Lotus Elan

40 Dcoe Tuning Part 2

PostPost by: steveww » Sat May 01, 2004 4:40 pm

Investing the time and effort to install my TJ is looking

What is "TJ" ?

I think finding the right set up on SU / Strombergs is a bit easier. I have yet to set up my Strombergs to take account of the changes to my twinc. I have done similar settings with SU on other engines. All it takes is some time on the rolling road and working through needle charts. I have a little computer program that helps with needle selection which makes this even easier. Anyway I quite like having a power bulge on the bonnet (hood) ;)

BTW: I am sure we have all enjoyed and found the weber tuning saga posted here quite fascinating. My thanks to Keith for taking the time and effort to do this. I have a lot of good tuning data for Stromberg / SU if anyone is interested?
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PostPost by: type26owner » Sat May 01, 2004 5:50 pm

Hey Steve,
"TJ" stands for Tecalemit-Jackson. It's a vintage mechanical fuel injection from the early seventies. We've had a 45mm racing version of it on our 1966 Lotus 41 Formula B since about 1970. They are very rare. The one Roger Sieling tipped me to purchase off of Ebay to fit into the Elan is even rarier. It was designed to replace the Webers on a stock configured Lotus Cortina twincam. First one I've ever seen with 38mm throttle bodies.

BTW, the funniest part is the installation instructions is where they warn you in big bold letters that NO ADJUSTMENT IS REQUIRED. Yeah, right. Geez, I wonder what chemical substance they were adding at that time to their Kool-Aid. ;) They went out of business pronto.
Regards,
Keith
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PostPost by: type26owner » Mon May 03, 2004 2:47 pm

Decided I'll have a go at finding some jetting combos which give the desired results. Maybe I should start by plunking in the stock ones first and see where they perform on the Weber Gameboard of guessing fun and excitement. Just maybe the grid is well stocked with stable combos which will work to perfection. Can't find any info which contradicts that hope. Don't want to quit now without at least giving it a try.

It would be very valuable to me if someone would replicate my one perfectly performing jetting combination at 11:1 and report back the results. This would confirm if the different types of fuels influence the mixture in a big way or not.

Here's the settings:
F11 emulsion tube - stock
30mm main venturi - stock
4.5mm auxillary venturi - stock
110 main jet
120 air corrector

Keith
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PostPost by: rgh0 » Tue May 04, 2004 9:24 am

Keith

Your settings are very much the same as the standard settings for the 30m choke car in my plus 2 manual (X050T0327Z)

Keiths settings:
F11 emulsion tube - stock
30mm main venturi - stock
4.5mm auxillary venturi - stock
110 main jet
120 air corrector

Standard Lotus plus 2 settings ( S engine spec not SE engine spec or S130)

F11 emulsion tube
30mm choke
4.5 mm aux venturi ( only option on a 40DCOE)
110 main jet
115 air corrector - This is only difference, lotus like it a little richer
45 F8 idle jet
pump jet 35 stroke 10 mm

However the settings for a 30mm choke standard Elan in my Elan manual (36/T 327) are siginficantly different from the plus2, hard to understand why Lotus did this. Maybe thus just got it right on the plus 2. My Elan came with these settings and it never ran well always too rich especially in mid range around town, and I ended up soon after I bought it moving to the 33mm chokes and sprint jetting which ran much better.

Standard Lotus Elan settings ( not SE or sprint)

F11 emulsion tube
30 mm choke
4.5 aux venturi
115 main jet
155 air corrector
50 F8 idle jet
pump jet 40 stroke 10 mm

I hope you figure out what all this means
Rohan
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PostPost by: type26owner » Tue May 04, 2004 1:51 pm

Rohan,
What it means is some idiot like myself has to try each jetting combination in turn and map out the flow domains caused by the two phase flow effects for these conditions. Where they are on the jetting X-Y grid and the mixture strengths. Hopefully somewhere out there on the gamegrid are ones with a mixture of 13:1 and 15:1 which stays within one pound per hour of air over the entire dynamic rpm range. Got to ask myself why this has not been done before and put into the public domain. Sure would make it whole lot easier to do the jetting if there existed a mixture map.
Regards,
Keith
p.s. Makes me wonder if there already is a secret treasure map lost in the archives at Lotus or Weber. Anyone out there well connected with those folks?
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PostPost by: type26owner » Wed May 05, 2004 4:59 pm

Just to use as a benchmark. Swapped out the F11 emulsion tube for some F7 ones just to see the affect but left all the other settings the same. There was no observable difference. The engine ran perfectly well at 11:1. Emulsion tubes are NOT a first order effect. It's the jetting which dominants the mixture's behavior.

Here's the settings now:
F7 emulsion tube - non-stock
30mm main venturi - stock
4.5mm auxillary venturi - stock
110 main jet
120 air corrector

Keith
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PostPost by: type26owner » Thu May 06, 2004 3:11 pm

<a href='http://www.teglerizer.com/dcoe/dcoe_a_letter_from_an_end_user.htm' target='_blank'>http://www.teglerizer.com/dcoe/dcoe_a_lett...an_end_user.htm</a>
First, economy seemed to improve when GOING UP in jet size.

I believe it! Perfectly consistant with the expected behavior due to jetting to a two phase flow emulsion stable domain with enough bandwidth so the rpms stay within it. Now the trick is to reduce the number of adjustment knobs (jetting) to only one if possible or to modify the surface tension properties of the fuel to expand the bandwidth of a domain (a real long shot but I have such a product to try). I'm pleased when I've got some ideas to try.:rolleyes:
Keith
p.s. Bingo! Thought of a way to solve it on the way into work. :)
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PostPost by: JACKJABBA » Sun May 09, 2004 4:01 pm

Keith, which idle jets have you ended up using ou t of your reamed 45's and what pump jets?

Regards Jack
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PostPost by: type26owner » Sun May 09, 2004 9:32 pm

Hi Jack,
I've got 40F9's installed but they're still way to fat. Finding the idle jets are not all that important. They only cover a small 500-800 rpm range from off idle to when the mains are in full control. This is good because they will be insensitive to any two phase flow effects over such a small rpm range.

Try my settings! It will run like a scalded dog. Just be aware it will be running to fat at 11:1. BTW, with such a small 120 air corrector the engine is likely to experience the momentary hesitation occasionally caused by the atomized fuel suddenly wetting onto the induction manifold at WOT. This effect is likely intensified by having the thermal settings wrong on the cooling system.

I'm at a point in my thought process that I've realized only one of two conditions is true. Either the Weber engineers did not understand two phase flow or what I have in mind to solve to this problem won't work. Go figure! :rolleyes:
Regards,
Keith
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PostPost by: type26owner » Sun May 16, 2004 1:59 am

Started studying the main jet stack in the carb 'well' and realized the float level positions the fuel level just about 1mm below the passage leading to the auxillary venturi. Measuring down the well hole with the probe end of a dial calipers was a joke to achieve any accuracy. Anyone have a better way (more accurate) of doing this process?

There is an circular array of eight 1mm air passage holes which point up at 60 degree angles and are equally spaced and a reduced diameter section of the tubing with a lower sloped shoulder that the holes penetrate on the F11 emulsion tube. The shoulder is just even with the passage.

One of the tricks I thought of doing to tweak the two phase flow was to insert a sleeve into the auxillary venturi passageways to reduce the diameter. Unfortunately that passageway was machined into the carb body from the bottomside and the hole through the bore is plugged off. There's no clear shot to insert a sleeve from the top of the well, the way the jet stack gets installed. Got a few others tricks to try though. :blink:
Keith
p.s. How do I paste a 3D model picture of the jetstack into this forum? The help isn't.
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PostPost by: BillGavin » Sun May 16, 2004 10:38 am

Keith

Use a pipette in the jet well.
SK carbs used to sell one for measuring fuel level/float height.

Watch out for miniscus effect; the amount of fuel displaced by the pipette will hopefully be small enough to neglect, or will be consistent enough to neglect or compensate for.

- Bill
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PostPost by: type26owner » Sun May 16, 2004 3:44 pm

Thanks Bill,
Good idea! I'll give that a shot.

Been looking for any published fuel level values which compare to my measured value. This link is the only resource I can find.
<a href='http://www.centerlinealfa.com/tips/images/weber_info.pdf' target='_blank'>http://www.centerlinealfa.com/tips/images/.../weber_info.pdf</a>

If this is the correct height at 29mm it positions the fuel level 1mm below the lowest four air corrector holes on the F11 emulsion tube. I'm guessing the auxillary venturi would prefer to suck on just air until it exceeds the flow rate of the installed air jet and then finally some fuel would be lifted by the 6mm to also flow into the passage. Seems likely this setting would guarantee a huge flatspot.

So far I've been downsizing the main jet size from the standard one to lean out the mixture. Have come to realization this is probably the wrong tack to take. The negative side of doing that is the fuel level will drop in the well (maybe all the way to the main jet orifice) by a larger amount at WOT. This causes the mixture to lean out at the higher rpms because it's limited by the flow rate of the fuel through the main jet. Suspect the main jet must be sized larger so as not to allow the fuel level to drop by too much. Seems plausible that the mixture should be controlled by the air corrector which must be matched for the desired mixture ratio and also fall within a stable flow domain envelope. The air corrector must also be large enough so it does not restrict at high flow rates or the mixture would go fat in theory. Have not seen this condition yet. Easy to test it though.

Bought all the thingies necessary to machine up all the jets sizes I'm missing. So off I go to play today and do it. :)
Keith
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PostPost by: type26owner » Fri May 21, 2004 5:00 am

Found this link and was fascinated with the possibilities. This is very likely an excellent visual example of two different domains of two phase flow effects. My guess is the one on the left is an example of 'slug' type flow.
<a href='http://www.factorypro.com/products/emu_hi_disp.html' target='_blank'>http://www.factorypro.com/products/emu_hi_disp.html</a>
Could it really be this simple? Can one simply observe the mixture discharging from the auxillary venturi and observe a two phase flow change? I could evaluate a specific jetting combination at least for the no load condition by this process in just a few seconds potentially. Feels almost like cheating! I already have a jetting combination at 11:1 which would give me a visual baseline of what proper flow should look like. This I've got to try! :D
Keith
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PostPost by: steveww » Fri May 21, 2004 10:37 am

Very interesting. I note that these are for bike carbs. I have always thought that car carb development stopped way back in the 70s as fuel injection started to appear. However bikes are still running carbs, in fact constant vacuum carbs like Stromberg / SU. These carbs are being used in top level competition and are still being developed. If you want the best carbs surely bike carbs are the way to go. I know some of the mini racers use bike carbs and 8 port heads. Hmm... May be there is some mileage in fitting 4 big bike carbs to the Lotus twinc? The problem with that is you are off in to uncharted territory and working out what needles etc to use will be as painful as getting a pair of Weber 40 set up sweet <_< Not to mention mounting them up to the head. You can see why so many people just fit a fuel injection system with throttle bodies.
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PostPost by: type26owner » Fri May 21, 2004 3:08 pm

Steve,
Haven't really paid that much attention to the bike carbs but it appears to me that they just added another tier of jetting to solve the problem. What this appears to do is reduce the operating rpm range of each jetting system but at the cost of increasing the complexity of the tuning and the cost. That does not impress me as a wise engineering decision. My impression is it just makes it more user unfriendly for a rookie tuner as a matter of fact. :(
Regards,
Keith
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