Lotus Elan

Weber Carbs - Jet and Choke sizes

PostPost by: Gizze » Mon Feb 18, 2019 6:12 pm

My ongoing battle with my 1970 Elan Plus 2s Continues at a pace ...........
I now have a running engine although one that runs too rich. I have checked the following Weber 40 DCOE items:-

Choke: 30mm (Internal diameter) Measured with a set of vernier calipers.
Air Corrector Jet: 155
Idle Jet: 45F8
Main Jet: 110
Pump Jet: 35

My engine is standard.
My question is: Are these the correct standard components?
Thanks in advance
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PostPost by: nmauduit » Mon Feb 18, 2019 8:38 pm

had Miles' LTC book nearby : here is the page about weber jetting:

p1050388.jpg and
Miles LTC Weber jetting page
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PostPost by: rcfurse » Mon Feb 18, 2019 8:59 pm

BUT . . . . . . . . . all our cars are now running on unleaded which runs much "leaner" than the original leaded petrol. If you want to burn your valves out use the original settings! Otherwise go to a rolling road and have the car set up by someone who knows their way around Webers and does it at a variety of engine revs and loads with a lambda up the pipe.
Don't know where you are but there is a great guy up near Chester.
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PostPost by: Chrispy » Tue Feb 19, 2019 2:43 am

Being a +2 I assume it has C type cams (single grove). My engine is standard with C type cams and uses the settings as per Miles' book, but with 30mm chokes and the changed pump jets and air corrector as per the note. Haven't had mine on the dyno or a WB02 up it's bum but plan on it soon enough.

Mine appears to be running quite well when on the main circuit, around progression it's a bit flat at times (especially when cold). Once hot it's quite happy. Muffler needs replacement so once that is done I'll have a closer look at AFR's.
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PostPost by: 69S4 » Tue Feb 19, 2019 7:29 am

rcfurse wrote:BUT . . . . . . . . . all our cars are now running on unleaded which runs much "leaner" than the original leaded petrol. If you want to burn your valves out use the original settings!


Just wondering about the magnitude of that. My S4 runs fine on it with the - Stromberg - settings close to book. I also have half a dozen early to mid 70's motorcycles that have not had any temper tantrums on it either. A couple of the smaller ones need to be 'encouraged' to keep up with modern traffic so it's not a case of 'puttering' along; the engines have to be worked hard. The only difference I've noticed is that, depending on the carb design, some of them need the float bowls draining and refilling if they're not used for a week or two. Others though can be left for months and start straight away. Only thing I do is to avoid anything higher than E10.
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PostPost by: Gizze » Tue Feb 19, 2019 8:23 am

Thank you for the responses. So matching the various specifications:

Choke: 30mm (Internal diameter) Measured with a set of vernier calipers. As specified
Air Corrector Jet: 155 (Std=200)
Idle Jet: 45F8 (Std=50F8)
Main Jet: 110 (Std=115)
Pump Jet: 35 (Std=40)

So if I was to change to these would the engine run leaner (numbers generally increase but I'm not familiar with the nomenclature)?
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PostPost by: nmauduit » Tue Feb 19, 2019 9:59 am

Gizze wrote:So if I was to change to these would the engine run leaner (numbers generally increase but I'm not familiar with the nomenclature)?


as mentionned in posts above, there is usually more to it than just matching jet numbers (that may be the beauty and evil of the thing), and your degree of satisfaction (tuning figure of merit) has a role to play in the entire process... not yet specified.

to answer your question more precisely (yet in a general manner, as different jets get in action at different revs) :
fuel jets, the larger the richer
air jet (corrector) the larger the leaner
NB: the diffuser (part holding the main jet and the air corrector) has a major role to play in the mix

I would not worry too much about acceleration pump jets at this point (esp. not getting them larger), this is for smoothing the richness regime change when there is a sudden acceleration, fine tuning

also fuel level in the carbs (best measured from the jets brass opening) has a direct role to play in richness : this is important

last I'm a bit wondering about the 30mm chokes - though that should not prevent from getting a car in a decent state of tune (would only limit the WOT performance then)

I'm assuming all the rest is well (spark plugs of proper heat index, ignition set adequately etc), and in particular that idle settings have been carefully tuned (it can be surprising how far in the revs a LTC runs on the idle circuit, which fuels the idle via the springs screws but also the transition regime via the transition holes, up to 3500 rpm give or take 500)

if you are undertaking the tuning yourself and expect best help from the forum maybe you should provide more informations (esp. describe "too rich", including in relation with the rpm)
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PostPost by: Gizze » Sun Feb 24, 2019 7:36 pm

From the replies - thank you - I've spent a bit of time getting the float levels right first. Initially working on the float height measurement, then checking actual fuel height. So now that's set I'm looking at the jets etc.
From what I can see maybe the Air corrector Jet and Idle Jet may not be helping. So plan to try alternatives swapping 155 to 200 and 45F8 to 45F9 respectively.
To clarify the other points raised - My engine's a standard one for the car, as far as I know. An engine tuner who came round could not get the fuel mixture down to an acceptably lean mixture and indeed the plugs foul within minutes of the engine starting. He checked timing and spark so these should be ok.
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PostPost by: nmauduit » Mon Feb 25, 2019 8:52 am

Gizze wrote:From the replies - thank you - I've spent a bit of time getting the float levels right first. Initially working on the float height measurement, then checking actual fuel height. So now that's set I'm looking at the jets etc.
From what I can see maybe the Air corrector Jet and Idle Jet may not be helping. So plan to try alternatives swapping 155 to 200 and 45F8 to 45F9 respectively.
To clarify the other points raised - My engine's a standard one for the car, as far as I know. An engine tuner who came round could not get the fuel mixture down to an acceptably lean mixture and indeed the plugs foul within minutes of the engine starting. He checked timing and spark so these should be ok.


if the spark foul so fast (I understand 5-10 minutes - probably spitting black sooth from the exhaust) I would try a hotter set if applicable, like NGK BP5ES (normally I run BP6ES on the street, BP7ES on the track on my stock engine) : that is easy and take little time, nothing to compare the days one can spend on carbs jet tuning when something goes wrong without obvious explanation...

Also, I understand it runs smoothly at idle and the idle screws have been carefully set : you may push this setting a bit on the lean side for a try... all elements of the current tuning have not been described, but the way I see it I would first focus on the idle circuit, and try smaller jets there (they govern richness up to 3500 rpm give or take a few hundred)
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PostPost by: Grizzly » Mon Feb 25, 2019 9:51 am

As above, start off making sure your ignition system is spot on and your running the correct plugs (i am using NGK BP5ES and BP6ES for a run myself) also make sure you have no vacuum leaks and have the carbs well balanced (SK synchrometer is the best way to do that) and make sure the choke springs are in good order (chokes need to be tight or you get lots of unwanted fuel knocking about)

Then you need to sign up to the Yahoo group sidedraft_central and read Mr Franck's white paper.

I am running a sprint setup with 33mm chokes and to be honest was chasing my tail for a long time until i found the jets weren't far off but the pump jets were dumping huge amounts of fuel in and the pump jet demand ball and seat were leaking/dripping making everything appear rich. If you find your self here remove the Spill valves and make sure the small bypass holes in the side are not blocked to start with.

You can get it close just going off the white paper but to really dial it in you need a wide band / data logger or a tuner who has them.

Good Luck!!
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PostPost by: Matt Elan » Mon Feb 25, 2019 11:31 am

I know nothing about Webers or Dellortos (but will be learning) but I do know a bit about spark plugs.
Two things - 0n modern plugs the ceramic insulator on the nose of the plug has a smaller glazed area so is more prone to fouling, and is harder to clean properly when it has fouled. So if you are setting up the carbs I'd buy a couple of sets of plugs to use.
Secondly there were (and still are) lots of counterfeit plugs on the market - while they may look original they will not be to spec and will foil any amount of carb adjusting. I would not trust any plug bought from eBay or where ever no matter how good the packaging (and price). I only use reputable suppliers for my plugs.
Just like to say this is my current experiances - I've not goy any interests (shares etc) in any spark plug manufactures :D
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