Lotus Elan

poor fuel consumption

PostPost by: jeff jackson » Thu Jul 03, 2008 8:16 pm

Hi All,
I have finally lost patience. I have rebuilt the TC big valve with new just about everything, pistons valves etc...
I have overhauled the Dellorto carbs with new gaskets, seals etc, balanced using a carbtune. All jets etc.. are what they should be. The floats have been set correctly for float and droop level.
My ignition is spot on, and the internals of the dist have been replaced.
My problem is that the engine runs rich. I have turned the idle mixture screws in pretty much all the way, but the plugs and exhaust are always sooty black, and my fuel consumption is pants, around 10 -15 MPG.
I have an aftermarket Stainless exhaust that a PO has put on and I have been told by one or two sources that it cause lack of back pressure, resulting in an over rich mixture.
Question is do I fit a smaller idle jet from 50 to a 45? Change the back box on the exhaust? Or is there something I have missed?
Any suggestions gratefully received.
Kind regards
Jeff 72 +2
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PostPost by: garyeanderson » Thu Jul 03, 2008 8:54 pm

How heavy is your right foot? how much junk in the trunk (boot)? Plus 2 or Elan? Where do you drive? Hill country? City? High altitude? What tire pressure do you run? Are the tires alined properly?

My 5000 pound F250 diesel get 19 mpg almost all the time towing the 2 wheeled trailer with the Elan loaded. The Elan with 1700 cc and 1300 pounds is in the 15mpg range when out for a drive with Beauregard :)
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PostPost by: bill308 » Fri Jul 04, 2008 2:01 am

Jeff,

The color tune shoud have taken care of the idle and off idle mixture.

Which Dellortos do you have?

Assuming you have the DHLA-40 (no suffix), these are the early Dellortos. They have 3-progression holes and are normally set up to get on the mains as soon as possible.

If this is the case, I'd check the main jets and consider moving down (leaner-smaller orifice) 1 or 2-steps and see how the engine responds.

It would be ideal if you could get you hands on an LM-1 wide band air/fuel meter to see what your mixture looks like as you drive. A dyno or rolling road test, with a gas analyzer, is a good alternative.

Any possibility you have a non releasing brake issue or a fuel leak somewhere?

Just some thoughts.

Bill
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PostPost by: 1964 S1 » Fri Jul 04, 2008 3:02 am

I'm gonna jump on the question answering bandwagon too...

1. ...do you have a vacuum leak or clog?
2. DITTO: CHECK FOR BRAKE DRAG. Can you roll the car freely in neutral after five or ten miles of driving?
3. Can you remove the silencer and throw a twenty dollar cherry bomb on the exhaust temporarily if need be, (they sound great.) Put a gallon of gas in the tank and see how far you get? (spare fuel can on board of course.) Backpressure affects mpg immensely on crossflows.

Eric
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PostPost by: steveww » Fri Jul 04, 2008 8:50 am

If you can get your hands on a LM-1 and really see what the mixture is doing as you drive this will give you a much better idea of what is going on. Personally I have never got on with the colour tune things, I never found them to be very accurate.

Alternatively pay for a rolling road session.
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PostPost by: tdafforn » Fri Jul 04, 2008 9:08 am

Ditto with the colourtune..
Set my twink up using it, then checked the AFR with a wideband meter and it was >20:1 at idle, off idle flatspot and popping on overrun. Re-tuned using the meter to 12:1 and checked the colour tune (very yellow flames) and now the car drives well.
tim
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PostPost by: ppnelan » Fri Jul 04, 2008 2:16 pm

It's worth making certain that the chokes aren't stuck on/leaking.
Also that the needle valves aren't sticking open from e.g. dirt in fuel or too high fuel pump pressure (see www.lotuselan.net/forums/viewtopic.php?p=86716).

:arrow: Matthew
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PostPost by: jeff jackson » Fri Jul 04, 2008 8:03 pm

Hi all, thanks as always for the replies.
First of all my brakes are good, I have renewed these as well, and the car is easily pushed in and out of the garage by me.
There are no dead bodies or concrete in the trunk, and I don't tow a caravan.
However, I agree with you that the colour tune is crap, but the carb balancer is superb. I may have a vacuum leak, but that would make the mixture too lean wouldn't it? ( too much air).
The Dellotro's are DHLA ( no suffix) so yes the older types. I again agree if I could get the elan on a rolling road, and reduce the jet sizes, I may stand a chance of getting it running as it should, before I go completely mad.
Again thanks for all the suggestions, they are much appreciated.
Kind regards
Jeff
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PostPost by: peterako » Sat Jul 05, 2008 8:23 am

Hi Jeff,

There is light at the end of the tunnel...it will be worth it!

I get approx 40 mpg from my +2, mixed urban/rural and granny/fast driving :D

Now....if only I could stop petrol evaporating when the car is NOT in use :?

How many other perfomance cars can give you such thrills with thrift :D

Now...if only we could get the family, bicycles, boards and baggage into the Elan for our trip to France.....

Peter
I is an Inginear....please excuse my speeling!
'73 +2S 130/5
Opel Zafira, Some bicycles, Wife, Kids, Cats, Dogs....chickens....cluck cluck...goldfish...ducks flew away!
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PostPost by: jeff jackson » Sat Jul 05, 2008 9:33 am

Hi Peter,
I are a Enginuer two.
I have just ordered 45 size idle jets, but I am wondering what can of worms I am now opening, do I now need to change air corrector jets etc..etc.. Where will it all end?
Jeff 72+2 ( wishing for 40 MPG)
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PostPost by: Roy Gillett » Mon Jul 07, 2008 8:46 am

Jeff,

You say you have screwed the idle screws all the way IN. Aren't these screws air bleeds? I have always thoughgt so. So all the way IN is as rich as they go not as lean as they go. It is also important to ensure that the butterflies are closing completely on tickover. If they are not in my experience all the other tuning goes to pot.

Unhook the throttle cable and ensure tickover doesn't alter.

My +2 returned 39 mpg over a recent 1200 mile trip, which included a 3-day competetive rally !

As Peter says, in my experience it is the only car that uses more petrol when parked up than if you use it!
:?
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PostPost by: denicholls2 » Mon Jul 07, 2008 5:14 pm

The Weber style carburetor is designed to be very efficient at fuel delivery. This is the opposite of being very efficient at fuel consumption.

Move those linkages and you can dump a whole lot of fuel in a big hurry.

There are plenty of tuning guides on the web or in this archive; start by getting the very basic basics right. Changing jets is not for the faint hearted, and yes they must be paired with the right idle, main, and air correctors in place. Right is usually defined by knowing what wrong is and buying a righter choice until everythng works.

It is likely cheapest to pay someone who knows how, the bits are pricey. But you can get there yourself if you like to fiddle. Locally, an hour of dyno time (more than enough to get basic tuning done) is about $100; doing it before buying bits will at least give you current AFR through the range. Note, however, that a stock horsepower run will all be on the mains, not helpful at all for economy tuning.

If you're willing to drill a hole in the exhaust, an AFR meter will tell you a large amount about tuning state under different conditions. This will cost you somewhat more than the dyno runs but works ever after.

My Europa on a single DCOE45 gets anywhere from 25-35 MPG, depending on driving conditions and weight of foot. I know it's a bit rich but haven't had time to play more. The stock settings offered lean stumble on the highway, so I jetted the idle one richer and that problem went away. I believe the right answer was a richer air corrector instead.

The same engine pre-Hermes was attributed a regular 50 MPG on the stock Solex. Your mileage may, therefore, vary a lot. You can tune Webers/Dells to be reasonably efficient, but most prefer the performance side.

In proper tune with a reasonable foot, I would expect at least 25 miles per US gallon in an Elan. Having taken one ride with Gary in his Elan, this is consistent with his 17 MPG number. :wink: But don't even start messing with the carbs until you are certain all other components related to spark and timing are right. A retarded spark can have a much greater impact on economy than a jet size.

I'm surprised to hear the Dells favor the main jet. In all but spirited driving, the Europa with Webers runs on the idle. I'm confident it has the right mains; when needed they come on like gangbusters. But they don't come on much for my driving style except up through the gears. For example, a steady 65-70 MPH, where my lean stumble appeared, is on the idle jets. Twink may differ a bit, but I suspect not a lot except there are twice as many accelerator pumps to deliver fuel.
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PostPost by: bill308 » Tue Jul 08, 2008 3:17 am

Roy,

You wrote,"You say you have screwed the idle screws all the way IN. Aren't these screws air bleeds?"

I assume you are referring to the mixture adjusting screws. These screws control the volume of emulsified fuel/air to be mixed with air flowing past the butterfly valve at idle. They do in fact meter the fuel content. Open them up and the resulting mixture will become richer.

Some late Webers and Dellortos do have an air bypass circuit to balance air flow within a carb, at idle. I don't believe these later carbs were ever fitted to Elans as OEM. In any case, they were not intended to set mixture. Properly used, only one in a carb should ever be open, that one on the low flow barrel.

Bill
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PostPost by: bill308 » Wed Jul 09, 2008 12:29 am

denicholls2,

Efficient fuel delivery and good fuel consumption are not mutually exclusive. Good vaporization and distribution enhance fuel delivery, good for both performance and economy.

The problem comes in the tuning. Set the air/fuel ratio (A/F) in the 12:1 range and you get very good perfomance, but poor economy. Set the A/F ratio in the 14:1 range and you get very good economy, but reduced performance. Note that these are mainly jet and or air corrector changes. All else is assumed the same.

The DHLA40 (no suffix) is primarilly a perfomance carburator. It has 3-progression holes and is very tunable. The 3-progression holes limit the range where you are on the idle circuit. I'm not sure exactly where the mains come in, but lets assume its in the 2800-3000 rpm range with a small throttle opening, likely lower with a larger throttle opening and higher with a small throttle opening. Exactly where it comes in depends on the chokes you are using, the auxilliary venturi, the float height, cams, ignition timing,and the amount of throttle opening. They are reportedly best when they come in a little early for perfomace purposes, which is what you want in a race environment.

The later carbs, DHLA40C, E and most of the rest (emission carbs), have additional progression holes that allow idle the circuit use to be extended to higher rpms. If tuned to enable this feature, the idle circuit can be tuned leaner for better economy, yet still retain a richer mixture when on the mains, to produce more power and torque at larger throttle openings. Of course they can also be tuned for a richer idle circuit and leaner mains to enable very good around town performance and economical high speed cruising. The C & E suffix versions can also be tuned, like the original (no suffix version), to enable the main to come in earlier as can some of the emission carbs which are generally less tunable overall.

I selected a set of DHLA40E's for my use. I was looking for a carb I could tune for good economy and performance, albite with jetting changes to optomize for good street and occasional track use. The E's have provision for fitting a manometer system to aid balancing.

By the way, contrary to what I wrote earlier, I understand the E versions may have been OEM on some of the late Sprints and +2's.

Somewhere in the archives (circa mid 2005) I detailed some of the more salient features of the different DHLA's and rank them for our use.

I hope this helps,
Bill
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PostPost by: andyelan » Wed Jul 09, 2008 8:16 am

Bill

Your comments are very interesting.

I get significantly better fuel consumption from my '74 Europa Special than from my '73 Plus 2. Now obviously the Europa is a good deal lighter, but it interesting to note that it's also on DHLA40E carbs whereas the Plus 2 on stright DHLA40.

Andy
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