Correct good way to describe it.
Retimed the cams Intake 110 thou tdc = 30/50 = LCA 100 Exhaust 70 thou tdc 30/44 = LCA 97 130 main 170 AC Baseline GPS rolling acceleration times from 60kph to 120kph in 3rd gear (Clock work autologger app) improved nearly 1 sec to 5.5 seconds with gains every where over 4k but no losses below 3k. Tried a 140 main to check the narrow band and the times dropped back to 6.2 sec, 830 mv 11 AFR. The 0.5mm/20 thou less exhaust lift at tdc is enough to preserve the civilized sub 3k behavior and allow the use of the 34 mm DCNF chokes but enough to allow the exhaust tuning to do its thing. I delayed the intake valve closing point to help support the higher rpm range given the improved torque of the advanced exhaust timing. The drivability is nothing short of brilliant. There was no diference in acceleration times with the short stubs & normal ones when using the previous 105 LCA exhaust timing. 120Kph is about 6.5k rpm and it is now obvious the car is much stronger from 5k to 7k with the retimed cams so results may now differ.
A long overdue post aimed straight at 911308. Big thanks for your work on the transition tuning. Even more thanks if you're able to answer some of these questions. Excuse the long post but I've been working on this for ages and you're clearly that much closer than I am to really sorting out the dcnf carbs on the 308. All of the following concern a euro 78 gt4, factory engine, low miles, great driving original car. The only changes from spec are a de-restricted airbox inlet and a (perhaps) slightly better flowing exhaust box. I say great driving but it does have the transition lean spot, exaccerbated on cornering. 1. Emulsion tubes. I have tried f36 and f24 emulsion tubes. While the f24 did help cover the transition hole and allowed slightly smaller idle jets, I just can't get the wide power band and top end power that I can with the f36s. With the F36, the fuelling is so perfect from 3500 to 8000. This is using 130 mains and 170 air correctors. Have you any experience with the F36 ? Or are all of your observations using the F24? Try as I might, there's less power and a tendency toward top end richness with the F24. Even when I get a good mixture at high rpm with the F24, it feels less responsive and the rev range over which I can get a perfect mixture is smaller. The current best setting I've got wiht the F24 is with 55 idles, 130 main, 210 air corrector. The F24 gives a greater volume of fuel in the well - this could be aiding transition. Any further thoughts on this and on F36s? 2. Float levels and needle valves. You talk about an improvement with 200 needle valves? Do you still consider that this is the way to go? Stable float level seems to me a major problem with the dcnf carbs on the 308 and certainly contributes to the transition leanness being worse in cornering. Solid mounting of the carbs also doesn't help with vibrations. I have measured fuel pressure and accurately set float levels only to observe some resonance where the floats vibrate up and down enormously at certain rpm. I have created some simple baffles (not yet installed) and also am considering foam lining the bottom of the float bowl (held in place by the drilled baffle) to help stop resonance and stabilise fuel on cornering. Any thoughts or experience on this? Should I try 200 needle valves? Anything to overcome this resonance. 3. Size and position of progression holes. Have you experimented with the size of individual progression holes? I see that you have added an additional hole at the top and plugged one existing hole but would you change this if you were to do it again? Would you use a larger size of any existing or new holes? You say "The smaller the holes and the fewer their number make for the richest transition" - can you elaborate on this please? I don't get this generalisation. You also say"Reviewing these hole sizes makes my blocking of the 1.6 mm hole and substituting a 1.5 mm hole upstream look quite conservative." Again, any thoughts on if you'd try it differently next time? Something that's interesting on the 3x, 4x series is that the last progression hole is smaller. The 7x series is what I'd think of as a more likely configuration with the size of each progression hole getting bigger from the 2nd to the 5th. My understanding is that the 3x and 4x series have the 0.8 - 1.4 - 1.3 - 1.6 - 0.8 progression holes (and only the 3x has the F24 emulsion as standard). I'm not sure what the 5x or 6x series have but will find out and report back if possible. 4. Idle jet and gam size. I have tried 50, 52, 55, 57 and 60 idle jets. Even 57 is too big for my car. I'm currently on 55s (with the F24s) but bottom end response is not so great, it's simply a little too rich off-idle. There's no fouling of plugs, just a not-so-sharp throttle response. I have drilled and tapped my carbs for changeable gam jets. Right now they're standard size but my feeling is that they need to be bigger, not smaller. After transition, at the high end of the idle circuit, it's rich again. Maybe the progression hole changes will improve that or maybe it will allow me to go smaller on idles again. Things to note on drilling progression holes - I'm making a steel guide for the drilling of transition holes and also have found that early DCOE progression hole covers have a fine thread and are the perfect size for replacing the original pressed brass plug - they look as if they could be original too. I see standard GAM sizes of 1.5, 1.6 and 1.7 on these carbs. 1.7 appears to be on USA spec cars only on the 4x series. I'm guessing this is more of an emissions thing. I'll start changing gam jets soon. 5. Ignition. A wise friend recently said to me "I'm sure you can eventually sort out the transition holes but you could save yourself a lot of bother and just get a programmable ignition system". I think there's a lot of truth in this but hey, I want to fix this anyway! I'm working on a programmable ignition system with a TPS which I'm confident will work wonders.......... but it's only going on the car when I'm that much closer to getting the transition right. What ignition system are you using and what plug gaps do you have? My next steps: Drill the progression holes - await your feedback to see if you'd try it differently. Go back to the F36 emulsions with the new progression holes and then see how the idle jetting and gam requirements have changed. Add this tps Throttle Position Sensor | Contactless - Penny + Giles - I'm planning to use this before installing the rest of the ignition system so that I can monitor which transition holes are being uncovered with more precision while driving. Photos of some of the above to follow when I get around to next steps!
Just to spend a few moments to consolidate the information. The hesitation does seem to be caused by lack of combustable fuel. Small idle jets make it worse as does low float levels (static or during hard cornering). We can try to reach up with the transition circuit , reach down with the main circuit or even improve the volumetric efficiency of the engine in this part of the operating range (cam timing ). Ignition timing is then optimised to the fuel mixtures achieved. The progression circuit modification probably does about half the job. It does however provide a much nicer driving experience. I suspect but cannot confirm that you can achieve the same outcome by blocking the third hole and enlarging the GAM without having to drill the upstream air bleed/? fuel delivery hole. Note the 5th progression hole in the euro carbs is 1.5mm not 1.8mm as previously posted. The position of the holes and their relative size to each other (not really their absolute size) is what maters. Once they are all exposed there is more than enough CSA to flow the fuel/air metered by the idle jet/GAM. Fewer & smaller holes just means less dynamic air bleed effect from upstream holes and a need to change the idle air/GAM sizes. The problem in the DCNF is the almost simultaneous exposure of holes 3 & 4. Emulsion tubes and float levels do at least the other half. I have no direct experience with the F36 but their design looks better than the F24. Higher impedence (less well volume and lighter fuel column) should make for more load sensitive fueling. The larger fuel well with the F24 does indeed help fueling at main circuit tip in. I have however moved my holes(the ones below the fuel level) up nearly 3mm to encourage earlier but now leaner tip in. I now have x2 1mm holes below these where there used to be x4 large holes. This reduces the the super richness at lower rpm. As far as improving the main circuit activation I would try to have your first ET holes at just 2mm below the fuel level. If your tubes look like the earlier post of the F36 then a 1mm drill would be your friend. Since putting in the 200 needle valves I had forgotten about the cornering issues with these carbs. They are the available answer to grose valves. The 200's are more sensitive to being over powered by high fuel pressure which leads to their reputation for flooding. Exhaust cam timing provides yet another half of the possible solutions. Up to 10 degrees of advance to an LCA close to 100 degrees makes a large increase in manifold vacuum at lower throttle angles but may reduce your rpm range by 500 rpm or so. Even 5 degrees = 80 thou at tdc might be enough. Now we have 150% of the solution for 32 mm chokes it is a case of choosing which one or two of the three solutions best suits. The first thing I did with my car was to use twin Crane 700 triggers and igniters controlled by a MAP based haltech programable computer. No amount of advance curve tinkering could get rid of the transition symptoms. Spark plug gaps of 30 thou is enough when using distributors. More with COP or multi coil crankfire setups. Look forward to hearing about your progress. Cheers Rob
A great resource from Paul Abbots site which discusses nearly alll the issues above. Performance Oriented Cheers Rob
I have just returned from a drive with the best combination of torque and throttle response I have ever achieved in this car. Two changes made. Firstly, 1.88 OD 1 inch long venturies in the exhaust collectors. Drove these for 500km with 32 chokes, 1mm lower float, 124/200 F24 ( 8 holes ) Fantastic drive with some lotus elans through targa stages but rich progression and poor fuel economy fuel range anxiety. Secondly, swapped to 34 choke 130/170 Shrouded F24 3 elevated hole & 1 anti siphon ET. Wow! Car pulls from 1500 rpm cleanly without that off cam funky hiss and burble of the webers in any gear. With this on my mind I thought I would post something I wrote earlier in the year. Information collected from many sources: The DCNF uses the 61450 tube style Carburatori Weber The differences are in tube diameter (outer & inner) as well as the number, size and position of the holes. The most tunable, progressive and fuel economical tubes are the F36 F21 F22 F30 etc. These tubes all have D holes just below the fuel level, which is what I ended modding into the F24. The larger outside diameter causes more impedence (stiction) which controls the cruising & low power mixtures more but will provide hinderance to main stack flow at 50Hp/cylinder IR setups at WOT, so a thinner tube is needed to put mixture control back at the main jet. It would seem a F36 needs about a 2 main jet size increase over the F24 due to the well stiction effects. The lesser number and higher position of the bubbling holes contributes to leaner and earlier onset of fuel flow as does the larger main jet. The main jet does have an effect over the whole rev range because it determines the dynamic fuel height in the well around the tube. Larger the jet the higher the level in the well for a given flow depression and thus earlier and richer the tip in as well as a change in the fuel column height above submerged holes etc. Bubbling ET holes work more effectively with the thicker tubes so fewer and or smaller holes are needed. The optimum hole size seems to be 1.2 mm with a submerged to air corrector area ratio of 3:1 Blocking holes for testing can be done with red/blue locktite wrap with aluminium foil & heat with heat gun to set ( messy and I prefer solder ) Dont forget the effect of the needle valves filling the float chamber size, speed and aeration effects. Most of the progression problems I noted were actually emulsion tube related. Over 80kph my car is running on the main circuit. With a standard 40 DCNF, a 50 idle gave perfect mixtures up to that transition point. The F24 ET required masses of air ( large AC ) to initiate flow, and a larger main jet to compensate. Thin tube, lots of air and bubbling holes deep below the fuel level giving massive tip in richness and poor cruising mixture control. Air through the main stack is a friend when it comes to initiating main circuit flow through bubbling and venturi effects , but acts as a brake to fuel flow at progressively higher rpm. The air is also compressible leading to a worsening of reversion richness at low rpm acceleration. If the circuit over delivers with one pass through the AV, a second pass with reversion makes it even worse. The least volume of air ( small AC ) through the fewest bubbling holes to get the job done will provide the most combustible and controlled AFR.. To have the DCNF/weber main circuit deliver a stable AFR over an rpm band of more than 4000 rpm is difficult. Leaning of the mixtures occurs eventually. Some fixes have included: - A high speed enrichment tube/jet from the float bowl to the velocity stack/bellmouth to deliver fuel. - A float chamber vent facing into the velocity stack intake flow to pressurise the float bowl at high speed with an adjustable bleed for fine tuning. - Enlarged accelerator jet outlet and bleed back hole, to act as an internal high speed enrichment device. Cheers Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Two thumbs way - WAY - W A Y UP!!!! Paul is a Weber god, and a great guy to work with. He helped solve the unsolv-able problem with one of my 308 DCNF carbs.
Sorry to drag up an older thread but I'm in the process of stripping down my carbs ( car hasn't been started for over 10 years) I have everything else apart but these FED Pistons are stuck fast in the carb body. I have soaked in penetrating fluid but to no avail. What is the best way to remove them? Cheers Jon
Throttle body cleaner or subaru upper cylinder cleaner ( the strongest and most toxic ) will dissolve most fuel/carbon tar. Gentle heating the carb with a heat gun may help but stuck might be the best position for them. They add very little to carburettor function as long as they don't leak air. Cheers
Thanks, will try some heat, do they not need to function for the choke to work? (I'm in Ireland so we do get cold weather even in summer!) Cheers Jon
The pistons need to move freely for the enrichment ( choke ) to work. Even in the snow, 3-4 accelerator pumps then 1/2 pumps every 2 seconds or so until she behaves seems to be what most would do. Others may well chime in. There have been whole threads on the subject. Cheers Rob
I'm in the middle of carrying out the progression circuit modification...... At this point there's not a lot more I can add about the theory or the outcome but I thought I'd show some details and some pics. After some consideration, a quick exchange of PM's and re-reading this thread very many times, I decided to make the new holes the same size and position as Rob did. I decided to make plugs for the unwanted progression hole from brass stock. My thinking was that this will be easier to reverse than other methods and a neat solution. I enlisted the help of a local clockmaker who should now add carb mods to the list of services he offers. Antic Daró · Services. Making the plugs was trial and error before we found the right interference fit. If I was to do this again, I'd drill the extra progression hole before plugging the third hole. When we made the brass plugs, we made a small pilot indent in the plug in case I ever want to drill them out. The plugs were installed with loctite retainer as well as the interference fit. I drilled and pulled out the brass progression hole covers and then tapped the hole with a 9x1.00 plug tap. Doing this allows the use of Weber dcoe threaded progression hole covers. I previously stated that these were early DCOE progression hole covers but that's wrong - this is the later type. I have a spare pair of dcnf carbs to play with but these are 36 dcnf's....... and I shouldn't have assumed that things like the progression hole bore is the same size! More on the tests on these spare carbs later. One thing that is present on the 36dcnf but not on the 40 is the vacuum take-off port. I find it very useful (for both speed and accuracy) to use manometers in setting up multiple carbs so I'm adding the vacuum port and take-off union. Photo below of this port on the 36's - updates and more pics on this mod very soon. While working on these carbs, I almost overlooked something obvious. The auxilliary venturi must be firmly seated and airtight against the bore. I always check this but almost forgot to do so because I already did so last time the carbs were out........ I'd just forgotten that was a long time ago and in the intervening period the aux venturis really had worked quite loose. Having removed the chokes and aux venturis, I reckoned there is some small but worthwhile improvement to be made here. Any marginal improvement to bring in the main circuit slightly earlier is good, I think. I profiled the aux venturis, matched and cleaned up the trumpets, carb lids and knife-edged the profile of the choke so that there is no longer a bluff edge at the critical point where it meets the bore of the carb. The same treatment can be done on the entry to the auxiliary venturi - this is even more important and it really has quite a rough finish and a bluff edge from the factory. Getting a better signal at the aux venturi will help. Let's see if these mods are worthwhile. More photos and mods to the carbs will follow soon. Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Very nice work! I assume the progression hole picture is rotated 90 degrees, which means the first hole uncovered by the throttle blade is left untouched to prevent off idle stumble. The very design of the weber venturi is not to flow the most air possible, but to direct the airflow to the centre of the carburettor bore around the auxillary venturi by using that blunt leading edge.This pushes the air more centrally to improve the airspeed & signal. Reshaping the venturi into an airfoil will improve air flow but alter the venturi operation ( efficient flow but decreased signal ) The next important dimension of the venturi is the size at the narrowest point ( waist ) and the relationship of the axillary venturi tail/ exit to this point. The main circuit activation can be tuned by altering this position. The strongest vacuum signal is about 2mm below the waist, but this provides the greatest obstruction to airflow. Above the waist gives the least air flow penalty but reduces the signal to the main circuit. The DCNF setup is at the narrowest point. Past the waist there is a straight taper to induce fuel shear to keep the fuel in the air flow, not the parabolic shape of an airfoil. My 34 chokes have a higher waist so the auxillary venturies sit deeper and have a better signal, but at the expense of some airflow. Cheers Rob Image Unavailable, Please Login
Yes, you're right on all those points Rob. I should have said that my intention was not to change the shape of the choke but more to clean it up so that it's closer to the theoretically desired profile rather than that of the manufactured product. What I wanted to remove was the perpendicular bluff edge on the choke which is necessary for mass production but is best as a radius, albeit a very small one. Removing the casting imperfections can only be a good thing. My concern is that there are several points which can introduce turbulence whereas I think we want laminar flow all the way in to the aux venturi........ These are very marginal improvements on the choke with the intention of getting the best of both worlds (flow and signal). The changes to the auxiliary venturi are also extremely minor but, contrary to the choke itself, we really do want to increase airflow or rather speed down the venturi. The entry to the auxiliary venturi is very crudely finished from the factory and those rough edges and distinctly bluff entry look like they would introduce turbulent flow just where we don't want it. Again, I'm not trying to make changes to the function, just clean up the compromises on a mass manufactured part so that it's closer to ideal. Whereas the changes to the choke really are going to be pretty marginal, I have some hopes that we'll get increased speed and signal in the aux venturi. The point about where the tail of the aux venturi sits in relation to the waist in the choke is really important. I'm not changing choke sizes so have not really looked at other possibilities. I did consider extending the aux venturis though, making them longer (on the entry side) but lack of space means it's not an option. Still a little way off getting this back on the road....
Getting close to the point where I finish mods to these carbs and actually get on with some testing, tuning and reporting some results here. Before I put the carbs back on the car, here's a recap of work done with some photos, links and part numbers. Nothing so new here but adding more pictures of modifications to this thread could be helpful to someone. Block one progression hole, drill a new one further up - exactly as per Rob's mod. Tap the progression hole boss with a 9mm thread to accept a late dcoe progresson hole cover. Weber part no. 61015.009 Knife-edged aux-venturis to improve signal. (Some background reading on the subject: Carburetor Boosters - Tech, Explanation - Popular Hot Rodding Magazine ) Profiled aux venturi wings to give them a better-flowing aerofoil section. Improved choke profile and cleaned it up, particularly where it meets the carb bore to promote higher speed laminar flow. Machined bore, thread and seat for vacuum take-off ports. I ended up using dellorto drla vacuum take-off unions. DRLA Vacuum take off union | Eurocarb I also ended up buying o-rings from a local supplier because the ones from Dellorto don't fit. This mod makes setting up the carbs very much faster and IMO using mercury manometers in conjunction with a flowmeter yields better results too. Another good thing about adding vacuum unions is that we can use it as an air bleed, similar to the cold start piston lift test Rob describes. Machined throttle position sensor actuator. I abandoned the float bowl baffles and foam for now - various attempts and none of them good enough to fit to the carbs. Various pics below - real-world tests and in-car analysis in a week or 2...... Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login Image Unavailable, Please Login
Great work! I do like the vacuum port machining and look forward to your experiences as I am sure you do. Cheers
To anyone following this thread, you might have noticed my enthusiasm for these kind of carb tweaks as well as the slow pace at which they're progressing. So the great news is that the carbs are back on the car and the engine is running again.... with both frustrations and observations...... The frustrations: 1. Startup was followed by progressively rougher running, misfiring, backfires, carbon fouled plugs and lots of black smoke. I admit to being worried that I'd trashed my carbs for just a few minutes but it didn't take too long to see that the fuel had gone stale. (Did I already say these things take me too long?) The fuel had gone a dark colour, had a distinctive, waxy smell and complete loss of the volatile fumes which normally take your breath away. Drained and refilled with 98 octane fuel including draining the bowls. Cleaned the plugs and now I can see the potential - the motor's running well...... more on this below. I've still not driven it though because...... 2. My Ex's Volvo is broken down in front of the garage..... and for some reason I volunteered to fix it. This may be the very definition of frustration (she always was). But I have some interesting observations so far, one which I believe is critical. Observations (static running and tuning): 1. While working on the carbs I noticed that the male part of the ball joints on the linkage were all working loose. These are factory fitted into the throttle shaft levers and are crudely peened over - that's all that holds them in. This applies to the ones on the carbs and also to the bell crank that the throttle cable attaches to. I fastened them in with a few tig tacks and the problem's solved. I'd certainly suggest that everyone checks this when adjusting / balancing carbs. 2. I'm back on my best previous jetting so far (although not best for progression). 32mm chokes 130M 170AC 55 Idle F36 ET 45 PUMP I tried 52 idles but clearly too lean throughout the whole range of the slow running circuit. Back to the 55s and checked CO/HC which seems good running the engine static up to around where the mains come in (which still seems a bit high but let's see how this works out). I have 57 idles to try if road tests require it but 55s really do seem right so far. 3. Here's the big one. The vacuum take-off ports have proved their worth. I had balanced the carbs in the usual way (basically the same as the Birdman tutorial). When I attached the mercury manometer, a few things became clear about the shortcomings of using a flowmeter and adjusting at idle and at part-throttle. And just to underline it, this isn't a critique of Birdman's method or tutorial - both are correct when using a single flowmeter. Adjusting each bank at idle (with linkage between banks disconnected) with the manometer on the vacuum ports corresponds to near equal flow readings too. So far so good. Adjusting at part throttle achieves more or less the same good correspondence between vacuum and flow - to be expected. But what this neglects is the shortcomings of the throttle linkage itself. When using manometers, I always check that opening the throttle causes each carb to come off idle at exactly the same time. When adjusting for perfectly matched flows between banks at part throttle, the much-more-crucial synchronisation of pick-up from idle is not synchronised because you're not measuring it directly. This is a symptom of imperfect manufacturing and imperfect linkage design. Very few cars have linkages which allow for slight geometry changes so that part throttle and synchronisation coming off idle are both perfectly achievable. What I'm pointing out here is that unless you're unthinkably lucky, if you adjust so that you have equal flow at (small) part throttle opening, then, when you accelerate off idle, one bank of cylinders will be on the idle stops, the other will be fractionally open. This, regardless of ignition and all our efforts on the transition circuit, will mean that coming off idle is never optimal. It might be easy to miss my point here - theoretically, if you adjust for equal flow at part throttle and you have adjusted your idle correctly, then both banks should come off idle in perfect sync. But in real life, the throttle linkage and its geometry are imperfect and this probably won't happen. Birdman mentions that the adjustment of pushrods from bellcrank to carb is very sensitive. It is. And this is indicative of how a very small change in adjustment can give you perfect sync off idle.......or not. And unless you have vacuum ports (or using multiple flowmeters - which is not ideal however hard you try), you're not measuring this off-idle sync. So those are my observations for now. I have freshly rebuilt carbs, fresh fuel, good sync, crisp response, promising mixture on the idle circuit, in-car gas analyser plumbed in and ready to go........ And, preventing me from reporting how it works in the real world is what's up on axle stands in front of the garage: a robust but broken old Volvo belonging to a young but fragile woman. (Or did I get that the wrong way round?) Hmm, I now have some motivation to fix the Volvo. More to follow when I've had a drive.
Hi magnesium. Thanks for your last post, especially item 3. I came to the same conclusion you reached with respect to the role the linkage plays in off idle performance in the same way you did. One is unlikely to see these linkage effects without a multibank manometer system giving real time, comparative data. I have advocated for balancing the carbs by backing off the idle stops until they are no longer in contact with anything and using only the throttle cable as the single point support for all the linkage. Balance should be accomplished solely in this configuration and when done in this manner, synchronization is carried over into the off idle regime. It is the low idle and off idle synchronization that is important for drivability. Only when balance is established in this way is it ok to nip up the throttle stops, not to produce any balance change, but merely to provide a backup, positive stop. In reality I think they do more harm than good so should probably not ever be used. They certainly are not useful for balance because as soon as the one of the carb linkage elements is off a stop before another, you have an imbalance problem. Only when all throttle elements leave the stop simultaneously, are you still in synchronization. The important thing to understand is there is no way to see this effect unless you are simultaneously looking at all cylinder pressures (vacuums) in real time and with sufficient precision to make on the fly adjustments to the linkage. An alternative to modifying your carbs is to remove the charcoal canister tubing manifolds that recirculate stored fumes and repurpose them with hose barbs that can supply manifold vacuum signals to a manometer system. When not used for testing, they can be soft plumbed back to the charcoal canister or simply capped off if you no longer use the vapor recovery system. Bill
You mention that the charcoal canister tubing manifolds can be used to hook up a manometer system. On my 1978 GTS, each carb does not have a separate tube. As I recall, the four rear carbs are on one piece of tubing and I think the four front carbs are on two separate tubes connected by a piece of rubber hose. So if you leave the tubes intact, could you just measure the front (combining the two tubes into one). And then measure the rear rear at the same time. In effect the front bank would be on one manometer tube and the rear bank would be on a second manometer tube. Ideally, 8 manometer tubes would better. As you stated with the throttle stops not touching and using the throttle cable with linkage attached, one could use two manometer tubes so each bank would be measured as a unit. Would this be worth the effort because you are still dealing with 4 carbs contributing to one measurement (as opposed to a separate measurement when using a manometer tube for each carb)? I do not want to cut the metal vacuum tube as presently installed. I'm thinking this method would allow some precision from front to rear synchronization. The difficulty would be if the measurements are off from front to rear, which carb do you adjust? Your thoughts.
Hi Jmaienza. With some effort, the tubing manifolds can be pulled from the intake manifolds. As I recall, they were bonded in with some sealant or adhesive.The resulting holes can be tapped and populated with hose barbs similar to those shown by magnesium. I like thick wall silicone tubing for vacuum signals as they slip on the hose barbs easily and pull of with a reasonable force. They also have a relatively high temperature rating. When done, a short piece of hose with a plug will seal the signal port or you can run soft silicone lines to the charcoal canister, consolidating all signals into one with T-fittings. I try to keep all the relative lengths equal like a good header system. The drilling into the manifold port, inside the manifold holes, are 1-mm in diameter IIRC, so this does dampen the vacuum signal somewhat, although it is still strong. I believe there is little benefit in looking a groups of tubes. One has to look at each one individually to gain significant benefit and ideally, all at the same time, for maximum benefit. Only in this way do you really see what is going on. Columns are pulsing up and down and when you make an adjustment, it affects all other cylinder signals. For instance, say there is a non contributing cylinder because there is not enough throttle opening and air flow. So vacuum will be high for this cylinder and when you open up the throttle valve for this cylinder and it flows more, vacuum for this cylinder will be reduced and the signal for this cylinder will be lower, so the mercury column will be correspondingly lower. Now when this cylinder flows more, engine rpm will increase and all other signals will be affected. What one wants to do is match the column movements across all 8-cylinders in real time and then adjust to the correct idle rpm with a single throttle cable adjustment at the cable adjustment screw. When everything is ok a idle, manually actuate the throttle cable and check transient response up to about 2000 rpm. If good, your are basically done. You may nip up the left side carb throttle stops to just touch the linkage if you want, but I'm not a fan of this. It may be that after a good drive and everything is cleaned out and stable, one should check balance one more time to ensure everything is still ok or make a final adjustment or two. If adjustments are now made, having not nipped up the idle stops will be an asset. The reason this method is superior is one never changes the load on the throttle linkage after adjustment. All adjustments are relative to the throttle cable, so the system is stable. Bring in the throttle stop and suddenly a throttle valve is not supported by the throttle cable, but by a independent screw and this changes the load state and will likely change the relative linkage associations. The multibank manometer system is the right measurement instrument and the throttle cable/linkage adjustment method is the right method. So, I'm now retired mechanical engineer having worked for 30-years in aerospace as a thermal/structural analyst. A big part of my job was verification and qualification testing of new products to verify that they acted as predicted. Instrumentation was always an issue and was always scrutinized with care. Were the readings as expected and were they correct was continually challenged. Lives depended on it. Even though all instruments were calibrated by metrology and up to date, they could be improperly used and could therefore give incorrect results. Put a thermocouple in a dead air area and it will not react as expected. Don't assume, challenge and verify for best results. Bill
Bill Thank you for the excellent explanation. Your answer was as I expected about each carb having its own reading. I assume the carb has to be off the car to put in the new tube. At this point, I am not sure I want to do this. My car has been together for just over 3 years after a 3 year rebuild. Putting on the carbs is quite a chore and the way it is running, which I estimate to be about 90% regarding carb tuning, I just may accept the present state of tune and just drive it. Your explanation and Magnesium's post have helped me understand how these carbs work and how to tune them. I only wish I had read this when my carbs were off the car. For what it is worth, I have functional exhaust sampling tubes which I fabricated from brake nuts and brake lines. I have often wondered if a modern gas analyzer could be used to sample each exhaust tube and adjust accordingly. Perhaps silicone tubing to connect to a larger diameter receiving end to insert a probe. Haven't figured out anything yet. To both of you keep up the good work.
jmaienza, Yes, the carbs really need to be off the car so you have access to the intake manifold. I guess the good news is you should be able to preserve your present linkage lengths and adjustments as a starting point, so should be pretty close. Functional exhaust sampling tubes are a tremendous asset IMHO. I am making up a set from stainless tubing and stainless fittings I had made up, but I need to replace the headers with a spare set as the currently installed set had the bosses welded shut. Silicone tubing may work for connecting to a gas analyzer for idle mixture setting as the flow rate should be pretty low so heat transfer should be correspondingly modest. Also some cooling will occur in the brake line portion of the line. I believe a simple CO meter is all that is necessary to set idle mixture with good precision. I suppose a Colortune could work too but that is more of a relative measure, color, rather than a discrete reading of lambda or A/F ratio. Also, I have never seen correlation between Colortune color and A/F ratio or lambda and how 0-10% ethanol affects color. Anybody know? The Ferrari tuning specification for CO at idle, at the sampling ports with no air injection, is 3% on my 1978 US spec car and this is relatively quite rich. This may correspond to the onset of yellow on a Colortune but I do not know. Gunson makes the Colortune and they also make a relatively inexpensive CO meter, which I also have, so some day I will do the header swap and be able to use my meter on the individual cylinders with the new sampling tubes. I have recently replaced my OEM muffler with an aftermarket Larini that I modified to fit. I had bungs installed in each inlet pipe and my first look at real data, which indicated I was pretty close everywhere except at high speed cruise, at a constant 100 mph. So, I ordered up some replacement air correctors to try and richen the very top end. There is simply no substitute for real and timely data. Bill
I actually tried the Gunnison Gas Analyzer, but could not get consistent readings. The unit is basic and needs initial calibration, which I did, but after many attempts the data I got was not consistent. Adding bungs in the exhaust pipe just after the header is a real asset. I have some in my exhaust system and had it tuned on a dyno with an AFR meter connected to the bungs, not the tailpipe. We were really able to zero in on a decent AFR as well as fine tuning the timing from my XDI electronic ignition which has two adjustable timing knobs to set initial and final timing.
Hi jmaienza. Is your Gunson Gas Analyzer a late model that uses a digital display? In the past, I used a Hawk CO meter and while it's data was a bit scattered too, I was able to get a pretty good tune. I believe vintage Heathkit exhaust gas analyzers work the same way. When checking exhaust gas at the tail pipe, there is always the possibility of dilution from outside air and it may not be consistent. There is also the case where a very lean condition could fool the meter into thinking it is very rich, IIRC. This might happen if there is miss firing present, resulting in raw gas getting passed into the exhaust system. Before I purchased a used LM-1 on ebay, I asked Innovate about using their tail pipe clamp on my 308 and they thought this was not a good idea because of dilution issues. Based upon this feedback, I bit the bullet and installed a couple of bungs. Marcovicci-Wenz Engineering in Ronkonkoma, NY, who built a special Lotus Twincam engine for me, highly recommended a tuning bung in the header collector, a good approximation to the muffler inlet pipe location on the 308. So clearly, the intent is to get the O2 sensor reasonably close to the exhaust gas source and in a place where it gets a good overall average exhaust gas mix, but more importantly, is not influenced by ambient air. I suspect temperature may also play a role. I too am using an XDI ignition, but the programmable version. Would you kindly share the ignition timing or curve you are running? Bill
I used the model with the digital display. I wasn't impressed. My timing is 18 degrees at idle rising to about 42 at higher RPM. I use a multimeter to take a reading of the pink wire of the XDI unit. I would like to get an AFR unit and I know the Innovate units are highly rated. I saw another brand that has two sensors and and it can track each bank at the same time, record and then play back on its own screen. Very portable set-up.
