) turn rich after picking up cylinder when screwed out from all the way closed.. Set with engine hot with both fans on and AC off.
    Gas CA 87 octane with 10% ethanol.
    Idle RPM hot with both fans on 980 - 1000
    Idle Rpm hot with both fans and AC on 900 - 950
    Engine vacuum at idle 19 -20 inches of mercury.

    A quick explanation as to why the carbs are completely closed at idle. Since I run the stock advance curve which provides 34 degrees advance at 5k rpm on the R1 points and the Pertronix doesn't use the idle retard R2 point circuit I have to advance the initial timing at 1K rpm from 3 degrees ATC (stock) to 7 degrees BTC which speeds the idle up therefore necessitating backing off the idle speed screws all the way to get a slow enough idle speed. Apparently there is enough air leakage around the throttle blades and the US emission system at high vacuum.

    Also the cam(s) adjustment is an attempt to reduce overlap and popping back at cold idle as well as provide a little more top end.

    Car runs great past red line with no flat spots.

    Ok now for the questions for you Weber gurus.

    I'm trying to understand the idle jet circuit. In reviewing the Weber diagram it looks like the only fuel being delivered at idle is via the idle jets with the amount being controlled solely by the idle mixture screws. So as far as rich of lean at idle the idle jet size or main jets have no effect assuming a satisfactory idle can be attained with the idle mixture screws and the progression holes are covered. Once the progression holes start to uncover the mixture is controlled by both the mixture screws, idle jets and starter jets. The main jets, air corrector jets and emulsion tubes have no affect until there is enough pressure drop to pull fuel through the auxiliary venturis. Is this correct?

    My concern is in reading the various threads about the carbs people say running to big (60 vs 55) idle jets will cause a rich idle. How can this be if the idle mixture screws control the idle mixture?
    I do understand that a larger idle jet would cause the progression mixture to be richer.

    How much affect do the idle jets and progression passages have at partial throttle? Is it enough to make it go rich during the CA smog dyno cruise test which is run at 2500 RPM probably in 2nd gear with some load?

    Does anyone know how much of a jet change if any would be required to compensate for gas with 10% ethanol? (I ran 60 idle and 130 mains before ethanol)

    Just trying to cover my bases so I don't get ensnared in CA smog prison.

    Its always passed before but I've had to change one of the cats so am a little worried.

    Thanks in advance
    Barry
     
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  2. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,848
    Atlanta
    Full Name:
    John!
    The idle circuit is delivered fuel from the idle jet. At idle with the throttle plate almost totally closed, it only gets fuel from the idle hole, which is controlled via the screw. I typically go to the next size idle jet if I need to have the screw backed out more than 4.5-5 turns. The idle jet is also responsible for the amount of fuel that is delivered to the transition holes. Those holes see fuel delivery through 3500-4000rpm, or thereabouts (also depending on how open the throttle butterfly is), to transition to the main jet. At partial throttle, the only jet doing anything is the idle jet by way of the transition holes. That emission test at 2500rpm cruise is pretty much a test of the idle circuit. From what I've seen, stock idle jet size is best for this. For best running in general on modern fuel, the idle circuit likes a 57 or 60.
     
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  3. bl10

    bl10 Formula Junior

    Jun 8, 2011
    440
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    Ferraripilot
    Thanks for the reply. That's what I thought. I'm currently running 60 idle jets and 130 mains. Since the throttle blades are completely closed at idle due to the ign lead there is no way it will use the mains at 2500 rpm in second gear on the dyno in fact it probably won't completely uncover the translon holes. The idle mixture screws are already 4 1/2 turns or so out which makes me leary of putting the 55's back in. I'm thinking the 10 percent ethanol fuel. (it may be higher knowing Calif). is causing it run slightly leaner thus necessitating the larger jets. It seems to me back in the 80's and 90's before ethanol I ran it with the mixture screws about 3 1/2 turns out with the 60's, however, that was with the R2 points working so the timing was 10 degrees further retarded at low speeds.

    Another quick question. Have you ever used one of those tail pipe clamps with a wide band O2 sensor to check the mixture. I've read pros and cons about them, however, since the O2 sensor would be in pretty much the same location as the smog sniffer I would think it would produce about the same results. The one I'm looking at is made by Inovate.

    Thanks for you replay
    Barry
     
  4. bill308

    bill308 Formula 3
    Silver Subscribed

    May 13, 2001
    1,224
    Windsor, CT
    Full Name:
    Bill Sebestyen
    Yes, this is what I would expect.

    I believe you are correct in all respects.

    Off Idle may go rich upon uncovering the first progression hole if the emulsion has more liquid in it because of the larger jet. If at idle, running solely on the needle valve supply, a richer emulsion could be compensated by leaning out or reducing flow with the needle valve.

    I'm not sure how to establish when the mains come in but I always considered you were on the progression circuit and transitioned to the mains around 3500 rpm. I would think you would be on the idle/progression circuit during a 2500 rpm steady state test and a light cruise load.

    My 78 is exempt from emissions testing in my state due to age. I am currently sorting out the jetting on my new 3.4 liter build that uses DCNF40's. I have now used the sniffer tubes successfully for the first time by reading the CO concentrations with a Heathkit ci-1080 gas analyzer. I was able to successfully adjust and compare the idle mixture for all cylinders. Two or three cylinders were overly rich at idle but were easily identified and rectified. I will now repeat the process with the air filter in place. I like to get the initial individual cylinder idle mixtures correct before evaluating progression using a wide band sensor like my LM-2.

    I hope this helps.
     
  5. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,848
    Atlanta
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    John!
    I never use a wideband that is connected just to the tailpipe unless the car is totally static on a dyno, too inaccurate and downstream. I always have a O2 sensor bung welded to the 5-8 bank long pipe usually somewhere near the gearbox. I jet for cruise (freeway driving) around stoic (14.7:1) and wide open throttle at 12.5:1 - 13:1 in 4th gear all the way to redline. Best done on a dyno to be safe but a person with a good passenger noting air/fuel reading can get you very close. With abrupt throttle movements the air fuel will dip into the 10:1 range due to the pump jet. Stock engines usually land around 200 air corrector and 140 main jet with 55-60 idles. Won't pass emissions like that in my experience though. Have to run it lean and hot, won't run great but it should pass.

    I cannot recommend having a couple MSD boxes enough for all carb cars. Will need a tach adapter but man do the MSD boxes help a ton. I mount them in the trunk just behind the left (driver side) rear wheel. Wires running through a small hole. Points still used to fire, but note the points last a ton longer when they're just firing an MSD box.
     
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  6. bl10

    bl10 Formula Junior

    Jun 8, 2011
    440
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    Thanks
    Yup that's what I'm afraid of with the tail pipe wideband. I don't have wideband bungs on the exhaust yet. Do the MSD boxes let you set the advance curve? I'm already using the Pertronix with their low ohm coils. Its got plenty of spark just no adjustability to the advance curve.
     
  7. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,848
    Atlanta
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    John!
    No, the springs in the distributor still control that. Not a huge fan of the Pertronix stuff but others seem to have success

    If you want full control of the ignition curve, the distributors themselves need to be deleted and a crank fire system, such as Electromotive used to have, though they are no longer around. Maybe ebay for the components or there has to be some other ignition box on the market that does the same.
     
  8. Howard A.

    Howard A. Karting

    Jun 23, 2010
    68
    Hershey PA
    Since it appears we have some Weber skilled people on this call I have an odd problem with my 1979 GT4 2V 2 dist. all stock, no second R2 switch, no air pump, all ports plugged (I believe).Timing is set to the book 6 degrees and 34BTDC at 5000. This car has never idled correctly. it either slows until it stops or when warmed up idles faster and faster. When cold will idle okay for the first few miles on the fast idle cam. By the time I get to the first light to wait for the green it is starting to speed up so it runs at 1200-1500-1800. Then off to where ever I am going at more open throttle and runs fine. Next stop and the idle slows and slows until I need to blip the gas or hold the throttle open with my heel. Carbs have received full kits 2 times including new throttle plate bearings with no change. ( no lead plug cleaning but new all other brass emulsion tubes etc. The throttle plate closures all match with .002 feeler gauge, Floats are all at spec height, I get a bit of back fire, I have seen gas dribble down inside 1 carb with only the fuel pump on, but swapped top plates with another and have not seen it again. Chokes are all closed tight as far as I can tell.
    Okay, here is where it gets odd. On an uphill warmed up the idle speed will go up to 2000 or more. On a down hill it will do the usual slow and slow. I have lowered the floats a mm or 2 and I have raised them the same to no avail other than make things worse. I have adjusted the idle timing a few degrees more and the a few degrees less. I have messed with the idle mixture, all are balanced for air flow. I believe idle jets are 65 and only out about 3 turns.

    The only thing that is influenced by tilt of the car is the floats that I can think of.
    This has been going on for 10 years or more.
    Help!
     
  9. bl10

    bl10 Formula Junior

    Jun 8, 2011
    440
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    Sorry this is so long and disconnected, just thinking out loud. In thinking about it after I wrote all this I doubt this is a carb problem. Either ignition advance or some kind of vacuum leak.

    I've just been through this on my 78 US GTS. I live in CA so have to run ethanol contaminated gas and pass tough smog test every two years.
    I failed smog due to my misunderstanding of how the air injection pump controller works but before I discovered that I went deep down the carb rabbit hole. I have been running 60 idle jets and 130 main jets for ever and it runs great, however the idle speed (to fast) has been a continuing problem from what I believe is caused by the Pertronix ignition not having the 10 degrees retard at idle provided by the R2 points in the stock system.
    It now idles great at 1K or a little less with AC and fans off and about 970 with AC and fans on.
    So in order to pass smog, before I realized I had the air pump messed up, I reinstalled the stock 55 idle jets and 125 main jets. The idle was so lean I couldn't adjust it rich enough. Popping back through the carbs like a shotgun. I had the idle mixture screws 6, yes 6, turns out and it was just terrible. I reinstalled the 60 idle jets and 130 mains (the mains have no effect on the idle) and all is good again and it passed smog with flying colors. (air pump hooked up correctly)
    Something to keep in mind is that the idle jets not only affect the idle mixture but the transition also.
    As far as the idle goes mine does take a couple of seconds to come back to a stable idle. When I coast to a stop it falls to 1100 then settles down to 1000. I don't have the dash pot installed.
    My carbs are synced to within an inch of their life. I sync them to where the flow is as equal as I can get it with the throttle blades closed then adjust the carb connector screws for min RPM.
    The flow is about 3+ Kg / hr on 3 carbs and 4Kg /hr on the drivers side rear. (which is the carb that has the brake booster vacuum line take off so there may be something there)
    The carbs have sealed throttle shaft bearings. (probably not necessary and al lot of work)
    The air balance screws are all the way closed.
    The idle speed screws along with the throttle blades are all the way closed.
    The carb linkage is adjusted to keep pressure on the drivers side rear carb.
    Double check the cold idle cam adjustment at it can cause it not to idle down when hot if misadjusted. .
    Timing is 34 degrees at 5K and 7 degrees BTC at 1K idle. Only one dizzy is used for advance with the Pertronix.
    Fuel pressure is 6PSI.
    Vacuum line to carbon canister is restricted.
    Vacuum line routing per Steve M.
    I adjust the idle mixture with engine hot enough to turn fans on by closing the mixture screw until the cylinder drops then back out until it picks up again then out 1/4 turn more. All idle screws are 3 1/2 to 4 turns out.

    Generally speaking I find engine temp makes a difference in the idle speed. On a long, 60 or 70 miles, drive at 4K RPM the idle will increase about 100 Rpm until engine cools down a little. Probably because I have the throttle blades all the way closed at idle and heat soak probably affects that.

    For mine at least 65 idle jets would be to rich.

    I wonder if maybe you have and dizzy advance problem. On the stock (points) system each dizzy's advance curve controls the advance for its bank. Since your idle is so inconsistent I would take a look at it. When I got mine running after sitting for 12 years and before I installed the Petronix I cleaned and lubed both advance mechanisms. The rear one was very dry and notchy feeling. I also used a micky mouse distributor machine utilizing a drill motor, dwell meter and tach to check both.

    Have you checked for a vacuum leak in the brake booster circuit. You might try blocking it off at the carb and see what happens. Not sure how well the brakes would work with no booster!

    Good Luck
    Barry
     
  10. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
    3,023
    I.M.H.O.

    How far are you out of spec on SMOG test?
    IDLE:
    CO / HC

    2500Revs:
    CO / HC

    What is the condition of your air pump? Did you ever measure the 'pressure" of the output? The spec is ~6psi when spinning fast. It must push past the check valves.

    Condition of the check valves? The crack pressure is ~ 1/2 psi. They both need to be close (for uniform flow to both banks).

    Lastly cats? Are they a known quantity?

    In CA, these parts must work!

    Then of course... The engine needs to be tuned reasonably well. NO missing. If it's stumbling while running, the air & cats might not be able to cope with SMOG load.

    High-ish CO at all the inspection tubes (like 3 - 5%), with HC <300ppm should get you there to blow zero's, or near if emission parts are doing their job.

    A good MSD etc helps all around! When the car is tuned well, it actually runs very well as well and runs clean. The one downside that concerns me is doing a track event. Even gently, spinning the 308 at 7,500 rev's bugs me when AIS and CATS are providing even higher EGT. The new cars with metallic substrate CATS and very high precision fuel delivery can get away with much more of this.

    How we love'm so!!
     
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  11. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
    3,023
    RE: Your comment on cam timing..
    "Stock cams set at 111 degrees lobe separation angle and 2 degrees retarded."

    Is this a stock setup? I'm used to following LC for each cam relative to the OM recipe.
     
  12. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,848
    Atlanta
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    John!
    6psi is way too much fuel pressure. 3-4psi absolute max
     
  13. Howard A.

    Howard A. Karting

    Jun 23, 2010
    68
    Hershey PA
    I found that I had a filter in the line just prior to the right side fuel tank that was clogged and the pressure dropped to 3-4. In the hopes that this would solve my idle problem I was thrilled to find this problem but to no avail it idled just as poorly fast slow etc.
     
  14. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
    3,023
    Idle poorly... Can you detail that? Won't idle? Returns to different idle speeds? Misfires? etc.
     
  15. bl10

    bl10 Formula Junior

    Jun 8, 2011
    440
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    2dinos - No stock is 109 degree separation and straight up (no retard or advance) I have it set that way for 2 reasons.
    1. I reduces the overlap slightly so It doesn't pop back much when cold.
    2. In theory, the wider cam separation angle gives it little more top end due to slight increase valve closing timing and the overall retard causes the valves to open and close later thereby making the engine think it has slightly larger cams.

    The down side is that it reduced cranking compression a little and reduces low end torque.

    Ferraripilot - The 6 Psi fuel pressure is at a dead idle with no load on engine (fans and AC off). I had a gauge on it trying to solve another problem and under full throttle going through the gears the fuel pressure would drop to 4 psi by the time I got to the top of fifth gear. I have a carter P4070 turbine fuel pump on it as the original died 30 years ago and was really expensive to replace. The carter flows 50 gph free flow at 4 to 8 Psi . I don't know how that compares to the stock pump. I have never had any trouble with the needle and seats leaking which is a symptom of to high fuel pressure.
     
  16. sltillim

    sltillim Formula 3
    Silver Subscribed

    Nov 22, 2009
    1,695
    San Diego
    I feel lucky everytime I see this with my '75. However I see all these tuning efforts yet few are going out and building a modern CARB compliant cat exhaust to bolt on and pass every two years. The money just seems so worth it. Instead of tuning, testing, tuning, testing back and forth, figure out how to get to the right AF ratio so you don't burn the cats and go get it tested. What's your time worth?

    This one is on ebay right now: https://www.ebay.com/itm/334658759397?chn=ps&_trkparms=ispr%3D1&amdata=enc%3A1RHSK_8hpQ22znSLMPln9Zg13&norover=1&mkevt=1&mkrid=21562-222008-2056-1&mkcid=2&itemid=334658759397&targetid=325425753764&device=c&mktype=pla&googleloc=9198345&poi=9041294&campaignid=21384589900&mkgroupid=164552185618&rlsatarget=pla-325425753764&abcId=&merchantid=114701160&gad_source=1&gclid=Cj0KCQiAi_G5BhDXARIsAN5SX7pBHBlPLaVazqQLzsuVdmYLuJsmU1z418VCvpcXx4xWXcqi_4rLfe4aAns-EALw_wcB

    You can bring an aftermarket exhaust as long as it has the CARB id's on it. Modern cats have to be way more effective than the 50 year old ones on there.

    Here's an interesting more recent thread: https://www.ferrarichat.com/forum/posts/146391379/ that states for pre OBDII cars all they need is a carb compliant cat(s) and they do not have to reference original equipment on the car - and I checked this here https://www.bar.ca.gov/pdf/smog-check-manual.pdf on page 40.

    In other words - build a modern exhaust that burns way cleaner, reinstall your air pump to get the 14.7:1 - the carbs likely won't do it on their own.

    I'm theoretical here. Am I right or wrong or way off? I am just thinking if I had to do this every two years....
     
  17. Ferraripilot

    Ferraripilot F1 World Champ
    Owner Project Master

    May 10, 2006
    17,848
    Atlanta
    Full Name:
    John!

    Hardi 11812 fuel pump is the one

    https://pmbperformance.com/products/hardi-fuel-pump-for-carbureted-911-and-914-6?variant=43194985414834&country=US&currency=USD&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&srsltid=AfmBOoo_DqQ_ntj842fxAjpvvb_anKDvUkG8paqi2sC9gvC9BzPwPQf_TyY
     
  18. Howard A.

    Howard A. Karting

    Jun 23, 2010
    68
    Hershey PA
    1. Has never idled on it's own at least 20 years. Either ran slower and slower until it stalled or ran faster and faster until it was 1500 -2000.
    2. Weird: On a hill headed up, went faster and faster, On a hill headed down slowed and slowed as above.
    3. Pops and backfires regularly especially #4. So I replaced the brake vacuum line and the check valve all the way to the front as well as all the vacuum lines up front.
    4. Pulled off the intakes and cleaned out the water lines, Flattened/lapped the mating face, new gasket and thin coat of sealant. all new carb gaskets, careful set of float heights.
    5. 4 new carb kits and Ignition and Coil wires. (a while ago).
    6. Just scrubbed and gapped the points, and tried to set the timing but was very jumpy at 6 degrees and 34 degrees (Front and Back). Didn't do a "Dwell Angle" ...yet. maybe this week. Break out my old SEARS Dwell Meter from when I had cars from the 60s.
    7. Not done: No New coils, condensers, and 12v wires, reset the open close angles as called out in shop manual
    Help
    Howard A.
     
  19. bl10

    bl10 Formula Junior

    Jun 8, 2011
    440
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    Howard
    The carb with the vacuum booster line (your #4?) does run a little different on mine. It flows slightly more than the other three no matter what I do and believe me I've done everything I can think of. If you have 65 idle jets that's pretty rich, especially if your running non ethanol gas. In the 80's mine was fine with stock 55 idle jets, however, once CA switched to gas with ethanol it ran like crap especially wen cold. 60 idle jets cleared it up. I think mine would be way rich on transition even if I could get it to idle correctly with 65 idle jets. Your comment about faster or slower depending on uphill or downhill seems to indicate carb/float problems, however, since the floats sit sideways I don't see how it would make that much difference. The stock settings are just fine on mine. If anything I would lower the floats a bit not raise them. Your comment about jumpy timing on both banks would be a concern. Mine is rock solid both with points and now Pertronix.

    I assume from your comments it runs fine at higher RPMs. So its only the Idle and maybe transition that is a problem?
    You have probably already done all this but here goes.

    Carbs:
    You mentioned your butterflies are set at 2 thou. Mine are all the way closed. I had to file the carb body on the #4 carb to get the carb to close completely.
    You mentioned the carbs are synced. What is the flow KG (should be 3 to 4) and is the dial (for lack of a better word) steady or bouncing around.
    Do you still have the thick fiber insulating gaskets between the carbs and intake manifold? (fuel percolation)
    Did you change the needle and seats when you rebuilt the carbs. Just because you can't see it doesn't mean it isn't leaking although mine still has original needle and seats as well as all gaskets. You mentioned dribbling fuel. Where was it dribbling from?
    Make sure the chokes are completely closed. Mine work fine but others have trouble with them no closing all the way and playing havoc with the idle. I believe its possible to install them incorrectly (Upside down) and or miss the slots with the controlling levers which might put them in an open position even when the levers are closed.
    If your car has the flapper in the air box have you checked its function.
    Did you change the needle and seats when you rebuilt the carbs. Just because you can't see it doesn't mean it isn't leaking although mine still has original needle and seats as well as all gaskets.
    You mentioned dribbling fuel. Where was it dribbling from?
    What do the plugs look like after idling a while. Mine are light brown.
    As Ferraripilot said 3 - 4 lbs fuel pressure is plenty especially at idle. I run a little more for a different reason.
    I have a need to run mine once is a while with the back end high on jack stands and the front end on the ground. The idle is pretty much the same as level.

    Ignition:
    If it runs OK at high RPM I doubt its a spark intensity problem but you never know.
    Have you verified voltage coming into the coils? (yellow wire) It should be full 12 volts.
    You can check the coils and ballast resistors with a Ohm meter.
    Have you checked and lubed the advance mechanism. They can get dry and sticky. The advance curve should be smooth up to 34 degrees at 5k.
    When you said 6 degrees initial timing I assume that is BTC not ATC.
    I have noticed that even with the Pertronix, low resistance coils and no ballast resistors the stock (original 78) plug wires make my induction timing light a little erratic.
    What spark plugs are you running . I run NGK Iridium as back in the day with the standard plugs if would foul one once in a while on long down hill coasts while in gear. My current set have 14K miles on them and work great.

    If you just let it run and heat up in the garage until the fans come on (which is when I adjust my idle mixture) does the RPM still drop/raise when hot?
    I adjust my idle mixture by starting at 4 1/2 turns out (60 jets) then lean it until cyl drops then richen it until it picks up the cyl than another 1/4 turn rich. If it pops back when cold I richen that carb(s) another 1/4 turn.

    As example
    Mine when I start it dead cold with no choke, carbs full of gas, pump 4 times idles at 750 to 800 RPM. Not real happy but keeps running and is drivable. Virtually no popping back through carbs. As it warms up the idle speed will gradually increase to about 1100 until it drops off the high speed cam at which point it drops back to about 975 to 1000 with no load and 925 to 950 with AC and fans on. Just had it out yesterday on a 210 mile FCA drive and idle RPM probably didn't vary more than 50 rpm all day with lots of stop and go along with freeway driving.
     
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  20. Howard A.

    Howard A. Karting

    Jun 23, 2010
    68
    Hershey PA
    I still have options. Maybe I can go look in my Carb stuff box and go back to smaller idle jets but I am 2 1/4 turns out all around with the 65s. If you have 0 clearance how does idle air get in? I did have a sticky advance slot on one carb, but went in and fixed that on one carb maybe I need to revisit, also on both.
    You say "If you just let it run" even at its best that would surely be less than a minute.
    "dead cold with no choke, carbs full of gas, pump 4 times idles at 750 to 800 RPM" this is the same until I get a mile away then I need to touch the gas to keep it going.
    Plugs are dark Brown after warm up drive loop. No flapper in the air box, carbs are now open top while fiddling with them.(idle is the same as with Air cleaner)
    Have you verified voltage coming into the coils? No but I did notice that my inductive timing light causes the idle to drop. ????? Click slow, click faster?? Maybe I'll check the coils and ballast resistors with meter.
    Chokes, install them incorrectly (Upside down) and or miss the slots with the controlling levers which might put them in an open position even when the levers are closed. I recall the chokes moving correctly when it was apart 10 years ago. Also place to look /check
     
  21. bl10

    bl10 Formula Junior

    Jun 8, 2011
    440
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    Howard
    My idle mixture screws are about 3 1/2 to 4 turns out. My comments had more to due with the transition. As the throttle blades open they uncover "transition" fuel passages which are controlled by the idle jets. The richer the idle jets the more fuel passes through the transition passages.
    By letting it run I meant start it and let it run until its up to operating temp. Somewhere around 190 degrees depending on the thermo switch in the rad. I make sure mine is to temp by insuring both rad fans are on. Takes at least 10 min or more as it has to get hot enough to open the thermostat before much hot coolant gets to the .rad.
    Keep in mind that as it heats up the mixture gets richer (hot intake air) hence the "popping back" when cold unless the chokes are on. (not a good idea according to most F-charters), On mine the difference between cold and hot idle mixture is substantial. At least 1/4 turn on the mixture screws.

    In my experience, I'm an old drag racer, the only thing that causes the engine to pop back through the carb is a lean mixture or retarded timing. Try snapping the throttle open on an engine that has a faulty accelerator pump and see what happens. Just don't stand to close or you'll have to regrow your eyebrows.

    As far as how it idles with the throttle blades completely closed. There is air gap around the area where the throttle shaft attaches to the throttle blade. Not the throttle shaft itself as mine has sealed bearings in it (probably a waste of time and money as the stock setup has sealing washers). The gap is not much but its enough. The reason I have to close the carbs at idle is the lack of R2 points. The Pertronix has no facility for that other than a 4 degree retard below 800 RPM for ease of starting which is probably the reason mine isn't real happy when its ice cold. With the R2 points the timing is about 10 degrees retarded at idle which really slows the engine down and requires use of the idle speed screws. My idle speed screws are set so a 2 thou feeler gauge drags when the throttle is closed just so the the if I snap the throttle closed it hits the stop. Mine also takes a a couple of seconds to settle into its idle.(I don't have the dash pot installed) Not sure why but Oh Well. If you do have the dash pot I would remove it and see if it makes a difference.

    After all you've done so far I would lower the float levels some if you haven't already tried it.

    Where are you picking up power for your timing light. I have a separate hot always power terminal wired off the starter. I use it primarily for a remote starter button but it also runs any gadget I hook up. (timing light, aux tach, voltmeter and so on. I was working on a 78 the other day and ran the timing light off the coil supply and it did act a little funny. Not sure if the idle dropped, however in your case since its noticeable I would double check the power circuit for the coils. (yellow wire). If you have the stock fuse box I believe the fuse for the ignition is the one furthest left on the left fuse box. Steve can probably chime in if I'm wrong about the fuse.

    Something else I discovered on mine is that the carb linkage that connects the banks works better if there is no tension on it when the engine is hot. I think the aluminum expands more than the steel linkage and if it is to tight its possible it will hold the carbs on one bank or the other slightly open. I set mine with a little "slop" at the carbs also.
     
  22. Sergio Tavares

    Sergio Tavares Formula 3

    Nov 15, 2018
    1,308
    Full Name:
    Sergio Tavares
    Pierce manifold now includes Carb Defender additive with rebuilds
     

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