Standard DCNF carburettor modification Part 1 of 2 55 idle, standard progression circuit and tuned to best lean ~ 3 turns out. The 125/200 M3 wins with the most responsive midrange 2-4 k rpm with slight hesitation in hard left corners only. ( R4 & R5 logs ) 130/200 M3 was just less lively below 4K. ( R1 ) 125/170 M3 Strong midrange but transition weakness and WOT dip of 10.5 ( R3 ) The 125/200 M3 ( 2 x 1.0 2 x 1.5 mm holes ) solves the transition issue in my car as a stand alone single reversible modification. The AFR above 6k WOT heads below 12 but mid range response is very good. The 130/220 M3 couldn't even transition to the main circuit ( funky lean ) I am testing some idle jet holders with adjustable air bleeds to find the Gam equivalent to see if the richness of the standard progression drillings can be improved Part 2. Cheers Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Rob, great update, glad to see you're still learning! I have stripped my '78 Euro down to do the valve clearances today and checked the jetting on my car - which to the best of my knowledge has never been changed. I have F36 ETs - 130 mains, 200 AC and 50 idles. You seem to have F24's - is this a difference between US & Euro cars? I'd be happy to have a play with a set of F24's as per your picture if you think that would be worthwhile. However, can you confirm the diameter of the new drilling you have made? Would a photo of my F36's be useful? Looking forward to pt.2
James, Some experienced F36 discussion. http://www.ferrarichat.com/forum/308-328/259880-more-emulsion-tube-questions.html For transition however, the first set of 'below fuel' holes should be 2mm below the fuel level for the earliest bubble effect. The fuel level can be measured through the air corrector as described in the https://www.ferrarichat.com/forum/308-328/430518-sputtering-carb-308-a.html thread. Part 2 - will finish road testing tomorrow and looks like the parts you have will be really close if used with Gam restrictors.
Rob, thanks for that. The emulsion tube discussion is interesting - seems to suggest that to get the best out of the engine F24's are better - but no actual data to back this up... Is it likely to be worth the £160 that the ETs will cost? FWIW I'm not dissatisfied with the outright performance, but just want the car to run as well as it can with better behaviour at part throttle openings and maybe some better MPG too. Great tip on measuring the fuel level - I'll see if I can get a suitable syringe & cannula - not so easy over here where the NHS have a stranglehold on anything medical! Looking forward to your further results, thanks.
Quote from Paul Abbott from Performance Oriented Remember that the idle/progression circuit takes in air for fuel emulsification just as the main circuit does with its air correction jet mixing air via the emulsion tube with the fuel. The idle/progression circuit is a little different; air is injected into the fuel in the fuel gallery from both the air bleed (little brass "pill" pressed into the top of the main throttle body just inboard of the idle jet and the progression holes above the edge of the throttle plates which are at atmospheric air pressure. As the throttle plates sweep past the progression holes; those below the edge of the throttle plate add fuel and since the number of holes above the throttle plate has decreased then the amount of air available to emulsify the fuel has decreased which enriches the mixture I can find no simple fix to the tuning of the standard progression circuit. The fuel delivery is non linear and changes to just the idle jet and Gam were not enough to bridge the transition weakness completely without excessive low speed and cruise richness. 50 idle 0.8mm (aluminum mig wire) Gam blocker - cruise AFR around 14. Stumble with S8 tube & untested with M3. 50 idle 1.0 mm (paperclip) Gam blocker 130/200 S8 cruise AFR 13-13.4 same as 55 idle with S8 and slight stumble 2-3 k in 4/5th gear. 125/200 M3 really just too rich at cruise 11.5-12 and no better than a straight 55 idle and no Gam blocker. Best modified progression 55 idle 125/200 M3 Good - Standard progression 55 idle 125/200 M3 - richer low speed & cruise. Possible = Standard progression 50 idle 0.8mm Gam occlusion 130/200 M3 I suspect any of the Fxx emulsion tubes would transition better when the the first set of below fuel holes are positioned around the 2mm under the fuel. It seems reasonable to maintain the same number and size of holes (to maintain the relationship to the AC size) but just move them up in position. I left 2 holes at 1.0mm so I could enlarge them later if needed in the M3 ET. Results will vary with cam timing, compression ratio, fuel octane and exhaust system. I will update with the effects of the magnaflow X system with and without merge collector changes. Cheers Rob Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Splendid work Rob, thanks for sharing it. I'm working away for a few days now, but I'll try to get my F36s out next week to have a look and see whether it looks possible to make the same mods as you have to the F24s. Expect a post on here some time! While I've got the car stripped down to do the valve clearances I might even look at whether I'm feeling brave enough to mods the progression circuit... Any tips to ensure that I get it exactly the same on every carb?
Notes & practical considerations for the adventurous - Progression circuit /ETmodification The aluminium plugs covering the progression drillings are pressed to within 2mm of the carburettor body. When a normal twist drill (the same diameter of the plug) first penetrates, it must not be used to further enlarge the hole in the plug. Instead swap to an end mill style or modified twist drill which has minimal point angle. I used the outer margin of the progression access hole to guide the outward wander / placement of the new hole and concentrated on the vertical placement, such the the lower margin of the hole was at the upper margin of the hole below. Size the hole from the ones in the body = 1.4 mm The exact position of the upper hole is not as critical as the working holes below. It is functional as an air bleed until transition to mains is starting/leaving cruise when it just converts the modified circuit over to a standard circuit but with a larger functional idle jet size. The blocking screws will need to be short to prevent stickout and throttle blade hangup on the inside of the carburetor body. The emulsion tubes discussed above have been multi-directional for ease of installation. Directional tubes allow yet a further change in tube sensitivity or response. Installation and the clocking of the tubes then becomes important and probably just too complicated except for the most obsessive with larger choke sizes. Image Unavailable, Please Login
Rob, that's brilliant, thanks for sharing. As soon as I've had a chance to sort out my valve clearances I'm keen to have a go at this. Based on my objective being to move one hole on my F36 ETs and jetting as per your advice do you think it would be foolhardy to try without having an AF meter?
Good question on AFR. AFR measurement is a great learning tool and diagnostic guide, but once the easy lessons are learned it can sometimes be more of a distraction than of real day to day use. The danger to be avoided is lean under load leading to high cylinder temps, burnt exhaust valves or detonation. The most probable cause for these are cruising in the transition lean spot with AFR 15 at a load level requiring about 150kph. This just doesn't happen in a normal situation because you are on the main circuit. Retarded ignition is a different story which is why you should service your mechanical advance mechanism if you are using a distributor/s. All the changes in progression circuit are at low load as are the changes to emulsion tubes, leaning out low load and filling in transition weakness onto the main circuit. None of the changes I made caused leaning at higher load, but a bias towards progressive richness at high rpm was more of the problem which I will try addressing in the New Year when the invasion of German vehicles ceases. In summary, you will feel the difference and the overall safety of the fueling will be grossly unchanged from where you started. The picture attached is from a NGK BP7 run at 13.6-14 AFR for 900km. Watching the plugs which will tell you more about a blocked main jet (lean at load ) than AFR from the opposite engine bank. Cheers Image Unavailable, Please Login Image Unavailable, Please Login
That's reassuring, thanks. I managed to get out to my garage for two minutes tonight to whip one of my F36 ETs out. I haven't had chance to measure it up yet but it's certainly a different beast from the standard F24 - though more similar to your modified one. Apart from the difference with the bottom drilling, it almost looks like a standard F24 with your extra drilling but no other drilling blocked. I'll have to measure up and see how it compares to your diagram. Image Unavailable, Please Login
The thermo concrete muffler has been retired. OEM but terrible internal welding and flow restrictive design. Yes I did look inside. The new exhaust is on and the effect is nothing short of sensational. My wife came for the first drive, to see what I was up to, and had to endure my bouts of laughter and exclamation. The exhaust volume is no louder than the original and there is no drone whatsoever according to my wife - it just sounds different. The exhaust thread is good reading, but I went for the easier fit /fabrication magnaflow X where the flow intermingles from each side. 2 mandrel ( 2 ¼ for 250-340 Hp ) into reverse cones to fit 2 ½ magnaflow acting as a termination box to slow the gases prior to exit. Theory aside, you no longer have to push the revs up over 4k , they push you and just keep going. This is now what a ferrari should be like. Just get rid of that OEM concrete POS. Of course one side-effect is overscavenging which really means less reversion and consequent less over fueling at low rpm. The transition weakness has returned ! Cooler cylinder head temps makes for a much more forgiving ignition curve and power delivery. Modified progression best idle +3/4 57 idles gives AFR 14+ cruise with 125/200 M3 flat 12.4 WOT. ( no progressive richening over 6K ) The primary circuit requires more fuel so 60 idles next. I am bench testing some different AV tube changes to see if the initial vacuum/ low airflow sensitivity can be improved. Cheers Rob Image Unavailable, Please Login Image Unavailable, Please Login
Update: Changes can be made to help bring in the main circuit early but the fuel droplet size and distribution at lower air flow is inconsistent and burns poorly. Without turbulent mixing, an AFR of 12-12.5 seems to be needed to obtain good low rpm response. For this reason I turned more to the transition / idle circuit because of its higher degree of fuel atomisation . A 65 idle (reamed )with the modified transition circuit gives AFR cruise 13.8 up to 80 kph then falls to 13 from 80 to 120 kph. A little rich but the upside with the 122/170 M3 normal AV & 32 Choke was perfect transition in 4th and 5th gear pulls from 2K rpm. 11.5 dip with WOT at 2k in 4th gear then steady 12.2 climbing to 12.7 at 7k+ air temp 28 Celsius. No AFR lean spikes or flat spots with light load or heavy load transitions. It behaves well in hard cornering due in part to the extra fuel from the idle circuit which is not float height dependent. I know it seems unreasonable to expect the DCNFs to transition in 5th gear at 2K up hills with out hesitation or cloud of black smoke from the exhaust but it is now possible in this car and it is just great to drive. Observations : Float height set to 48mm including the bump of the float. Higher impedance thicker ET tubes require fewer bubbles to lift the fuel. As the holes below the fuel level are moved upwards this reduces the amount of fuel displaced (pumped) and reduces the rich tip in ( like the F22 emission tubes). Less accelerator pump like effect of the ET and slightly earlier transition. Major issue for these carburettors is the AV/booster signal at low air flow and the fuel presentation/emulsion they provide. I tried making the AV less efficient with a trip ring which increases the venturi signal but introduces turbulence/low pressure to break up the fuel droplets on emergence from the AV. With a 34 venturi this provided the same AFR curve with the 125/200 jets as with the 32 venturi (12-12.4 AFR) but did improve the response from 2K rpm medium to heavy acceleration . More interestingly the car tolerates leaner mixtures without protest. I did some runs with 116/170 jets (reamed to mm size) which shows the transition leanness return but did some full throttle runs because the car ran so well. Only later did I look more closely at the WOT at steady 13.8. to 14. The car has previously protested when approaching these numbers at WOT. Unfortunately there are no available annular boosters for these carburettors and most owners have 32 chokes. With most of the early AV flow passing over the downward throttle edge, a scalloped trailing edge could well help break up the fuel at low AV air flow and shallow throttle plate angles. The car runs so well now that I will only add these profiled edges if inside the car for another reason. Another valid approach which is being explored on Side Draft Central is to use larger GAM air bleeds with the large idle jets to reduce the relative vacuum change with throttle position to make the fuel delivery less peaky and more flow dependent. How this works in horizontal non immersed idle jets I will leave to someone else. Now I need to wear some tyres out.
Mission accomplished. Final piece of the puzzle was finding a 150 needle valve in one of the carburettors. This explains the miss (and lean spikes in the AFR ) with full throttle at 4.5 to 5k. Swapped to 200 size needle valves all round expecting to have rich surge or flooding issues but nothing of the sort. The larger needle valves maintain the float levels better or even higher on twisty roads with high cornering loads meaning all the cornering hesitation that was previously improved is now gone. This is probably why Gene Berg often used grose ball valves for their higher fuel flow rates. I have now restored all my ignition timing advance and have a gem of a car to drive. Cheers Rob
This thread is awesome. Honestly, a bunch of it is currently over my head because I haven't had the chance to dig into the carbs yet. Exhaust looks great too - I have a magnaflow the previous owner put on myself...although with cats. Tubi standing by for replacement, but want to get a baseline on all measurements before replacing - just as you did here to great results!
Weber purists read no further. It turns out that the trip rings I tried out on the AVs are actually called skirted boosters. These are used in applications where the chokes are too large and the booster(AV) signal needs to be amplified.. A useful side effect of the skirt is a wider fan of fuel and more complete break up of fuel at lower air speeds. This helps fuel around the throttle plate and presents it in a more combustible form. This improves throttle response and AV signal (tip in) but will reduce air flow. So where does this leave the DCNF. My initial runs with these skirted boosters last year were impressive. 16mm OD 1 mm thick, copper water pipe 0.46cm2 Some numbers for the cm H20 vacuum generated by the AV by a shop vac. Std vs skirt 32 Ch 8.5 vs 11 cm CSA 8cm2 34 Ch 7.5 vs 9 cm CSA 9.1cm2 36 Ch 6.5 vs 7.5 cm CSA 10.2cm2 So mathematically a 34 choke with the skirted booster approximates a 33 Choke flow with a stronger AV signal than a 32 Choke. How does it work in the real world. Just like the maths, improved torque from 2.5k and and revs through like 34 Chokes. The plugs came out clean white so I shall jet to 130/170 from the 125/170. More lively around town due to the main circuit transitioning earlier into the lean idle circuit that I currently run. Interesting stuff. Image Unavailable, Please Login
Really great stuff there. I'm enjoying this. BTW, I built an exhaust system about identical to yours although I used the slightly smaller Magnaflow muffler and resonated pipe tips, sounds awesome!
Some shameless cross promotion of threads but I have just retimed my exhaust cams to an LCA of 105 and have picked up a lot of lower throttle response and manifold vacuum below 3K. This may be just enough of a change to remove the DCNF flat spot in its own right. I am a bit too far along the path of changes to confirm this as a single fix, but it provides a very nice response. Steep driveways without having to rev and slip the clutch! Cheers
Brief update With the revised exhaust cam LCA of 105 I removed the skirts on the boosters to see if the transition weakness with the 34 chokes would return. The exhaust/intake note is altered under 2.5 k rpm in 5th gear, but minimal loss in drivability. The up side is perfect throttle response and drivability with an extra 500 rpm gain from 6.5 k to 7+ k rpm where the engine keeps pulling. So I now have a 308 that drives better with the 34 chokes than I ever managed to achieve with the 32s. I would be almost certain that retiming euro exhaust cams to between 100 to 105 LCA would fix most dcnf progression problems with 32 chokes and f24 tubes and 57 idles. Where now? I have so much midrange that I will try some short intake stubs so see what difference the the increased clearance to the air box lid makes to the top end breathing of the engine. The next belt change will have the exhaust centres back to 100 to see what that uncovers. Cheers
Just had to try the short air horns. https://www.ferrarichat.com/forum/308-328/183960-dcnf-air-horns-kermit-ring-explained.html My experience is much the same with 5k to 7k being a special place to be. I am more aware of the switching on to the main circuit below 3.5k (transition feel), but it doesn't up set the car balance like it used to do. Strangely the was no change to the fuel mixtures. This may be due to me shrouding the the air corrector jets so there is not a bernoulli effect of the incoming air altering the pumping of the emulsion tubes. The real test will be swapping back in a few months to feel the difference. Image Unavailable, Please Login
I hope you're keeping a notebook to document you efforts. Very nice work!! I too have spent many hours seeking perfection on tuning the 40 DCNF Webers. At the end-of-the-day, I feel like it's just inherent with how these things function given the emission requirements put upon these devices. A way of saying, "I'm done!" What year is your 308? I have a theory that older DCNF's work better with less of the drivability problems. I also believe the recipe for the progression holes are the key change. Also - (dumb question) - What's "GAM"? Thanks.
The thread has grown somewhat as I tend to use it as my notes. Gam (idle air bleed) mentioned in post 1. along with car details. From Bill 308 Recipe of progression holes from throttle plate upwards. 4x series 0.8 1.4 1.3 1.6 0.8 Float 48 mm Michelotto gp 4 7x series 1.5 1.15 1.5 1.6 1.8 Float 48 mm eg 1979 GTB The smaller the holes and the fewer their number make for the richest transition, which is used mainly in racing carburettors. Reviewing these hole sizes makes my blocking of the 1.6 mm hole and substituting a 1.5 mm hole upstream look quite conservative. With the end result being the sum of the changes, other parts of the configuration I have not mentioned include beveled insulator gaskets to help promote turbulence to the air stream at low throttle positions (done to help break up the fuel as it exited the AV booster at the onset of main circuit activation) and textured intake manifolds to help fuel re-entry to the air stream and perhaps band aid poor intake shape/SSR and cam events. Lots written on these topics but strong opinion on both sides and no consensus. I will leave them in the unproven snake oil category. Image Unavailable, Please Login Image Unavailable, Please Login
Yes, yes, yes. The progression mod works. My Engine configuration is slightly different to Robs' car but the result with the progression circuit is the same. I have just rebuilt my engine. It is a euro spec single distributor dry sump 78 GTB. The engine has 10:1 comp, a larger cam with excellent mid range 4000 to 7000 RPM and larger intake valves. I drilled a 1.55mm hole using a jet drill by hand and blocked the third hole. I was running 60 idle jet 3.5 turns out before the mod and this was smooth but thirsty and smelly. Now I still run the 60 idle but 4.25 turns out (and can actually use the choke from cold start to help idle when cold for about a minute if I like). The difference is it is much leaner on progression but no hesitation at all and there is no over fuelling at low throttle angle around town like there used to be and much cleaner and sharper throttle blips on down shift. My next step is to fit 34mm choke and re-jet. This will be a while before I get to this as I have spent the last 8 months rebuilding: engine, gearbox, differential, brakes, cooling system (w. pump, hoses, radiator), fuel system including all fuel hoses and carbs. So its time to enjoy for a while For some reason I cannot attach photos to my post otherwise I would have. If anyone has any ideas? The only down side to the progression mod is that it is not at all easy and definitely not a DIY for everyone. It is a time consuming and a very delicate task, so extreme care must be taken not to damage the carbs. And definitely don't entrust just anyone to perform this task. A good machine shop is probably a good place to start looking or someone that has a lot of carb rebuilding experience. I did not use screws to block the third hole, instead I made some very small plugs with a head on one side and fitted with slight interference, heated the carb body up to about 60 deg c and gently placed pins in with Loctite stud lock. (be carful not to distort the carb throat when placing pins in). You could use epoxy like JB Weld to block the holes as this is the difficult part of the job and it will last many years but it will eventually soften.
Good work! Not rocket science, more like brain surgery perhaps? The "Go Advanced" tab and then manage attachments and upload etc will get the pics there Enjoy
