Lotus Elan and Lotus Elan Plus 2 Forums

Having a changing mixture strength behavoir issue within the rpm range of the progressive holes. As the rpms rise the mixture gets fatter. This is independent of the size of the idle jets installed. It's independent of the engine loading too. Mixture difference is typically when at 1500-2000 rpm = 12:1 and at 2500-3000 = 10:1. Now the question is what to do to change this so the mixture remains constant throughout it's entire range. My options seem to be to change the flow rate of the progressive holes or the pressure gradient forces acting on them. The force is influenced by main venturi just upstream and is my only standard tuning option provided by Weber. I've got a stock 30mm main venturi installed right now. I'm reluctant to change the main venturi because this affects the main jet side of the tuning possibly in a big way. That could get very expensive to swap and suck all new emulsion tubes and auxillary venturis. I might try to fabricate some inserts which fit into the access pockets on the backside of the progressive hole to reduce the size of the third progressive hole and solve it that way. Can't realize the benefits of having obtaining the best fuel economy and have every cylinder fire cleanly at any time with this condition present.

BTW, Figure_5 on page 7 titled 'Mixture_Strenght' of the Weber Tuning Manual does not make sense to me unless it's just refering to the main jets. It's does not say one way or the other though.:unsure:
Keith

Geez!!! John Passini not withstanding, how th' hell are you gettin' ~accurate~ air/fuel ratio readings at those RPM's?!?!?!

That said, why can you NOT cobble up a set of tubes to get you where you want to be??

this is faaar beyond a 'simple' Weber tweak. 'specially if "fat gooood, lean baaaad!"

and finally: you're trollin' here, ain't ya? ;-}

I'm now curious to find if you get an optimum solution. I have this MGB fitted with a Weber, y'see.....

Welcome Dr E,

Geez!!! John Passini not withstanding, how th' hell are you gettin' ~accurate~ air/fuel ratio readings at those RPM's?!?!?!

Check out my earlier posting titled 'Affordable Wide Band O2 Sensor' and you'll know how it's done. Suggest you read everything I've posted here to get up to speed.

That said, why can you NOT cobble up a set of tubes to get you where you want to be??

Man, if you've got the answers, I'm all ears. Please explain your solution in more detail. To give you some perspective in my opinion this stuff is at least tens times harder then the thermodynamics issues I've been dealing with recently. There are so many variables that interact in very complicated ways that I can't measure which makes this is a bitch to solve. The bottomline is you have to swap and suck to confirm anything.

this is faaar beyond a 'simple' Weber tweak. 'specially if "fat gooood, lean baaaad!"

and finally: you're trollin' here, ain't ya? ;-}

Yep, it isn't simple stuff and not in my field of expertise. My line of work is the engineering design of scientific instrumentation. Here's a link to one of my x-ray microscopes to establish some creditability. I'm just doing this stuff for the fun of it and because I can't believe the experts.
<a href='http://unicorn.mcmaster.ca/highlights/stxm532/stxm532.html' target='_blank'>http://unicorn.mcmaster.ca/highlights/stxm...32/stxm532.html</a>

I'm now curious to find if you get an optimum solution. I have this MGB fitted with a Weber, y'see.....

I suspect the issues I'm resolving are common on most cars that came fitted with Webers. Also that the cost incurred for the manufacturers to apply these tweaks to each car's individual needs would soon drive them out of business. Guessing only a niche high end outfit like Ferrari or Aston Martin would do this routinely.
Regards,
Keith Franck

Thanks fer th' welcome.

I wish I had better Weber answers.

Yep, it isn't simple stuff and not in my field of expertise. My line of work is the engineering design of scientific instrumentation. Here's a link to one of my x-ray microscopes to establish some creditability. I'm just doing this stuff for the fun of it and because I can't believe the experts.

There weren't much doubt as to your credibility... I was a bit stunned as to the depth of the info. Webers are (IMHO) part empirical, part f?n VOODOO. I've been playin' with 'em since the early '70's (Lotus', Alfas, MG's, V12 Ferrari's)... and still have nowhere near the level of intimacy you've exhibited in that ~one~ post. It took me aback a bit. Change any one jet and it has effect on the whole range... I will peruse your prior postings to try and "catch up," but fear I'll be left in the dust, still clutchin' th' mojo bag I use as a Weber tuning aid. ;-} I'm at 'caveman' level here: A ColourTune, manometre, plug cuts and me ear! I'm fascinated by this info, as in the moth/flame relationship.

I suspect the issues I'm resolving are common on most cars that came fitted with Webers. Also that the cost incurred for the manufacturers to apply these tweaks to each car's individual needs would soon drive them out of business. Guessing only a niche high end outfit like Ferrari or Aston Martin would do this routinely.

I believe you're correct, and with regard to expense I know you are. Further: I'm not convinced those lads (E.F.'s or D.B.'s) had it any better sorted than the rest. With time, patience and a sack-full of $hiny Weber bits there always seemed to be a bit better idle/resopnse/high-end/mid-range, etc. one could find, but the result never met the effort in a cost/benefit sense. You're on a journey I will observe with interest and respect. Thank you.

Well then, let me reveal the selection criteria I'm going to use for choosing the size of the idle jets. Kind of had one of those moments last weekend when it all became quite clear and the light lit up real bright. Discovered my engine is most sensitive to any mixture changes at 1800 rpm with a no load condition. 13:1 turns out to be it's sweet spot. Found this by swap and sucking on a range of different size idle jets and doing this same test over and over again. After tuning for the best idle with the mixture screw with a set idle jets I'd then raise the rpms to 1800 and listen at the tailpipe for a clean exhaust note. If it was missing I'd go in turn to each idle mixture screw and fatten or lean it by 1/6 of a turn and go listen for the resulting exhaust note. In this way I could tell which way mixture wise to go with the idle jet selection. When I installed the idle jets that got it to 13:1 it would miss by an equal amount whether fat or lean on each cylinder in turn when this test was done. It had a perfectly clean exhaust note when the mixture screws were set back to the neutral position though. Kind of makes me feel geeky thinking I'm now the Weber equivalent of a horse whisper.
Keith
p.s. This is the last item to complete on my to-do carlist which I've had for the past eight something years.

Hey Keith, looks like we are in the same sort of field of work, I work for Hitachi on SEM's. You should be use to the voodoo factor, as TM's are a black art, or so you TM guys keep telling me.

Starting to make some progress with my webers now that the new dizzy is fitted. What a difference that made!

Jack

Gee, now that I think about it I've worked on all flavors of microscopes at one time or another. It's just provided good training to do the more important work like sorting out the problematic behavoir of Webers.

Realized the solution to reducing the flow of fuel out of the third progressive hole is real easy to do most likely. I'll try bending up a 270 degree loop of wire which fits snugly inside the backside access pocket of the holes. On one end it will have a sharp 90 degree bend which fits down into the hole and the reduces the flow by virtue of it's diameter and the turbulence it creates. The other end will be bent to stick in towards the center of the loop and and slope up to contact the screwed-in threaded cap so it is held firmly in place by it's springyness. I already have a huge selection of small diameter music wire which I've used for similiar purposes to tune the flow on the Tecalemit-Jackosn fuel injection system on the 41. Should be a piece of cake!
Keith

It occurred to me driving home there should be a measurable way of determining the correct phasing of the throttle plates relative to the first progressive hole to see if the desired idle speed can be achieved without a hesitation. This should nearly be totally insensitive to the actual size of the idle jets which are installed too. By elevating the idling speed well up into the range of the progressive holes like about 2k rpm and then closing off all the idle mixture screws one can slowly back off the rpms to determine the minimum the engine can run on via the first first hole alone. By comparing that rpm to the same condition applied to a benchmarked properly adjusted carb system a determination can be made if further adjustment of the throttle plate position is needed by drilling air bleed holes or chamferring of the plate edges. The other indicator which could be used is to measure the minimal sustainable rpm for combustion is the amount of clocking travel in the idle speed screw from there to the chosen idling rpm. With this info it might be possible for someone to do these adjustments successfully without requiring a Wide Band O2 Sensor. Would anyone like me to benchmark those values?

The reason this works is the contribution to the mixture from the idle mixture screws to sustain an idling rpm can be assumed to be a mathematical constant regardless of the size of the idle jets which are installed because they always should have enough capacity to oversupply enough mixture at 2K rpms and below. It's contribution then can be safely subtracted (closing the idle mixture screws) without skewing the results uncontrollably. This allows the threshhold rpm to be compared to a benchmark value since the weaker mixture result is canceled by the benchmark's throttle plates being repositioned to the first hole when it's idle mixture screws were likewise closed. Determining a tolerance is more tricky and I don't have an answer for that one. Perhaps like hand gernades getting it close might be good enough though. This assumes all the other variables which affect combustion are nominal like the cylinder compression values.

To but this into perspective there always is an idle speed with the stock setup where there will be no hesitation caused by a weak mixture when coming off idle. Lotus just left it so that point happens to be around 1500 rpm.
Keith

For the last week or so, reading these posts from Keith et al, I've been re-motivated. Currently I only have the Alfa Spider to play with: a few years ago (four, I think) I put together a spiving 1750cc, then proceeded to replace the lump of a 2 litre it had when I got it. I had a set of 40 DCOE 18's just sitting around ;-} so... on they went (after going thru 'em on the bench, and jetting them for the 1750 as per Alfa's OEM settings). I've been driving the car since then as my daily commuter and done nothing but change oil on schedule and plugs once. It doesn't idle as well as it might after it gets up to temp, and I've basically "put up with it" 'cause I'm not too bothered by a lumpy idle. But now that I've been lurking in here I'm gonna see if I can "fuss" the mix down to a good smooth idle with Keith's info as a guide. The progression holes are likely not right for the Alfa's setup, but with some differing guage wire and the weekend to meself... who knows. Even Herself may be more at ease driving it around if it'll come off idle a bit more smoothly.

Thanks for the great ideas, Keith!

P.S: The AFR meter is now tops on my "Mommy can I have one of those, PLEEEZE?!?!?" list as well.

Dr Entropy,
I'm very pleased to have helped. I realize there's only a few folks out there that are going to technically savvy enough already so my little contribution will help make the light bulb light up bright for them too. Let me know if there's anything I can help you on. Remember I'm pretty good on the theory part but short on first-hand experience (I've done this only once). Good luck!
Regards,
Keith

The O-2 metre won't arrive in time for the weekend, but I'm still gonna have a go with the wire thingie. That's an idea I'm ~ashamed~ I hadn't had. Not given it a thought (kinda like the B-B in the vacuum line)! I consider meself a fairly clever lad, but that one plumb escaped me. I've spent the last 30-some years as a Lotus owner, at one time with three (S-3 Elan, +2, and a MkI Cortina) as our only means of transport. In recent years (the last 15) I've earned me living wrenching... British cars mostly in the first 5, then with a restoration shop in sunny Sarasota for the rest. Damned demanding, as we took it seriously. Now I've slid into I.T. as a consultant (small business networking) and kinda miss the car thing. That was the reason for my sniffing out this forum again.

Segue: Th Beeb just reported M.Schumacher has the Ferrari in Malay race well positioned, apparently...

sorry-- easily distracted in my dotage.

I'm looking forward to trying the tweak. I'll report back.

Got the mixture through the full rpm range of the progressive hole to remain the same value using the wire insert technique. Ended up needing a 0.8mm wire stuck into the third 1.0mm hole to reduce the aperture area by 60%.

Found that the wire's loop shape didn't work well. Instead a triangular shape with just two legs creating a stable plane to sit on the bottom of the carb housing's pocket worked like a charm. Needed two right-handed ones and two left-handed ones to clear the other progressive holes. Piece of cake! B)

Make sure those caps are not leaking in air. They're very prone to cause a miss at idle which can't be tuned away. Maybe I'll actually see a measurable increase of the fuel mileage with this modification. :unsure:

Should mention that the choke mechanism is necessary for cold starting now.
Keith

Keith,
Needing the chokes for starting? Wow, I think you're going to have the first 'green' Elan. Very nice!

Greg Z.

Hi Greg,
Funny you should mention this. Took my wife's Corvette C5 to get it smogged yesterday on the dyno as required now by California. Realized if I had an old 3-gas smog machine to tune the Webers I'd have half a chance now of passing the Elan under the current requirements if they require it again. No please, Arnold, No! :rolleyes:
Regards,
Keith

Hoping ultimately by getting the combustion process to be cleaner I can reduce the carbon deposits. Might be able to extract a few more horsepower from the engine by bumping up (advancing) the total ignition timing by a few degrees if it will not knock or ping on the same unleaded pump fuel. The carbon sometimes acts as a glow plug at WOT and can be the primary limiting factor in some engines. We'll see if it applies to the twincam too. Have to first remove the O2 sensor and feed it ChemTool for about four tankfuls to find out. B)
Keith