weber jetting for high density altitude

[GLB]

Hello again. This is a belated continuance of my previous posting about taking my early Elan (0045) to LOG in Salt Lake City and a new question. My original plan was to take the Elan and my son take the +2S to LOG. As some may have seen the pictures, the +2S made it to Las Cruces NM, about 50 miles when it overheated from a failed head gasket. I was really surprised as that car had been on several long trips, over 5000 mi total, with no problems. It is now fixed and I know why it failed but that is another story. We got the +2 home safely and decided to carry on in the Elan. My friend Karl offered his truck and trailer but we decided on the adventure and with Karl leading the way in his Elise we left again, only about 3 hours late. In a way it was a gift because I got to spend a lot more time in the company of my son. It is surprising how much can fit in an Elan. We spent the day cruising up I 25 across the NM desert. Temps in the 90's F. Car was okay, about 90 C except in Albuquerque afternoon traffic, a little over 95 C. A little scary considering the size of everything else on the road. We left the interstate at Las Vegas NM and took to the two lane climbing all the while to spend the night in Alamosa CO. This is a very remote part of the country with no towns and little traffic. I was glad to have the company of Karl in a reliable Lotus. We stayed in a cheap hotel and left the next morning for Denver. We climbed higher into the mountains, over several high passes, The traffic increased but the car did fine. Made it to Denver and stayed with Karls son. The next morning the car started and made it about 3 blocks and died. We pushed back to the house, all down hill and a diagnosis of weak/no spark was made. Bad condenser. I brought 4 old used ones with me. Out with the old, a new manufacture one and in with the new, probably 30 years old, a few dumbass mistakes on my part and off we go. We were going to meet up with a group of about 40 Lotus to caravan to Salt Lake City but we missed them because of the condenser. We did make it to LOG, got a first place in type 26 class and made it back home. About 2300 miles. On the cover of ReMarque with Ross Robbins Elan this month.

I had had enough of the Elan for a while and did not drive it for a few months. When I did it had a high speed misfire. Changed the plugs and all is well. I took it out last week, about 40 degrees F and it ran great. Better than ever. When I changed the plugs they looked a little rich to me. Now the question which started this post.

I live at 4000 feet. I assume the jetting specified in the Lotus shop manual is for England, cooler and closer to sea level. I have some experience in small planes with density altitude and figure the jetting could be leaner for where I live and operate. The type 26 manual for my stock, not SE engine specifies 30 mm chokes, 115 main, 200 air and 45f9 idle. That is how it is set up now. Just on a lark I looked at the +2 manual and it showed 3 carb set ups. comparing only the 30mm choke set up it specifies 110 main, 155 air and 45f8 idle. The main is of coarse leaner, the 45 f8 is one step leaner than the 45 f9 for the idle jet. I assume since it was specified for the stock set up for the +2 it is not too lean, but it is leaner than what I have. I don't have any hesitation or flat spots, but the other day it just ran so crisp I feel the mixture was closer to ideal because of the cool temperature. These cars have been around a long time and I can't be the only person running at a higher altitude/ temperature. Does anyone have any suggestions based on experience? I have tried Keith Franck hypo jets and his subsequent idle jets for less idle manifold vacuum. He very kindly sent me a selection to try and I can change the plug color from white to sooted up black. They accelerate well but I always had a steady state surging that I could not cure. No surge with weber jeting. Any suggestions appreciated. Gary

[stugilmour]

Gary, great job on getting to LOG. Unfortunately the Border Revenooers wouldn’t let a Canadian contingent attend this year.

I am also at ~4,000 ft. Well, really 3,500 ft at home base and 4,500 ft at turn one. Talking to drag race guys, they said they would actually jet closer to 5,000 ft here due to low humidity.

I found this page helpful. Re-reading it, it pretty much says hang in there, try leaning things out, and you are not alone.

https://lcengineering.com/tuning-weber- ... -altitude/

Currently running Keith’s W jets and VP tubes. Running OK now, but took a ton of trial and error with an AFR gauge.

Not sure on the surging, but may have read somewhere it can be due to carb balance issues?

What fuel level are you running? Not sure if this would be a possible cause of surging (assuming Keith’s jets all want a higher fuel level). Fuel level more associated with the classic stumble at 2,500 to 3,000 RPM perhaps.

Plus 1 one running great on a cool damp morning! More power and the engine just feels happy.

I think Ross is closer to 5,000 ft at home base. He also runs the Franck tubes, not sure for his idle jets. Said he just installed them as per Keith’s instructions and all good. Sounded hopeful!

HTH

Stu

[GLB]

Thanks for your input, Stu. I am running 25 mm fuel level per Kieth Franck. The carb sync has been checked at 2500 as well as idle. No vacuum leaks i can find with propane. new leather shaft seals. The surging went away on returning to weber jetting. I think I would need an AF meter to get spot on, so for now I am using the seat of my pants, which is often deceiving and reading spark plugs. When I was tying Kieths hypo and W idle jets I had his O 6 emulsion tube because he had not come up with the VF tubes yet. I could not really tell any difference between them and the stock weber tube. I was using the same main/air jets, 115 and 200, What combination do you have now? I read the article you referenced and wonder about the idle jet selection. Why go from a 45 to a 40 F9 instead of progressing to a leaner jet, like a 45 F 8 or F 11? No one seems to be able to tell me how much leaner the progression of jets are, there does not seem to be any rhyme or reason to the system other than the first number indicating the size of the orifice. I ordered a jet set of 45 F 8, 110main and 155 air from Pierce and talked at some length to them. He discouraged me from a 40 idle jet but could not really give me an idea of the relative change from one jet to another. He did explain how the idle jet worked, more or larger air bleed holes, length of the fuel orifice and size of the drilling where the air mixed. He also said as a rule of thumb the main air jet should be about 50 larger than the main, ie 120 main 170 air. I guess it will be trial and error until I put in a AF meter. Gary

[stugilmour]

This is what I landed on before the sticky winds of winter arrived:

The car is a 1969 Lotus Plus 2. The engine is an Omnitech 1950 cc version of the original Lotus Twin Cam. The Stromberg head has been converted to twin Weber 45’s by John McCoy. Head is mated to a new Crossflow Tall Block. The build includes 2” TTR big bore exhaust, oversized TTR airbox, and MSD electronic ignition. I am not sure of the cam details, but believe John uses high lift with relatively short duration. Similar engines dyno at about 180 hp, although I do not have a dyno chart for my specific engine.

Carb details to date are:

Weber DCOE 45G 151
Number 002 17 on name plate
Assuming 36 mm Chokes
Assuming 4.50 Auxiliary Venturi
Assuming 045 Pump Jet
Assuming 040 Pump Exhaust
Assuming 2.00 float needle
Confirmed four progression holes present
Idle Jet: Was 55F9 and running badly, Now seems OK on W50-3 (3 air holes open)
Idle mixture screws adjusted to ~2 1/2 turns out (mid-range).
Air balance bypass screws needed fiddling on number 4 & 1
Main Fuel Jet: 135
Air Corrector: 175
Emulsion Tube: Was F16 and running badly; Now seems OK with the under 2000 cc size VP tube installed
Fuel float level set to 25 mm below jet mounting platform

Next steps in the spring

Confirm W jet air holes open optimal
Confirm main jet OK at WOT
Adjust air corrector size, although transition to main jet seems very good right now
Take a trip to sea level and see how things go.

Looking over my notes, the recommendation of going to a W50 idle jet seems to be right on. Will have the 55F9 to try if required at sea level. If needed I would be OK with buying a W55 set for sea level use.

I am thinking I will be able to run smaller air correctors. My understanding of the VP tube design and tuning method is to use the air corrector size to adjust when the main circuit comes on and use the main jet size to be optimized for WOT. What I will be trying for is better fuel economy by seeing if there is good drivability at steady state cruising without bringing back the transition stumble.

I just ran out of time this fall. We get so cold here it was totally changing the required tune.

Agree you will probably need an AFR to fine tune yours at altitude. Question; does yours run OK at sea level? I have only tested at 3,500 to 4,500 ft so far.

All the best.

[Billmack]

Get a cheap afr meter or find a friend with one. Too many variables you are not in control of. They even have them for gokarts now. I have a michron on my gokart engine dyno. My buddy has made part of his business going around with a portable afr unit

[TBG]

As I understand it the AFR needs to be measured close to the cylinder head and not just stuck up the exhaust - am I right? If so one needs to put a tapping in the exhaust manifold and this is something not done easily. D

[nmauduit]

As I understand it the AFR needs to be measured close to the cylinder head and not just stuck up the exhaust - am I right? If so one needs to put a tapping in the exhaust manifold and this is something not done easily. D

The signal is better when the wideband sensor is closer to the chamber (at least 60cm or so, as gases a very hot), but one can still get an idea with exhaust removable setups, only not at idle or lower revs because of turbulences and backflow : in that case use longer observations periods so that you can average the noise.

[stugilmour]

My wide band O2 sensor is installed in the exhaust pipe just after the two into one Y pipe. My Spyder frame is open in this area which allows it to be inserted vertically. The rather large wires are run through a small opening in the large rubber tunnel access cover.

For tuning purposes I used a clamp style bung, which has since been replaced with a welded bung when I had the exhaust finalized.

Typical O2 sensor mount.

Not sure how difficult this location would be with a stock chassis or on an Elan. The sensor itself has the same threads as a typical spark plug but is about twice as long with the wires.

Two items from the instructions to prevent fouling or damage to the sensor. The sensor should be as close to vertical as possible and it should not be left in place without being powered up (to prevent fouling). There is a heater circuit that warms the sensor up, which is required to be connected, or the whole thing should be removed and replaced with a threaded plug. The model I have requires periodic calibration in free air, but I understand there are better units available that do not.

I mounted the gauge in the product’s cardboard packing box along with a portable tach used for timing. The various wires are run through the passenger side firewall (already had a extra hole for an electronic ignition controller). I tuck the box behind the seat when not in use. Wires are long enough to be able to set the box on the windshield while working under the hood.

Depending on how stable the tune becomes (primarily when I change altitude) I may leave the thing in place permanently and come up with some sort of concealed mounting for the gauge. I suppose a gauge could be mounted in the later Plus 2 dash in either the clock or ambient temperature hole, but they look pretty ugly to my eye!

Yeah, I guess it is quite a hassle! Just some ideas.

[mr.vman]

Gary,
Curious the spark plug change rid the missfire. How many miles on the spark plugs? What caused the missfire? Mixture strength? Weak ignition? Heat range? Those Webers have large acclerator pumps. Perhaps before removing spark plugs, Cruise at steady speed, no accelerator pump action to give a rich mixture. At RPM, clutch in, turn ignition off, coast to side of road. So called, "plug cut". Remove and read plugs while burning your hands and potentially getting run over by a semi on Interstate 10. This would give a more acurate view of spark plug condtion during cruise. If you have a rich air fuel ratio....rich mixtures can sometimes have the engine run cooler, leaning the mixture out at cruise could give hotter temperatures. What about oil consumption? Lotus is famous for using oil. No valve guide seals, rings? Could too much oil be getting in combustion chamber, hence sooting the spark plugs. With the rear axle ratio the car has, constant high RPM. How about instead of the standard NGK 7 go to a hotter NGK 6 spark plug?
Many of us can only dream of having our Weber carburetors transition smoothly, no surging during cruise,(or even drive around the block with out a problem) nice drivability like you have. And excellent performance you cars have. If you do jet leaner, watch temperatures and drivability. By the way, your S1 Elan is a special car, probably oldest Elan in USA....if that information helps jetting?
Steve V. In Arizona USA