carb accel flat spot - diagnosis/fix | FerrariChat
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carb accel flat spot - diagnosis/fix

Discussion in 'Technical Q&A' started by snj5, Sep 4, 2005.

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  1. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    Russ Turner
    #1 snj5, Sep 4, 2005
    Last edited by a moderator: Sep 7, 2017
    Of course I could be all wrong, but for you weberphiles out there: Picked up the car with new cams from Norwoods. Drives great, but there is a new slight short miss in the 3500 to 4000 range during cruise with very small throttle movements. Was always for just a moment, and would not occur with larger throttle movements. It was mostly annoying as I was cruising around 70 + mph and small throttle application would cause a little quick miss the the mixture would catch up. Initially I thought that perhaps the new cams needed a richer idle jet to richen up transition. To check where we were, put on the LM-1 in car A/F meter.
    Turns out that the mixture on the circuit is fine, even a bit fat. What looks to be happening is that at smaller throttle movements, the small accel pump jets I have don't respond well enough initially unless the pressure of a large throttle deflection loads the accel pump circuit. To review how this works in a DCNF, the accel pump shoots in to cover any transient leanness until the main circuit catches up to the new vacuum situation; when the pump diaphragm is deflected from throttle movement, that volume is the distributed through the metered accel pump nozzel and a fixed bypass opening in the carb. It is the combination of these two 'resistances' that determines how much fuel goes where. If you look at the graph below, made while highway driving around 3500+ rpm, you see the five and take firing of the accell pump. When the throttle is opened slowly, you see the lean miss up to 17 (my 3.2 begins to audibly miss above A/F of 16) then the mixture recovers as the normal circuit catches back up to the new vacuum level. Sooo, what I need to do is at low throttle deflections, I need to get more fuel to the accell pump jet. So, I reduce the resistance by using a larger accell jet taking some of the flow away from the bypass. The idle/transition/main circuit seem to be delivering enough fuel at steady state, so the accel pump is the object of my attention. Check out the LM-1 plot from today, with time on the x avis and A/F on the y axis.
    Will put in a larger accel pump and see what happens. I makes sense as similarly jetted DCNFs (e.g. Ghibli) have much larger accel pump jets than I am using.
    best
    rt
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  2. snj5

    snj5 F1 World Champ

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    #2 snj5, Sep 9, 2005
    Last edited by a moderator: Sep 7, 2017
    I spoke with Gron Wednesday at Pierce; He disagreed with my accell pump idea and supported with the theory that the 135 main jet was too small for the current 36mm venturi, and the problem WAS NOT the accell pump as I had thought . When told what I was running, he stated that my main circuit was not coming in soon enough leaving a hole. When told of my venturi and main jet size, he said that could be part of the problem, as a 36 mm venturi needed about a 145 main jet to start flowing soon enough. What he suggested was to go to a 145 main to see if that brought the main circuit in sooner, and consider going back down to a 34mm venturi to increase the air velocity if the hole was still there. He relayed an almost exactly similar experience on a hot cammed 356, where you went up on the main jet, then backed off on the idle/transition.
    Son of a gun - it worked!!!!!
    Gron was right.
    Went up to a 145 main (F24;210;53) and you could feel the main circuit come in earlier!! Clinically, the flat spot is about 80% gone - if you try hard you can induce it sometimes a little, if you are looking for it, but the anoying recurrent flat spot at 3500 - 3800 rpm 5th gear cruise is GONE. I almost don't believe it. I am now backed off on the idle/transition from 53 to 50 to lean out the low end cruise richness of the mid/low 12s - Sure enough, the cruise A/F, while still variable came up generally across the board about a point. The car now seems to like about 75 mph in 5th where it settles out around 13.7 to 14.2 depending on load. Another temptation is to go back to a 34mm venturi which would lower the transition to the main circuit a couple of hundred rpm, but the current improvement is amazing. While slow throttle opening before poked a hole to 17, now it just hits 15 and the main kicks in, you really do not feel it unless specifically looking for it, and even then just under very occaisional circumstances.
    The last thing I may do is back down on the accell pump jet from the current 60. When I floor it, the A/F drops to 10 or so and lasts for a while. When I flash the throttle sitting still, I do get the blue smoke out the back suggesting very rich mixture. I have a 35 and 40, and my records show even a 308 LeMans with 36mm venturis used a 40 accell pump jet. So perhaps my Ghibli analogy below is in error. May do that tomorrow, and if all goes well will disconnect the LM-1 and hold what I have for a while.I would like to get to a dyno for a quick check of the high end full load mixture to look at the air corrector jet, now at 210, as it is not safe to do that on a public road.
    best to all
    Russ

    <edit add> - I don't say this much anymore as I hear my car all the time, but the sound of a Weber carburated 32 valve Ferrari 4 cam V-8 with a opened up airbox, hotter intake cam, open flow through exhaust and Ansa Daytona resonators is better than any K-jetronic car ever could for me... :)
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  3. snj5

    snj5 F1 World Champ

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    #3 snj5, Sep 9, 2005
    Quick follow up - just tried the 40 accel jets - seemed to work well with less 'dip' and smooth wide open progression.

    Just a summary: Had a flat spot in the transition to main circuit after installing new cams with 36mm venturis, 135 main jets, F24 ET, 190 A/C, 53 idle and 35 accel pump.

    Turns out that can lower the transition to main circuit by increasing main jet size as well as increasing velocity with a smaller venturi. Also allowed use of a smaller idle/transition jet for better cruise.

    After some variations, going to a larger main jet of 145 eliminated most of the lean hole without downsizing the venturi:
    Final: 36mm venturi; 145 main, F24, 210 A/C, 50 idle, 40 accel jet.

    I am getting very efficient at rejetting DCNFs, lemme tell you . And what a sound!
    best
    Russ
     
  4. bill308

    bill308 Formula 3
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    #4 bill308, Sep 9, 2005
    Russ,

    It's likely that Le Manns competition car competitiors didn't worry about this issue too much for racing purposes. I think it's important for street driven vehicles.

    I suggested in your other thread you try the 34 mm chokes to bring the mains in earlier due to their more powerful vacuum signal. We both understand the consequences of potential loss of top end power and I understand your reluctance to go this route.

    Some other thoughts:

    I think you are at the point of needing only small adjustments.

    1. Did you use an 8-bank manometer system to balance the carbs off idle? Sometimes only 1 or 2 barrels are a little off during transition to the mains.

    2. Check that the float levels are all equal, one may result in a little lower fuel level than the others and cause a lean condition on affected cylinders.

    3. You might consider raising the float levels 1mm. This should extend the progression circuit and bring the mains in a little sooner, thus covering up the little lean spot.

    4. Are alternate auxliary venturies available that will bring the mains in a little earlier? The purpose of the auxliarly venturies is to amplify the vacuum signal for the mains. I don't know if they are available for DCNF's, but I recently had a correspondance on this issue with a knowledgable supplier in the UK with regard to a set of Dellorto's I ordered for my Lotus Elan. At issue was the fact that the used, but pristine carbs I purchased, were fitted with -2 auxliary venturies as normally fitted to Alfas, whereas Lotus specified -1 venturies. The operational difference was that the -1's brought the mains in sooner for a given vacuum signal and this is what he suggested and what I opted for in on my Elan.

    5. Step up one more main jet size to enrichen the lower end of the mains, as this has already shown postive results, but later compensate by fitting a larger air corrector to lean out the upper end. It is my understanding that the air corrector primarilly affects the upper rpm range, thus changing the A/F ratio slope for the mains.

    I can't say which of the above suggestions will be helpful to you, but at least you have some ideas to think about and potentially try.

    Good luck.
    Bill
     
  5. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    #5 snj5, Sep 9, 2005
    Last edited by a moderator: Sep 7, 2017
    Bill - brilliant post, I learned a lot. I do not have a manometer (yet), I am using the trusty old SK. I really like the idea of the float adjustment and will check and perhaps adjust. I am running the stock Ferrari 4.5 secondary venturis, while most other dcnfs use a 3.5. Do you think this would make a difference?
    I actually do have a set of 150 main jets and 220 a/c jets already, and may look at trying those.

    Below is a LM-1 A/F graph primarily showing cruise with the new 50 idle/transition jets. The difference in 53 and 50 is actually impressive - certainly more than I would have predicted. This Ferrari does not have any lean missing until above A/F of 16 under load.

    Again, thanks Bill for the great advice.
    Hope this is entertaining for some
    best
    rt
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  6. Birdman

    Birdman F1 Veteran

    Jun 20, 2003
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    #6 Birdman, Sep 11, 2005
    Hey Russ,
    My 77 308 does something similar. I have a slight hesitation when I apply the gas gently, but none when I stomp it, suggesting a similar issue. I get the occasional misfire too, but rarely. Maybe every 10 minutes, only when I apply the gas gently. This seemed to appear when I gutted the airbox, (due to the increase flow?) or it was there all the time and I didn't hear it through the "muffling" in the box. The car runs great at constant throttle settings. I'm running 135 mains. My car was misfiring at idle/low speed a LOT until I upped the idle jets from 55 to 60. Now it idles and runs perfectly around town. I'm thinking maybe I need to try going up a size in the main jets too. What do you think? Would 140s be worth a try? Other than even worse gas mileage, it shouldn't hurt anything if its not the problem. I need to get one of those A/F contraptions you have! (Car has an Ansa exhaust and "Russified" airbox and will shortly have an electromotive ignition since my ignition package just arrived from Nick!)

    Birdman
     
  7. bill308

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    #7 bill308, Sep 11, 2005
    Hi Russ,

    Thanks for the kind words.

    I did some checking in the Haynes manual and it appears all the 308's used a 4.5 auxiliary venturi with 32 mm chokes. The 1975 GT4, 208, used a 3.5 auxiliary venturi with a 29 mm chokes.

    I believe the 3.5 mm dimension is the diameter of the narrowest part of the auxiliary venturi nozzle channel. This should provide a stronger vacuum signal and bring the mains in earlier. Perhaps a 4 mm auxiliary venturi might be a better choice, if available, and you decide to go down this route.

    Personally, I would go back to basics and ensure the float levels are all the same. I then recommed to balance the carbs using an 8-bank manometer system. I did a small write up with pictures on the other site some time back(See the link http://www.*****************/discus/messages/3111/971.html and scroll down about 2/3's of the way). Ensure the barrel-to-barrel balance within each carb is correct. Now pay particular attention to the off idle balance, up to about 2000 rpm, by gently pulling on the throttle cable and observing the transient, simultaneous responses of all vacuum signals. This test is impossible to perform without the manometer system. Adjust the intercarb ballance adjustments and the linkage rods as necessary to obtain balance across the board. When the off throttle balance is correct between all carbs, then set the idle balance stop screws. When performed in this manner, the off idle balance, having previously been set, will remain set and the idle stop screws will just maintain the status quo. When carefully set up in this manner, you should reliaze improved low end response as when starting from a standstill.

    For the following, I would try just one change at a time and evaluate the results before trying something else. I find it best to be very methodical when doing this kind of work.

    If the small flat spot remains, try adjusting the floats to allow a 1 mm higher fuel level. Again, the goal here is to extend the progression range and bring the mains in earlier.

    If the small flat spot still remains, reset the float levels to their stock settings and try the richer mains. This should richen up the main circuit uniformly across the whole rpm band. If this remedies the flat spot, dial in the upper rpm mixture if necessary by changing the air correctors to lean out the effects of the larger main jets.

    If none of the above get you where you want to be, change the auxiliary venturies to the next smaller size or increase the fuel levels in conjunction with the richer mains.

    Good luck and please let us know your results.
     
  8. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    #8 snj5, Sep 11, 2005
    Bill

    Thanks for your reply; After a 1 hour out and back today (in the rain!) I was stiill annoyed by more transition stumbling than I had previously described, and really the car was only 'happy' when well into the main circuit at cruise (>4000 rpm) or higher. I guess that is what I should expect from larger venturis and reduced relative velocity. Since this Ferrari is used as a trip car, usable drivability is important, so back to the garage...
    So, I got home and after she cooled off a bit I re-installed the 34mm venturis with the same jetting. In search of most air velocity available, I also went back to the standard length 308 air horns instead of the D1 short stacks although not sure if it makes a difference. I verified the float height settings and every float travel and position was the same. LM-1 was not connected, so do not have numbers until later this week. Subjectively it was a lot smoother throughout the entire range, but the rain had cleared so cannot directly compare. The stumble was not really apparent as it had been, and I looked hard for it..

    So, new current jetting is 34mm venturis, 145 main, F24 ET, 210 A/C, 50 idle, 40 accel jet, and -11 pump cams and stock air horns. I know I can probably get as much street tractable hp as I realistically can expect with the 34mm venturis - the low end is great with them.

    I am also convinced this car likes to run in the low 13s and below for smoothness, so depending on what the LM-1 says this week, I have a set of 150 mains and 220 A/C that I may try. The 40 DCNFs only have a choice of two aux venturis, the 3.5 and 4.5 being the only factory offered pieces. I will probably stick with the 4.5 for now. I am also learning that the numbers are just a part of the tuning - seat of the pants and listening counts as well.

    Bird - there are a lot of guys in the archives who say their 308 runs better with 140s - they are cheap and easy, so it may be worth a try. I would.

    Again, many thanks,
    rt

    OBTW - where do you get an 8 cyl manometer?
     
  9. bill308

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    #9 bill308, Sep 11, 2005
    Russ,

    I think you made a wise decision refitting the 34 mm chokes.

    I used 2-Motion Pro Carb Tuners, 2 x banks of 4, like the motorcycle folks use, and attached them side by side using a couple of short pieces of wood and rubber bands. I believe you can order them direct at:

    http://www.motionpro.com/Docs/servicetools.html

    I think they will sell direct but I bought mine by comparison shopping online by searching on "carb +tuner +deluxe". A good motorcycle shop may stock them or should be able to order them for you. Prices vary a good bit. They also have a less expensive version but the pro version is higher visibility and won't rust.

    The link on my privious post was censored so I'll try once again

    http://www.fer****-talk.com/discus/messages/3111/971.html

    If you haven't seen it and the coded link above is censored again, pm me and I'll send you the complete link.

    Bill
     
  10. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    #10 snj5, Sep 11, 2005
    do you need manifold taps to use the manometer, or can be used down the carb throat?
     
  11. cavallo_nero

    cavallo_nero Formula 3

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    #11 cavallo_nero, Sep 12, 2005
    hey russ
    I am surprised to hear you are running 210AC jets. everything i have read, states to DECREASE AC jets when increasing Main jet size. I am running 140 mains and 175 AC, and my top end surely likes it - according to the dyno anyway. just a thought, although your A/F readings look spot on - hmmmm.
    john
     
  12. snj5

    snj5 F1 World Champ

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    #12 snj5, Sep 12, 2005
    Last edited by a moderator: Sep 7, 2017
    John -
    With the increase in mainjet size, it works by getting a decrease in resistance to flow so the vacuum will pull more gas through the main circuit sooner, richening across the entire band. As Bill states below, the richer top end can be leaned a bit by the A/C, the larger a/c sizes allowing more air in for a relatively leaner mixture. I will definitely agree with you that these cars seem to like it richer than leaner based on my experience.
    Below is a dyno pull with 34mm venturis, 135 mainjet and 190 a/c. So since I went up to 145 to bring the main circuit earlier, will shade up on the a/c to keep the top end from being too rich as the cams come into their own.
    best
    rt
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  13. bill308

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    #13 bill308, Sep 12, 2005
    Russ,

    Yes you do need manifold vacuum taps. I modified and used the existing charcoal canister purge line orifices. The hard line was gently removed and the orifaces in the manifold were tapped for a hose barb. After balancing, functionality of the charcoal cansiter purge system was restored with silicone flex hose and plastic T's. Should I post pictures of the installation?

    Bill
     
  14. snj5

    snj5 F1 World Champ

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    #14 snj5, Sep 12, 2005
    Yes, please, if possible.
    Many thanks
    rt
     
  15. bill308

    bill308 Formula 3
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    #15 bill308, Sep 13, 2005
    Russ,

    I'll get them together and post them.

    Bill
     
  16. bill308

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    #16 bill308, Sep 13, 2005
    Russ,

    I'm going to attach a Powerpoint file with the pictures and comments. If I'm unsuccessful or if anyone can't open this file, let me know, and I'll try to post *.jpg pictures. I've found in the past that a lot of detail may be lost with *.jpg's though.

    Bill
     
  17. bill308

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    #17 bill308, Sep 13, 2005
    Last edited by a moderator: Sep 7, 2017
  18. bill308

    bill308 Formula 3
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    #18 bill308, Sep 13, 2005
    OK, that worked, but the notes are pretty, small so I'll repeat the text below.

    Picture 1

    The hard plumbing was removed with the aid of a piece of wood placed through the hoop and tapped on the side with a hammer. The tubing was not damaged. High temperature RTV or perhaps Locktite could reverse removal if you change your mind.

    Picture 2

    The holes in the manifold have been tapped in this photo.

    This barb was tried by rejected because of throttle linkage clearance issues with the soft plumbing attached later

    Picture 3

    You can see the factory hole to evacuate the vapor canister in the throat. The small diameter helps dampen the vacuum pulses. Ensure it is clean with carb cleaner..

    Picture 4

    The correct barbs are installed and oriented for neatness. The soft plumbing is hooked up ready for carb reinstallation

    Picture 5

    Tubing from the barbs are collected on one bank here. I tried to ensure equal lengths so all cylinders share equally all vapors from the vacuum canister. Vacuum tubing gets progressively larger as the branches approach the canister. Given the small diameter of the holes into the manifold, this is probably not necessary. I can be a little anal at times.


    Picture 6

    Two, 4-bank manometer systems are rubber banded together ready to be hooked up. I found these tubing lengths on the short side which limited placement and orientation of the manometers during the tuning process. Longer would be better. The tubes are numbered and hooked up in order with the #1 cylinder manometer tube to the far left. The tubes are necked down to small diameter silicone tube ends using a plastic reducer.

    Picture 7

    This photo shows the status at the beginning of the balance process. The lower vacuum in cylinders 1-4 reflects a larger throttle opening. The difference between cylinders 1 and 2 is the inter-barrel difference to be balanced with the air bleed needle valves. The high flowing barrel air bleed should always be closed and the low flowing barrel adjusted up to balance. Tightening lock screws after adjustment will alter the final settings so one should anticipate this and adjust accordingly. The good thing is both hands are free to adjust as necessary. The status of all cylinders is known in real time. You will see the effects of adjustment to any cylinder or carb and it’s immediate impact on all others. Be careful not to rev the engine to a high rpm and slam the throttles shut. This will cause a high vacuum condition and could in the carbs ingesting mercury, especially if the manometers are not maintained substantially vertical.

    Picture 8

    After adjustment, the vacuum signals from all cylinders fall between 24-26 cm of mercury on my car. This balance condition remains pretty much the same off idle because I set the balance off idle and afterwards adjusted the throttle stops to just maintain this position. There is some sloppiness in the linkage so the idle balance can fine tuned a little. After balancing the engine idled much better and idle mixture adjustments were easier. The car ran much smoother with noticeably better low end response.

    Picture 9

    Hard Plumbing of the Vapor Canister is Replaced with Soft Plumbing

    I installed 90° brass barbs with 1/8-NPT threads at these 8-locations. The holes in the manifolds are slightly undersize for this tap, but with a lot of care and lubricant, very nice threads can be cut by hand. Other threads are possible. Don’t go too deep or you may bugger up the tiny port to the carb throat. A bottoming tap may be useful to finish up but I didn’t find it necessary. The principle mechanical load is from attaching and removing the vacuum lines. Clean out any chips. I used Teflon tape to ensure sealing and accommodate the orientation of the barbs angled down to clear the rear bank throttle linkage.

    Hopefully, I didn't miss any text.

    Bill
     

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