Lotus Elan

Weber jetting

PostPost by: Geoffers71 » Tue Jan 26, 2016 2:50 pm

I've read the numerous previous posts on this subject, but remain confused (my regular state these day :roll: ) about my particular problem. You will all have read my restoration thread on my 65 S2 (EOK) that I posted ad nausium over the last 2 years, ( you haven't ? ....then better get to it :D ) This car was restored from boxes of bits after being stored for the previous 40 odd years. The engine that came with the car was not original, being I suspect from a Plus2 of '67 vintage. When I had the head restored I found that one of the cams was an SE and after advice I fitted this to the inlet side and at the same time had the larger inlet valves fitted. The carbs (DCOE 18s) were very gummed up and were eventually ultrasonically cleaned and the car tuned on a rolling road. However I have have never been totally satisfied with their performance. There is a stuttering hesitation from a steady idle at 900 rpm to when the main jets come into play. At the same time, and possibly unconnected with this issue, cylinders 2 and 3 aren't pulling their weight as a plug lead pulled from these two makes little difference to engine revs. Accuspark ignition, new ht leads, dizzy cap, rotor and plugs.
The car is going back to the tuner on Thursday for him to take another look.
The hesitation at small throttle openings would probably be due to the carbs still having the original Slow running jets, and would hopefully respond to a larger jet. We will look at that on Thursday. I'm assuming that the current jets are standard i.e. 45 F.9, but I don't know how to determine this. Examining the jets shows the number 35 stamped on them, but nothing else. Does this mean the jets are even smaller than 45s? In view of the SE cam and the larger inlet valve, should we go to the SE spec idle jets of 50 F.8 ? Also Air Corrector jets are I assume standard i.e. 200 rather than 150 for the SE and choke size is 30 not 32 for the SE
Regarding the cylinders 2 and 3 not pulling their weight. I've noticed on a few occasions that I can get a loss of power when driving normally, as if it's running on 3 cylinders. This "feels" electrical, though not sure. Checked everything electrical but no improvement.
So, can anyone make helpful suggestions re: what does the 35 signify stamped on the idle jets?
Would going to 50F8 be a sensible step to take? (can't go down the route of trying every conceivable combination of jet size at this point.
What effect of not reducing size of air corrector jet?
Would these carb issues be the source of the problem for the poor performance of 2 and 3? ( bearing in mind the old adage that most carb problems turn out to be electrically based)
Any other useful suggestions gratefully received
Apologies for the length of this post, I do go on a bit :mrgreen:

Not competent enough to rule out the possibility that these two things may be connected in some way.
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2002 Elise S2 (now sold :-()
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PostPost by: Esprit2 » Tue Jan 26, 2016 8:05 pm

Check out the attached Excel file.

Which chokes, jets, air correctors, emulsion tubes, etc., are currently installed? Each is stamped with either their size or a part number, so pull them out and record what is there. Other than to say, "go with standard", no one here can diagose the problem or make a recommendation without first knowing exactly what is in the carbs now.

The 35 stamped on the idle jets is their size, too small, and not original to any Twin Cam engine. The 50F8 is standard, and a better place to start... then tune from there.

50 is the idle jet size (meters in the fuel), and F8 is the air corrector (meters in the air). The factory determined a balanced set for the engine. You can make small changes to one without also changing the other, but for any significant change to one, there should be a proportional change to the other. If you find it necessary to change one, like going a little richer on the idle jet, then you should also re-evaluate the idle air corrector. At this point, 50F8 would be a far better starting point than is 35 with anything.

The choke size sets the overall vacuum condition in the carb, and is the foundation for all jetting that follows. Three different chokes were used for different states of Twin Cam tune, 30, 32 & 33 millimeters. Determine which chokes you have now, check the attached Excel sheet for the correct jetting for that choke, and adjust from there if road testing shows it's necessary. If you have 30mm choke carbs, then don't use 33mm choke jetting just because you installed a new cam or something. You may need to adjust the jetting somewhere after you make some change to the engine, but don't just plug in a jetting package for a different carb/ choke... instead, adjust for the choke you have. Take a 30/ 32/ 33 set-up package that fits your existing carb/ choke. Then you may have to tweak an idle/ main jet, but at least work from a sound starting point.

For now, the 35 idle jet is just wrong.

Regards,
Tim Engel
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Weber DCOE - Jetting - Factory Set-ups, Lotus.xls
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PostPost by: mbell » Tue Jan 26, 2016 8:17 pm

Hi,

I'd guess that your cylinder 2/3 issue may be triggered by imbalance in air flow been the throats of the carbs. Which could be triggered by a twist in the shaft or poor throttle plate positioning. Have you checked the airflow of each throat using a STE SK Synchrometer or similiar?

From the sounds of it I think a larger "idle" jet would probably help. At small throttle openings the fuel is delivered via the progression holes from the idle jet and the mixture is controlled by the idle jet size. The idle mixture is controlled via the screw so you can get a good idle mixture despite the wrong idle jet.

On choke size, it is kind of your choice and the main effect of the choke is the effect on low speed tractability v power at high revs. The bigger the choke the high up the rev band the power goes. So whether to change or not depends on how you want the car to drive.

Other things to check are the fuel level in the carbs, it seems 25mm from the top of the jet holder in the main jet hole is the magic number. Also the position of the throttle plates relative to the progression holes at idle, the throttle plates should not leave any progression holes exposed on the engine side.
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PostPost by: StressCraxx » Tue Jan 26, 2016 8:19 pm

Hello Geoff,

Weber tuning has restored my humility multiple times in the course of my Elan ownership for the last 37 years....

First of all, the engine should be in good shape, even compression and valve adjustment in spec. The ignition timing and advance must be in spec as well. If they are not, no amount of jetting and rolling road time will help.

The fuel levels need to be 25mm from the top of the main jet wells, as measured with the engine at idle with a main jet removed from each carb. You can use a thin strip of paper marked at 25mm to verify. Keith Franck's white paper has a "how to".

The carbs need to be balanced, not just carb to carb, including cylinder to cylinder. If not the stumble off idle is worse. Mine were off and required additional attention, including replacing the leaking leather seals.

A 35 idle jet (unless it was drilled to a 45 or 50) is too lean. Try first a 50F8. because that's what was recommended with SE cams. If you are using equal length headers instead of the cast manifold, it also figures into both the slow running, main jets and air corrector size.

Hopefully, the above should minimize the off idle stumble. For further reading have a look at Sidedraft Central on Yahoo groups, run by Keith Franck. Join, and download his white paper on the basics. There is lots of great discussion and Keith is very helpful with his new Hypojets and E-tubes.

Regards,
Dan
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PostPost by: Esprit2 » Tue Jan 26, 2016 9:09 pm

mbell wrote:On choke size, it is kind of your choice and the main effect of the choke is the effect on low speed tractability v power at high revs. The bigger the choke the high up the rev band the power goes. So whether to change or not depends on how you want the car to drive.
New chokes are ?12.95 at EuroCarb Ltd, and you'll need 4. Then, once you change choke sizes, you'll also have to change some of the other jets as is appropriate for the new chokes. Re-sizing the chokes isn't inexpensive, but yes, it can be done easily during a rebuild.

However, if you simply want to use the carbs you have with only the necessary tweaks, then start with the original jetting package that is appropriate for the size chokes that are currently installed, and adjust from there.

When you start modifying the engine, it's important to keep the three-C's in balance... carbs, cams, compression. Going big on one without also modifying the other two won't get you the full benefit you might have expected. If your engine is otherwise pretty stock, but you put in one new cam, that's not a big change.

Regards,
Tim Engel
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PostPost by: Esprit2 » Tue Jan 26, 2016 9:17 pm

+1 on balance being critical. An otherwise perfect engine will run like crap if the carbs are just a little out of balance. If you don't have the skills or tools to balance the carbs, then at least make certain that your mechanic has done the job.

If the throttle shafts are twisted, the book repair is to replace them. But with a little care, and preferably a 4-tube manometer, they can be twisted back to straight. But do not try to 'adjust around' twisted shafts. Fix 'em. Then balance the carbs before attempting any jetting tweaks.

Regards,
Tim Engel
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PostPost by: Geoffers71 » Tue Jan 26, 2016 11:17 pm

Many thanks for all your advice guys, much appreciated. I think most of the things you've mentioned have been seen to. Carbs have been balanced using sync device, float levels spot on, butterflies reveal progression holes correctly, best position of mixture screws is 1 1/4 turns out, valve clearances all in tolerance, compressions good and all equal, timing spot on and advance works as per book, as stated all electrical hardware has been replaced and checked, good strong sparks at least at idle. Not ruling out there may still be a problem here though.
Taking on board all that has been said I'm more and more convinced that the idle screw size is the main culprit. If the 35 really is the size of the idle jet then clearly it's too small. How they were 35 in the first place is a mystery :?
So the first thing to do is to replace these with at least 45s (probably 55s as suggested would be the best start point). Car is back on a gas analyser and CRT type thingy on Thursday, so we'll see what transpires. Anything else I should ask the mechanic to check? Really appreciate all your inputs, many thanks.
1965 Elan S2 (26/4726)
2002 Elise S2 (now sold :-()
1970 Scimitar GTE
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PostPost by: Chancer » Wed Jan 27, 2016 12:00 am

They could well have been drilled out, unlikely to be smaller but larger jets can be soldered and drilled to smaller sizes, I have done both of these myself when I didn't have the right jets to hand.
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PostPost by: simonknee » Wed Jan 27, 2016 12:02 am

It sounds like you have gone through all the stuff required to correctly balance (or your tuning shop has). If somebody else has done this have you double checked yourself? One simple test you can try is to up the idle speed from 900 rpm to 1100 say (try a few different, higher idle speeds). Do you still get stumble? If you don't then it could be that the position of the butterflies with respect to the progression holes is actually wrong. And the fact you feel that some cylinders are not contributing to the party makes me think that the balance is not as balanced as it could be yet.

You definitely want to start with a set of 50F8 for your SE spec cams. Remember that the "idle" jets are providing fuel right up to 2500-3500 rpm. You'll be surprised - the name idle is something of a misnomer. To demonstrate take out the main jets and drive the car around gently (no WOT) on a nice flat road. You can get a surprising amount of rpm before the engine cuts due to the lack of the main jets!
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PostPost by: simonknee » Wed Jan 27, 2016 12:03 am

Chancer wrote:They could well have been drilled out, unlikely to be smaller but larger jets can be soldered and drilled to smaller sizes, I have done both of these myself when I didn't have the right jets to hand.


Chancer by name... :twisted:
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PostPost by: simonknee » Wed Jan 27, 2016 12:04 am

Oh I might have some 50f8 that I can send you. Only use Keith Franck specials now...
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PostPost by: simonknee » Wed Jan 27, 2016 12:10 am

You do need to be at SE spec so you do need to swap the cokes from 30 to 32 and the air corrector jet from 200 to 150. I'd check that the emulsion tube is really an F11 given that your random idle jets. Basically a DPO did not know what he was doing and thought that tiny idle jest were the cure. Oh dear.
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PostPost by: webbslinger » Wed Jan 27, 2016 3:08 am

Sorry if this got posted twice. It's frustrating if you have multiple problems, but in regard to cyls 2 & 3 being unresponsive to pulling the plug wires: is it possible there's a bit of debris or the tip of your idle adjustment screws are broken off? If you have the very fine brass ones they are easy to break off in the seat. Sounds like more than that but you will get it.
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PostPost by: simonknee » Wed Jan 27, 2016 7:33 am

I just re-read your original post, Geoffers, I think you can ignore me when I say test to try a higher idle speed. This is to test for an off-idle stumble due to incorrect butterfly positions but you say it is "hesitant all the way up to the main jets".

I think the "35" idle jets have to go before you try anything else. If you have SE spec cam then you need SE spec jetting in the carbs. Miles quizzed me more than once, before we even fired up the rebuild, that had I put the correct SE spec chokes/jets in the carbs when I went from B cam to C cam!

(Note: "off-idle stumble" means the it bogs down just at the point you are trying to move off, you are going off-idle speed BUT not off the idle jets. It took me a while before I realised this so others may be confused too).
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PostPost by: simonknee » Wed Jan 27, 2016 10:02 am

I checked my spares and you are welcome to try out any of the following...
45f8.jpg and

45f9.jpg and

50f9.jpg and


Note that they all have both the jet size and the air size stamped on them. If yours only have 35 then I'm not sure they are even genuine weber ones really.

Though if the bug bites then go for these...

h20.jpg and


Keiths Hypojets (uses a hypodermic need to get precise metering) These are shown without their corresponding carrier that allows for air adjustment too.

Oh and where are my 50F8?? Turns out I only have two - I remember now that the carbs came with the front and back fitted with different idle jets. This was somebody's attempt to fix what was actually an airflow balance problem. I must have been lying to Miles when I said it was correctly jetted. I suspect I had the 50F9 in. :lol:
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