Lotus Elan and Lotus Elan Plus 2 Forums

I have a 1969S4 with Weber 40`s/18.It runs good with no apparent problems ,hope I don`t regret saying that.My fuel consumpsion averages 18miles /gallon,not good??.On checking the jets I find I have non standard jets fitted namely Main jet 100,Air corrector 165,Emulsion tube F16& Pump jet 45.I don`t know what cams I have .Would a return to standard jets [a]improve my fuel consumption ? [b] give any adverse proformance implications?

16 to 20 mpg per imp gallon is about what I get on my tc Webber head 7 and the Webber head ?lan ---depending on how much I value my drivers license as to how deep my foot is in it ---ed

If the air/fuel ratio is 12:1 at partial throttle cruise than 25 mpg is more likely with a 3.9 diff. The emulsion tube primarily controls the mixture at low rpms and your F16s has only 6.3 sq/mm of air bleed holes above the fuel level. I'm running similiar sized main and air jets but modified F11s that have 11_sq/mm of holes above the fuel. Bet you're really fat (~9:1) at partial throttle and just pouring the fuel in there.

Does your engine die for a moment while turning lefthand corners by chance? Particularly when the ambient air is chilly?

Thanks for you info .I havent noticed any dieing of the engine under any circumstances.My plugs are very sooted up ,the soot is very easily to clean off, could the low MPG and the fouling plugs be caused by the F16 emulsion tube?Did you modify your F11`s and what did you do ?

could the low MPG and the fouling plugs be caused by the F16 emulsion tube?

99% probability I'd guess.

Did you modify your F11`s and what did you do ?

I drilled two more 1mm holes in the top. I did this because I can measure the AFR with a digital meter that records the datastream. Just tweaking what Weber has provided based on their description on how this stuff works in the tuning manual. It does help to be able to read between the lines on an engineering level though.

Gotta disappear now have an old Holley two-barrel which surges at highway speed. Opening up the EGR valve first to see if it's plugged solid with carbon after 200k miles.

I'm back. The thought occurred to me while doing some Hoovering (to please the wife) that you could just drill 6 more 1mm holes in each tube to fix the problem. Emulsion tubes are readily available and cost ~$20 each. The F16 and F11 tubes are nearly identical except for the air bleed holes. I know this because the Weber Tuning manual has fairly good cross-sectioned drawings and a table which lists all the sizes of every feature of every jet and tube they ever made. Handy book to read! Mine is rather tattered from usage now.

Thanks for your very interesting reply .I shall findout the cost & availabilty of F11 tubes here in the UK befor I start drilling the F16`s

John,
On my 67 elan with a 3.55 rear end gear, has been getting 25 to 33 MPH depending how heavy my foot is. Mostly 25

Keith,
the fuel level gauge has not arrived as of last night. I haven't checked today. Hopefully the post office has not lost it.

Hi Sarto,
Don't look for the optical fuel level gauge thingie to arrive by the mail since I did not send it. It was my understanding after the long chat we had that you would just make your own up. Perhaps my memory is failing though.

Triumphelan,
The risk of damaging the tubes is low. Any holes that need to be eliminated can be soldered or epoxied closed again.

Keith, I did attempt to make one and it does not slide as easly as yours. I used a nylon nut that I drilled out and split. If the nut is to large it won't fit flush down in the carb space. You used a plastic nut affair that I couldn't find anywhere. It does work, I'll just be very careful on my readings.
Then we'll see how much more power we can get and see if we can maintain 20 to 28 MPG, which would be great.

Thanks again!
Sarto

Sarto,
Let me tweak the way you view this stuff a little. The amount power you can get from the fuel is limited by the quantity of air there is to chemically combine it with. If you provide it with just enough or more fuel to react with every last oxygen molecule then you've reached the power limit. Adding more fuel just wastes the extra portion but if it's way too much like Triumphelan has it then the heat from the burn is reduced by the latent heating effect of the extra fuel. In otherwords it robs the cylinder of peak pressure by evaporating the extra fuel. This can really reduce the fuel mileage by a ton. Your original jetting got you lower then the 12:1 AFR to achieve maximum power but it was dumping too much fuel at WOT and was fluctuating allover the place at the cruise level.

Are you sure you didn't have a flatspot? Do you remember me running up the idle speed to check for a flatspot before I tore into the carbies? Maybe I just forget to check for it! With that 200 air corrector you had I would expect it to have a nasty one. Should have had you do the WOT test at 1500 roms while I was riding along in your car. Take was a mistake on my part.

Keith
To arrive at the fuel/air ratio of 12:1,what calculations do I need to make ?.I have measured the hole sizes of air correctorjet 2sq m/m,main jet .095sq m/m& the 7 holes in the emulsion tube,total 5.5sq m/m.Whats my next move?

Keith
Main jet should have been 0.95sq m/m not .095sq m/m

John,
There isn't anyway to calculate the AFR directly. Guessing that would be a problem for a super computer and only for a static condition. Weber tuning goes straight into the realm of chaos.

I'm sharing my limited experience measuring the AFR with a http://www.tuneyourengine.com instrument. The interaction of the jets and the emulsion tube is my focus point at this time. I'm just beginning to see the light now.

100 main jets are really small ! What chokes do you have ? Have you checked that your main jets are what they read, - and haven't been drilled ?
Dag