A work in progress. I wanted to fix the engine hesitation when rolling on to the throttle in the apex of bends or downhill sweepers . A bit harder with the throttle in a high gear up hill and you might hear a small chirp before the main circuit took over. This chirp was preignition due to hot lean afr 16-17 mixture burning faster under load than a rich cooler mixture. I will make the following assumptions - Properly working distributor/s, HT leads, spark plugs clean fuel etc,etc Carburettor synchronisation (Birdman tutorial) Fuel enrichment devices closed fully or disabled. The usual logical sequence to reduce the fueling gap between the primary idle circuit and the main circuit follows the well trodden path - 1. Raise floats to 48 mm on the 40 DCNF this puts the fuel at 2-3 mm below the auxiliary venturi outlet. Higher than this gives flooding from fuel slosh during cornering. 2. Properly radius the auxiliary venturi (AV) right to the outer edge so it captures as much air as possible to increase vacuum signal, and make sure it seats well and tightly in the carburettor housing. 3. Best rich idle or best lean idle plus ½ or ¾ turn richer. This increases the overall fuel discharge area to the progression circuit without idle jet upsizing. I found the easiest way for me to have consistent mixtures between cylinders was to use the Lift test. To perform the lift test, remove the fuel enrichment (choke) cover to expose the pistons behind. When these pistons are lifted 1/8 inch they provide a controlled air leak to the running engine. If engine speed increases the the mixture is rich afr <12. Slight rise then fall afr = 13. A drastic fall in speed seems to be around afr 14- too lean. 4. Oversize the idle jet to supply enough fuel for the air flow at the point of leanness. This results in the classic weber fuel consumption figures and cruising afr of 11-12 due to over delivery with lower throttle plate angles. 5. Tilt the idle circuit fuel delivery curve with changes to the air bleeds in the carburettor body. The side holes in the DCNF idle jet are for fuel delivery and have no effect on AFR unlike DCOE jets. Smaller holes richening the mixture at higher rpm similar to main air correctors. The brass fittings are around 160 and can be removed and tapped for grub screws with differing hole sizes for subsequent tuning. 6. Emulsion tube modification. This could take pages but it appears there is a relationship between the Air corrector (AC) size and the number, size and position of holes in the emulsion tube with respect to the fuel level and the AV position in the fuel well. This means hole changes above the fuel level, including directional clocking of holes to help direct air flow, can facilitate flow onset & modify well vacuum . Holes below fuel level change bubbly fuel viscosity, helping onset of action & enthusiasm of fuel to climb up the well. 7. Progression hole modification in the carburettor body. I would not suggest that everyone try this. There is little information about this other than the comment from Gene Berg (VW fame) that the 40/44 DCNF had progression holes that were too large and in the wrong place for modification. He chose 42 DCNF bodies which could have the smaller holes tweaked to the correct sizes. I fear he was correct. The test bed for me is my 79 GTB 308 - Fresh Wisco 81mm bottom end 9.8 CR 1mm oversize SS intake valves Port matching with mininum 34 mm hour glass pinch point. Euro camshafts 110 / 90 thou lift In/Ex at TDC NGK BP7ES Haltech E6S running dual distributors. Inovate wideband at front bank collector With 32 mm chokes and the first 4 changes things were close. Idle jet at 58 was acceptable but thirsty. Possibly a 55 Idle and a modified ET would have been right but I did not test this. I was distracted by the 34 chokes . With 34 chokes 120 main 170 air (even with unmodified ET tubes) the main circuit fueling was just so good. Flat 12.6 WOT. The engine note was clean, no weber chuffing and over fueling with reversion at low speed and eagerness to rev through 7000 WOT. I couldn't give these chokes up but the transition leanness around town was annoying as was smooth driving in high gears. I am sure you have heard just learn to drive around it. I drilled a 1.5 mm hole as pictured with the lower edge at the level of the upper edge of upper most hole. May be too large but I wanted to see an effect! The result is not what you would expect but confirms what other people have found in modifying similar configuration IDF / Solex carburettors. The upper hole, well away from the throttle plate, acts as an air bleed. This causes further emulsification and weakening of the idle circuit mixture up to the point when the throttle blade passes the hole. At this point the the the hole is exposed to vacuum and starts to contribute fuel to what had been a weakened mixture. My car now needs a 60 Idle jet and requires some continued accelerator use on first few minutes of warm up due to the more normal fuel mixtures. Idle screws are now at 4+ turns out for idle afr 13.5. Around town and cruise afr around 13-14 with no popping. The good news is no hesitation any where at any rpm or load. It pulls like a fuel injected car with the 34 chokes. This almost closes months of experimentation which got me close (modified emulsion tubes), but only the holes got me so close. The bad news is three sooty black spark plugs. At very small throttle openings (1500 rpm with no load) the reversion pressure pushes air into the lower progression holes, forcing pulses of fuel to dance out the upper hole that was drilled. This corrects by 1800 rpm and is only an issue with 3 cylinders. Those cylinders have 2 thou tighter valve lash so perhaps 4 degrees more cam duration. I have 1/8 bsp plugs over my progression ports if I wish to access and further change / plug/ modify progression. If these cylinders can't be sorted I can plug the progression drilling and I will try between a 90 to 120 air bleed in the idle circuit. Updates to follow. Rob References - Weber mod and jetting for the Ducati 906 .pdf Restricted bubbles / small AC: Keith Franck Sidedraft central Extra bubbles / Large AC: TheSamba.com :: View topic - Wideband Results 1quickS - Pelican Parts Technical BBS - View Single Post - Weber IDTP to IDA conversion and link Sparkplug reading observations by Bruce Robertson. QuickChange by Smartcarby Guy Croft DCNF tuning thread. Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Update. Blocked the third progression hole which totally transformed the idle circuit. Clean plugs and a stable AFR. Now running 55 idle tuned to best lean and 32mm chokes. AFR is 13.8 to 14.4 every where from idle, light acceleration, to 70 mph cruise. A small squeeze more on the accelerator brings in the mains and AFR drop to 10.5 -11.0 then to steady 12s (still a little rich). AFR was mid to high 13s with the 34mm chokes, too lean and I didn't have any 125 main or 155 AC to try. The 55 idle jets run like a CIS injected car that lacks the big rich accelerator pump effect around town, but just gives linear acceleration with pedal movement. Clean & lean. The main circuit just tips in on demand which makes it a pleasure to drive. The rich tip in to mains could be reduced by blocking some holes just below the fuel level. The third transition hole in this configuration could be enlarged slightly to fatten up the small throttle transients around town for a more lively carburettor feelor a 57 jet might just be an easier (reversible) option. Lots of fun to drive now, power and economy, so I will leave further tweaking for another time. Rob Image Unavailable, Please Login Image Unavailable, Please Login
I see you're running F24 tubes, I worked with them a massive amount and just couldn't find a point where they didn't dump in massive amounts of fuel through even the smallest pump just. 120 mains sounds like a good small point to run well with this setup which I have never tried, great job figuring this out!
These topics put many people to sleep, so I am always happy to hear someone has found value in them. Sidedraft central has a wealth of information on emulsion tube dynamics if you join and search the topic bubbles. My thoughts with respect to the DCNF are mostly as follows :- Fuel must be elevated from the fuel well up to the AV outlet and then mix with the passing airstream. The elevation required varies with fuel/float level. 2 mm below the auxiliary venturi outlet leaves the fuel meniscus over 1mm higher just at the level of the F24 upper holes. A number of forces act on the fuel well to pull fuel out. 1. Static vacuum from the AV which is diluted by the air bleed from the air corrector letting air into the well. 2. Dynamic venturi suction created by the rushing of air (from ET holes above the fuel level) out and down the AV. Classical weber tuning has mainly used the second effect. Larger air correctors allow more air volume and venturi suction to enable earlier initial main circuit activation but at the expense of leaner mixture onset, low rpm richness and upper rpm lean out. The larger main jets employed to balance mid/top end fueling leave an over rich bottom end which is often blamed on reversion when off cam. The flow area of the first set of holes in the F24 (and the F 9,11,15 etc series )is made to be as large as the largest usable 240 air corrector so the bottle neck is always stays at the air corrector to control mixture. Another approach is to increase the static vacuum in the well by reducing the leak from the air corrector. Smaller air corrector size helps along with a reduction in size of the ET holes ( to maintain air/bubble velocity ). When these ET holes are placed 2mm below the fuel level the bubbles released help displace and elevate the fuel to activate the main circuit. In my case it wasn't possible to cleanly get onto the main circuit with out a hybrid approach. The first ET holes are below the fuel level but I have one 1mm hole above the fuel level which is clocked to face the AV hole in the carburetor body. The advantages seemed to be the ability to run a main jet 2 sizes smaller than previously and not have the rich mixtures below 4000 rpm. Sharp throttle response was another bonus. The F24 ET pictured has a ring of 2x1.2mm and 2x1.0mm holes between the existing holes. The upper holes are filled except for 1x1.0 hole remaining which works like a 1mm AC until the bubbles burst from the submerged ring of new holes 2mm below the fuel level. It also prevents siphoning from the ET well. The 2mm silver ring between the jet and ET is to elevate the new holes to the correct position in the well. The holes should have been placed 3mm below the original upper holes not the 5mm I first tried. Is this the best solution? Who knows, but it allows for manipulation of the various parameters to adjust onset and amount of fuel when the mains are activated. Other observations :- The width of the ET also effects acceleration richness coming onto the mains ( aside from pump jets ). Once you have WOT and the main circuit is active the ET's role is probably over. Conjecture:- Less air through the AC and fewer bubbles seems to produce more stable AFR perhaps due to there being less compressible air. The compression and decompression of this air causes a pumping effect and a varying enrichment through the rev range. Cheers Rob
Rob, awesome weberology, reading every sentence trying to digest it. I cant resist the urge to ask you expert advice. I have 4 - 42 DCNF's on my Jalpa in the same basic set up as the 308. I am having an issue with the 7/8 cylinder carbs, when the motor gets hot, i cant get the 7 or 8 cylinders to stumble when I run the idle mix screws in and out. Its like those cylinders arent participating. I have check everything i can think of (new fuel pump, new MSD, good wires, good plugs, good spark, rebuilt carbs, cleaned idle circuit, check needle, check float level). The location of the old fuel pump was right by the header, which cooked it (common problem), but the fuel lines are still close to the header, and the 7/8 carb is the first in line for the fuel feed. The question is, what would be the symptoms of the gas being too hot in that first carb, or is that even a possibility. I just cant think of anything else to check at this point.
The real experts are on the sideline eating popcorn. The thread was somewhere some cumulative experience could be stored that related to the DCNF. I always make the following checks with fuel systems. fuel delivery at operating pressure (temperature) is greater than 600ml/min per 100hp. So 1.5 liter per min free flow is just acceptable to 240hp Float level checked in a running engine. More a problem with DCOE which are shaken to death or froth at high rpm. I found with the DCNF I just remove the main jet stacks (AC,ET &jet) in a warm engine, restart and upon shutdown measure the height down to the fuel level. A rusty vernier caliper works. I use a 4mm fibreoptic scope down the fuel well to view the level below the AV hole. Functioning fuel return to tank. A dead headed system can vapor lock or flood the carburetors. I have not seen heat cause a problem that didn't affect all the carburettors and totally mess up driveability. You will see spluttering of fuel out of the AV after engine shutdown and subsequent heat soak which is normal. Non functioning (contributing) carburetor idle screws ? no fuel too lean no air carburetor sync. Stethoscope, flow meter, borescope, lift test etc Leaking fuel enrichment device remove the the covers and run the engine to check/listen for air leaks and compare the effects of lifting the pistons on different carburetors. Good luck Image Unavailable, Please Login Image Unavailable, Please Login
Thanks for this fabulous info Rob. That's a gutsy move drilling holes in the carb body, most appreciated!
Gutsy to say the least! I've thought plenty about trying to effectively fix this exact issue-anybody who has adjusted these carbs has-but this really is taking it to the next level. You should teach a masters level class in Maranello on this exact topic.
Rob, great to have you on the board. Can you tell me more information about what you are doing in the last picture with the choke cover plate off?
so lifting the choke tubes up and down, i get that, but looking for more information, what leaks, what symptoms to look for..... <i know just enough to be dangerous>
Having coined the "Lift test" I should really elaborate. The gold standard of best lean/synch and repeat is the first thing to do and if your carburettors are close then this will work perfectly. Lifting the FED pistons is a quick test to confirm that you have achieved the goals above. The drop in rpm with the introduction of air past the piston seal should be the same for all carb barrels. This is similar to SU carburettor dashpot elevation to check the effect of leaning the idle fuel mixture. No change in idle speed suggests a non contributing barrel, usually too lean to fire - a blocked idle jet or need more turns out on the idle screw or spark / spark plug problem etc. Acceleration indicates a super rich condition (idle circuit or low airflow / poor synch). I can do this check faster than pulling all my front spark plugs and it gets me to the problem cylinder quickly. Requires very few tools and reminds me of the old days when you worked things out with a screwdriver not a laptop. The real reason to mention test was to remind people to check, clean and test this part of the carburettor because its a pain to diagnose if crud lodges under these pistons and gives you weird tuning results. Carb cleaner and repeated action of the piston works fine. No fuel is delivered by this circuit when the cover is removed because it breaks the vacuum to the FED fuel jet and ET. The other reason was to see if anyone was actually still awake. Cheers
This is my saturday project. I ordered new fluid for my four column manometer, and will try this. I cant thank you enough, great contribution.
On my 308, i just set the idle mix screws at a consistent 3 turns out and everything worked just fine easily being able to balance the carb flow using a syncrometer, the balancing screws and the ac screws, without having to move the throttle plates. On the Jalpa (4 40DCNF), i am having a hard time getting even air flow without titling throttle plates forward, which is resulting in high idle. on the Jalpa, i have vacuum ports below the carbs, and am using a 4 column manometer as the syncrometer not does deal with the higher airflow of the 3.5L. My question is this, or something like this: should the idle screws only be used to find "best idle", or can/should I be using them to achieve equal vacuum on each barrel? I notice that i can change vacuum by changing the idle screw position. Thank you.
Hi Geno, At the end of the day I always think the result is about driveabitity. Best lean setting is about mixture strength (AFR). Manifold vacuum is the similar, but as an absolute number it is influenced by airflow ie synchronization errors and AFR differences. If the mixtures are similar you can synchronize with vacuum. A great technique. If synchronization is good the you can equalize the AFR with vacuum. The reason to use best lean is to establish a base line AFR consistent across the carbs. This base line can then be moved richer to aid drivability if required. Synchronization is about off idle response and helping rev matching down shifts etc. With the 40DCNF in the 308 the first progression hole sits perfectly covered at 900rpm with no supplementary air correction. Side to side ( L & R ) synchronization can be done at idle. I spend more time the balancing front to back on the left hand master carburettors off idle at 3000 rpm ( or easy to read flow number / vacuum level ). This improves real world drivability ,torque and potentially stumble/roughness. I would tee a common variable air bleed in to the manometer tubing to enable higher flow usage. Spend your time here neighbors permitting. Cheers Rob
Rob thank you this is all very helpful. One other question: When I use the best lean mixture approach, i get quite a bit of lean overrun. So, I rich up the idle until i get just the right amount of burble/pop on overrun. Does that surprise you that best lean idle setting give me lean overrun when lifting off the throttle from high rpm driving? I am running no cats. This happens on both my 308 and the Jalpa. Interestingly on the Jalpa, each throttle control has its own fixed rod, so you cant get either perfect setting at idle or perfect setting at off idle, but not both. As you say, its tuning for driveability. thanks Rob. PM me your address if you like Beer, and I will send you a few special ones for your help. Geno
Thanks for your offer Geno Backfiring can vary with the exhaust muffler/ driving conditions. It is commonly described in these cars with "spirited driving". This link sums up what you have observed better that I could have done. Deceleration Backfire or "Popping" - HMF Engineering Forums A higher idle speed might help as well by reducing the trailing throttle vacuum. Cheers Rob
911308 I see where you drilled the holes that you have a allen head bolt to block the well. They usually have plugs in there, did you cut the threads and installed those screws yourself to be able to play with it? You also blocked one of those little holes with a tiny screw, why?
In reflection :- My aim was to explore progression circuitry similar to that used on truly great carburettors such as the PMO, Mikuni, and dellorto. I wanted a reversible non destructive way to access and change hole geometry with respect to the throttle plate. The small shortened screws ( from an old laptop ) fit snugly and can reverse the effect of the new holes and more importantly block original holes as required. I tapped the carburettor with a tapered BSP for easy repeated access but in reality a plug for tuning and the replacement brass plug when finished would work fine. These plugs primarily stop air leaks so a non tapered thread with sealant or O ring would also be fine. Up stream progression holes are not so critical in their position as are the first 3, so careful freehand drilling was easier for me than jigs or drill press. Blocking the 3rd hole which opens simultaneously with the 4th reduces reversion pressure pulses from over richening (pumping) from upper transition holes.This could be clearly be seen with the fibreoptic scope whilst running the engine at various speeds. The upper hole weakens the idle mix at low rpm but contributes fuel at the transition point where it is needed. More observations : - Wide and narrow band O2 sensors currently read AFR mixtures similar to 50-52 idle jets in the standard carburetor. When I originally tried to run 50 idles I found the accelerator worked more like a brake pedal when trying moderate acceleration ( dangerous transition leanness) Transition mixtures are now similar to when I ran 60+ idles in the standard carburetor. With standard F24 tubes a 57 idle jet might be required. Because I ran the 34 chokes for so long it had clearly helped me learn what the idle circuit does by opening up the transition area problem and making it clear when it was improved upon. The F24 tube is of a relatively small diameter which leaves a reservoir of fuel in the well ready to be dumped on main circuit activation. This is aggravated with the use of larger main jets as was noted by ferraripilot. Thickening of these tubes 0.4mm ? Would reduce this initial AFR dip, but soldering and turning tubes or impedance sleeves I shall leave to another time. Cheers Rob
Update: 55 idles 125main 170 AC 32 chokes. 500 km mountain and plateau drive & tune. Reduced the AC's with 0.6 mm wires ( similar to a 155 ) to ensure a safe stable AFR at speed for initial tuning. Having dropped a main jet down a carburettor on the side of a road previously, AC adjustments with u shaped wire is quicker and safer to judge the effect of changes.. Guitar strings offer a wide variety in sizing. The idle / cruise circuit performs beautifully. AFR 14 to 15 cruise all the way to 140 kph (85mph). The downside is the higher spark plug temperatures when running so close to stoichiometric. This showed up as a miss when going to sudden full throttle between 3000-4000 rpm.( detonation - worse than preignition ! ) even with AFR mid 13. Mid range 4-6 k improved performance AFR 12 ( was 0.7 higher with 170 AC ). AFR now falling to 11 around 7 k. Rolling 2nd & 3rd gear foot to the floor overtakes showed brief initial leanness and stutter. So the leaner cruise mix causes hotter plugs, and uncovered too much early advance and mixture leanness.. WOT at lower speed reinforced the need for larger main jets and should be balanced by the 170 AC at the top end.. The original 120 mains turn out to be about 125 when flowed, so I have put in original 130 mains, bp8es plugs and taken out 3 degrees advance in the 3-4 k rpm range. Over fueling can cover a multitude of sins which become evident as this 'protection' is stripped away. It highlights the need to match the ignition timing to the fuel mixtures and the type of fuel ( burn rate ). Easier to match with adjustable ignition timing and even easier with EFI no doubt. Cheers Rob Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
