40 Dcoe Tuning Part 2

Identified two more issues to resolve now to get it all sorted correctly.

One is the cold start choke unit when engaged halfway causes the mixture to fatten to less then a 10:1 mixture but it does not raise the idle speed. If pulled on fully it will not assist the starting and it will not sustain the running after it does start. Since this unit supplies both additional air and fuel this should not be the correct result. It should rev the engine up to about 2k rpm and help not hinder the cold running performance. Suspect the stock setup is jetted way to fat.

Secondly, I’ve got a mixture leaning situation at high revs now. None of the tuning books properly address how to correct this condition BTW. By jetting the mains (100 w/Shell 91 fuel) for a mixture of 13:1 at ~3K rpms w/no load by the time it gets to 6k at WOT and at maximum load it’s at 17:1. This is not okay and I’ve already got tiny 140 air corrector jets installed with little affect on this situation. Appears the best adjustment to try first is to install larger main venturis. The stock 30mm ones are undersized according to the graph (pg22, fig32) in the Weber Tuning Manual. This is assuming that the one labeled ‘4-stroke sport engines’ is the correct one to apply for the twincam. By this graph a 36mm size is required which just so happens to be the largest size which fits and is available for the 40DCOE. So what I think is happening is the undersized venturi is generating to much low pressure at to low a rpm. This causes a very fat mixture at the intial rpm point of mixture’s introduction. When it reaches the high revs it then is restricting the airflow so much it impedes the auxillary venturi from reacting to a high enough vacuum to keep the mixture constant. By enlarging the main venturi it should reduce the vacuum level and delay it a low revs and at the high end it should allow more air to pass through so according to Bernoulli’s principle it could actually increase the vacuum level in theory. Have to test this theory. Just hope it’s this simple to solve. Can’t believe how lame the advised criteria on which the tuning books use to size the the main venturis. My engineering intuition tells me it’s generally true but for a different reason then what’s described. :frowning:
Keith
p.s. Hoping at this point the stock exhaust system does not impede the induction system at higher flow rates. Going to attempt to measure the pressure drop at the downstream point the auxilary venturi protrudes into the main venturi at least for the no load condition. It may be possible to establish a correlation between the pressure drop and the mixture strength. These two effects should be slaved together ideally.

Keith, 30mm to 36mm choke is a heck of a jump. Wouldn’t you be trading off a proper vacuum to getting too much air, hence leaning it out at high revs? Why not go to a, say, 32mm choke to see if your theory is correct?

Greg Z.

Hi Greg,
I’m aware of that situation and please realize I don’t care to throw endless amounts of money at this situation either. Contacted my friend Mike Ostrov earlier this week to borrow some venturis for testing purposes. He’s graciously offered me all the sizes he’s got socked away as spares. One set is of 33mm which I’m hoping is enough of difference so I tell if it’s the right direction of component to change in order to correct this problem.

Geez, a set of these venturis is nearly $100 for each set. Asked Doug at Pierce Manifolds how much a complete complete set of spares of every part offered for the 40DCOE and he guessed it would cost well over $20K to complete one. The real challenge here is to succeed correcting these problems without spending a fortune doing it.
Regards,
Keith

Been pondering this situation and have realized maybe this condition is real and the typical limitation of a carbie. Trouble is the experts are so vague on this subject that their comments are really quite useless for me to make that determination. All you get is a statement like ‘for getting the best drivability one should install this size main venturi’. Hey, wait minute here why is this so? What are the underlying physical laws that make that true? To make a broad sweeping statement like that without presenting any data or a complete examination of the system dynamics is setting off my bs alarm bigtime. All these tuning books suck. I wish one were written in a true engineering article format.

Now that I ranted about the quality of the info which we’ve all got to deal with these issues. Iit occurred to me there might be a simple way to cobble together a cure if I can determine it’s utimately needed. Since at WOT the mixture is going extremely lean then the obvious solution is to increase the available energy which operates the induction system but only at the WOT condition. This could be done by just increasing the pressure gradient between the air pocket in the float bowl and the main venturi. Can’t really do anything about the main venturi since I’d leave it sized for the mixture needs of a partial open throttle condition. What possibly could be done though is to slightly elevate the air pressure in the float bowl but only at WOT. The pressure source could be a small CO2 bottle about a 10 lbs size would be good for months of driving. It would supply a pnuematic control valve which I’d cobble onto the end of the Weber throttle spindle. It’s designed to provide the extra potential energy via a pressure increase at the WOT position and releases that additional pressure at any other spindle position. The control valve would also have a second stage which would operate at a few psi to close two more pnuematically operated valves which are fitted into the vent holes of the carb body which are there to breathe into the airbox. Possibly most of the pieces necessary could be purchased for a few hundred dollars or less. Best guess is the pressure needed to do the trick is a few inches of mercury at most. Just a thought. :slight_smile:
Keith
p.s. Hey Steve, if I can’t resolve these issues then I will be installing my TJ fuel injection system afterall.

Was able to enlarge the fuel passages on sets of 40F2, 40F13 & 40F11 idle jets to make them into the NLA 45 ones I needed. Drilled a tight fitting pocket into a block of nylon which accepted the engagement end of the idle jet. This centered the jet body closely to the quill spindle of the milling machine. The carbide drill would hold the hole size to a few microns of error in diameter no problem. Measured it with the pin gauges I bought from Pierce Manifolds. 0.45mm just happened to be smallest size on the gauge. Didn’t break a drill or ruin any jets in the process. Whew!

Found a set of 120 and 100 air corrector jets I’d forgotten I had. Installed the 100 ones to see what would happen. Seems like I’m able to move the errant slope of mixtures around the scale but not reduce the slope itself. Counting on Weber having done the engineering and applied the capability to overcome this condition. Just have to figure out how it’s done. :frowning:
Keith

Changing to a size 100 air corrector and a 100 main jet has apparently significantly changed the viscosity of the air/fuel mixture. Now when in 4th gear and at any rpm below 3k when the throttle plates are opened pass halfway the mixture goes extremely lean too. This is to expected if the mixture is trimmed to be too lean on the progressive holes by moving the plates away from them enough and the main venturi does not generate enough vacuum from the low airflow to extract the denser air/fuel mixture from the auxillary venturi. The stock jet settings were so overly rich this never happen before. Maybe this is what’s referred to as being ‘drivablity’ but not explained in any detail so one would actually know it. Go figure.

The baffling part is the Weber Tuning Manual describes a simple carburetor on pg.9 and the leaning at the high rpms I’m experiencing is contrary the basic principles too. No help there either. :angry:

Going to start over again and make sure the AFM is giving me believable info. It’s brand new technology and it’s possible I’ve stumbled upon a glitch here.
Keith

Does the leanness at half throttle show up as a stumble?

It always pays to be sure of the measuring equipment, but so far it seems to be giving good data.

Have you tried different Auxiliary Venturies? You may be able to get a stronger signal to the jets. You may also want to try different emulsion tubes.

You’re well into uncharted territory, it seems. With methodical work and the A/F meter, you should eventually win.

  • Bill

Hi Bill,

Would better describe like I turned the ignition off. Not good for throttle response but I’m getting fantastic mileage now. By backing away with these mixture settings one could definitely live with a small stumble by just not lugging the engine at low rpms but really benefit from the extra fuel mileage.

I’m going to send it back to the factory and pay to have it requalified again. Could be I’m just using it wrong too. I’ll cover those ducks next after I try the larger main venturis. This process could take a couple of weeks cause I’ve got other obligations to fulfill at the moment.

Nope. There are only two sizes available. The stock 4.5 ones are the larger ones. Have any of 3.5 ones laying around by chance? Again the written advice about what to expect from a particular emulsion tube is really not useful. I got the stock F11 ones and a set of F7 which are suppose to be fatter at the low rpms. Looks like the F16 ones are the direction to go possibly though but how does one to know without swapping and sucking. Even the Weber Manual is totally vague on this subject. :frowning:

Possibly. Going to definitely find the limits in no uncertain terms that I can understand and not blindly believe what’s been written. The experts could be wrong. I’ve succeeded many times in building devices that ‘the experts’ had deemed impossible. In my opinion the Weber designer really was a true master of his craft. I just need to kind of reverse engineer this stuff to explore the potential fully and find the true limits. Fun Stuff! :smiley:

Oh, forgot to add I removed the wires from the third progressive hole already. Can’t say for certain but after having swapped lots of idle jets the trend is the ones with a smallish well diameter universally do not improve the situation on my twincam. Haven’t the faintest idea why this would be the case , it just is.:blink:
Regards,
Keith
p.s. If only Carroll Smith had written a book on Weber tuning. He’s got my respect and nearly blind trust.

Okay, here’s my new theory. It’s so simple and elegant, it’s completely stupid and as far as I can tell completely overlooked by the experts. Hope it turns out to be actually true. The air/fuel mixture properties for each of the two jetting systems which must be met first is to match the fluid VISCOSITY by deriving the necessary ratio of air to fuel ratio via the jet(s) selection itself. Secondarily, the total quantity must be then be adjusted to get proper mixture to support the required type of combustion ALL THE WHILE MAINTAINING THE VISCOSITY. In other words one has to dance with the main venturi and the progressive holes viscosity sweet spot and then properly feed the combustion process. For the idle jets this means finding the best ratio for the viscostiy and then staying with that ratio. Weber has made this a complete nightmare with their random designations. It might actually be easier to modify to suit the needed jets then to deal with the Weber ones. The main jet and air corrector RATIO OF AREAS must be determined for the best viscosity and then both jets must be changed to increase or decrease as a matched set in order to get the proper amount of mixture to optimize the combustion. The limit here is to determine the necessary viscosity to best match the dynamic range of the airflow of the main and auxillary venturis to maintain a somewhat constant slope of the changing vacuum level to achieve a steady state mixture over the rpm range.

How this is done as separate tuning steps I have not solved yet. Hoping to make some headway otherwise it’s such a complex problem that tuning the standard way is almost impossible and so daunting I’m considering giving it up as a hopeless endeavor for a rookie like myself to even do it.:frowning:
Keith

That makes sense to me.

I don’t know how you would home in on the right series of idle jets, other than the usual method of an educated guess on the series and size, then trying it with the idle screws backed out a certain amount - what is it, 1-3/4 turns for the early, fine thread needles?

There are lots of combinations that will work at this point, so the next step is to make more educated guesses about the emulsion tube, main jet, and air corrector. Then more “suck it and see” until the transitions are right and the mixture good through the midrange.

If your theory is correct, there should be a matching of an idle jet series and an emulsion tube that will match the emulsion viscosities, at least in the transition zone. (I can hear Rod Serlings voice now, “Welcome to… The Transition Zone”)

None of this stuff is linear, so it seems that there will always be some aspect of hit-or-miss. I’m not altogether sure of the influence of the emulsion tubes’ characteristics - we have ID, OD, size of holes, number of holes, and distribution of holes, and maybe more. The idle jets are simpler, having just ID, air bleed, and delivery size, but still non-linear.

Complicating it all, the behavior of the jets is considered to vary considerably depending on whether they are factory-made or redrilled, as the radius at the ends of the holes will vary. Idle jets, as far as I can see, are just drilled at the factory, so they should be OK, as long as you can keep track of what size you have.

Something else to consider is that we’re working with a number of discrete steps for each variable, and their effects are non-linear, so it probably is not even possible to get perfect mixture through the range. We can, however, measure A/F ratio fairly precisely: this brings up the question “how close is close enough?”.

The cost of this exercise is also an issue: I have a lot of Weber stuff left over from my racing Mini and time trial Midget, but everything is in pairs, so I’d still have to spend lots of $$ to play around much with the Lotus.

Bill,
Everything clicks into place if you have the correct perspective and it then becomes a simple matrix to solve. To do it you just pick a size of main jet that is suppose to be the mean and then step through the range of air correctors to find the combination which does not lean out at either or both ends of the rpm dynamic range of the jet system. If one cannot meet this best viscosity condition then probably the size selection of the main venturi is not optimized for the cfm of the induction needs of the engine. This superimposes the two flow components and the slopes from the venturis to the mixture viscosity on top of one another as a best fit senario for finding a steady state mixture. It does not have to be linear. Just the slope shapes and magnitudes have to match up fairly well. Once the equilibrium is established then it’s just tuning for the mixture strength to achieve the desired ‘drivabilty’(I can be vague too :smiley: ). This is a best compromise fitting procedure. I’d be happy if I could halve the difference in the slope I’ve got right now. That is if the difference was only 2 lbs of air/hour instead of 4.

Suggest we figure out the conceptional solution before plunging into the practical aspects like the quality of modified jets. By doing this correctly one should be able to optimize both jet systems in like 20 swap & suck steps total. Tricky part is to optimize the size the venturis with no fuss or bother. I’m lazy and desire this to be so dead simple I can do it even after having consumed some beverages. :smiley:
Regards,
Keith

Talking of viscosity… The viscosity of fuel changes with its temp. How does the Weber correct for this?

I am still considering sticking with the Stromberg head and putting on a pair of SU HIF44. I have just received the dimensions from Burlen Fuel Systems so it is out to the garage with the tape measure to see if they would fit. Going to SU would give me a better choice of needles / springs. Now for the controversial bit :astonished: A well set up SU is as good as any Weber side draft. As far as I can see the Stromberg head is well designed and should have no problems getting the required flow. However I am not looking for absolute power as the car is mainly used on the road and the extra economy will keep the range good from a small fuel tank. Thus on a long trip my car will be much faster than Weber equipped cars as I will have to stop for fuel less often :wink:

Hi Steve,

The density difference between gasoline and air is about 570:1 so any viscosity variation of either due to temp is in the noise. What is key to this and is a first order effect is the mixing the right ratio of air to gasoline to modify it’s fluid properties to best suit the small potential energy from the vacuum sources. This dominates the functioning of the carbie.

IIRC it’s called latent heat. It’s got built-in cooling. Suspect it’s best to establish a thermal equilibium by having an enclosed engine bay and NOT overcooling the engine.

Not going to comment on your other topics cause it’s well outside my expertise and my interests as of right now.
Regards,
Keith

With the 30mm main venturi installed I can state that the main jet system starts contributing at about a 2k rpm threshhold when the viscosity is close.

Having run around exploring the lean mixture realm I’ve stumbled upon another problem I didn’t know existed. On a frequency of about one in five times when into the accelerator pump I get a bigtime cutout. Suspect it’s vibration related and I’m getting foaming or the check valves are not sealing as a result. The Weber Manual covers this condition in detail and I’m going to change the natural frequency as per their recommendations. Hope that is the cure. :frowning:
Keith

Been hitting the books and the found a description to a condition which perfectly matches my accelerator pump cutout problem. Compressing it a bit it says under some condtions when the throttle is opened quickly the atomized fuel will wet out onto the walls instantaneously and cause a weak mixture and a resulting ‘misfire momentarily’ is possible. Of course it does not describe what actually are those conditions! I think maybe I’ve met them though but the good news is it can and will be tuned away. :slight_smile:
Keith

Hi,
Had very much the same “leaning out at high RPM” problem with a spitfire that I had fitted twin DCOE40s to. Tried all sorts of wacky jet/immulsion tube settings to get it right. Ended up solving it with a 3 step process.

  1. bob wieghts on the dizzy had jammed (replaced).
  2. Webbers were knackered (bought from friend) (replaced)
  3. 200 quid on an afternoon on the dyno with an expert (great fun doing 90+ MPH in a shed on tethers and great to watch a pro set a car up)
    Result 35 mpg and a whole lot of umph!
    If I was doing it again (and may have to one day with the Dellortos on the +2 as it seemeds to bog a little at low revs, would go to the same guy again, well worth it.
    Cheers
    Tim

Okay, so why does the air/fuel emulsion viscosity dominate the carbie? Simple. Because at the optimium viscosity the lowest pressure differential is required to efficiently maintain a steady state flow of the emulsion. What this does is extend the useful range of the venturi system towards the lower rpms. The fact that there is more emulsive air/fuel being being delivered at the point of the highest vacuum causes the vacuum to be extinguished at a faster rate because the vapor is filling the void at the mean vaporization value (maximum) of the bell curve.

This means when the carbie is tuned correctly they quiet down considerably. Because of this fact and the temperature effect from the latent heat behaving as thermometer I can walk up to any working carbie and diagnose in five seconds if it’s out of tune and by touching it determine whether it’s lean or fat if I’ve heard a proper tuned one running beforehand.

When I disclosed my theory to my son yesterday I could almost see the light bulb above his head come on brightly. He put two and two together immediately and stated this explains why the muscle cars folks can tune their carbies using a induction manifold vacuum gauge to get the best running engine by adjusting it till the smallest pressure drop is achieved. They been doing it like this for a long time but he has not run across anyone who knows why this actually works. :slight_smile:
Keith

Wow! I reserve the right to be wrong on this one because I’m reporting raw data without replicating it first. It appears the emulsion viscosity effect is much more dominant then even I envisioned. Apparently it’s a bell curve with a FWHM of 0.1mm or less of the air corrector jet sizing. I was stepping right over it using 0.2mm increments on multiple tuning passes missing this effect each time because I believed the experts that said the air correctors had an affect primarily at the highest rpms and large steps were okay. THEY’RE ALL WRONG! Here’s a link that generally explains what the FWHM is.
http://www.noao.edu/wiyn/images/fwhm.html

Got my mixture to remain within 1 lb/hr of air over the entire operating rpm range. Good enough to rival any fuel injection numbers. I won’t have to fool around experimenting with any oversized main venturi sets afterall.

Now I can empathize with the ethical dilemma the Weber designer must have experienced. He or She most likely was ORDERED to keep this quiet. Makes me all warm and fuzzy to comprend it like the original designer obviously did.
Keith
p.s. I’m out of here soon so if you don’t understand what I’ve covered ask it NOW. Bye.

Keith -

Do I understand correctly that you were seeing the same behavior on both sides of the correct point? Were you able to find the correct point with standard sizes, or were you going to smaller increments with your drills?

  • Bill

Bill,
I need to purchase all the sizes of air corrector jets around the sweet spot to confirm and define the curve dimensions with precision. Applying a leap of reasoning though this explains why the available step size increments is only 0.05mm. The emulsion viscosity bell curve is actually a sharp spike and with any bigger steps you’d miss it. Who’d have thunk it would turn out to be razor-edge adjustment! Have not drilled out any air corrector jets yet. Not expecting it to be required.

Possibly got it wrong were I said the tuning must be done from the bottom up. Have yet to explore the impact this has on the progressive hole tuning segment. :smiley:

Speculating all of their commercial customer that bought these carbies and installed them knew this info and decided to screw their customers based on the added cost. Weber kept it quiet to not rock the boat. Got to love it when poor tuning practice makes for good business practice. Must have blown their minds to have designed the perfect carbie and not have it applied correctly intentionally. BTDT. :frowning:
Regards,
Keith