Weber jetting

[Geoffers71]

Simon, that's a very kind offer, thank you. However I believe my tuner chappie has a full set of jets and probably has 50 F9 in stock. If not he has the full range of reamers so could ream out the existing jets to the right size. Thanks anyway.

[simonknee]

No worries, best of luck.
Remember it's 50F8 to start with though I settled on 45F9 for my C-cam/standard port engine. Before I got the Hypojets that is.

As I recall Keith has a dim view of reaming/drilling out jets. His approach is all about precision and also he wanted in-between sizes - which is why he employs hypodermic needles!

Glad I did the big old thread elan-mods-f31/hypojets-and-tubes-the-way-t25922-30.html back in the day. I'll be able to work out where I got to - it's how I found that I had previously fitted 45F9. Next step for me is to try 33mm chokes as I remember... now where have those got to.

[Geoffers71]

Well the car was with the tuner for 2 1/2 hours yesterday before getting the engine into an acceptable state . To begin with it was clear that the mixture screws were very very sensitive to adjustment. It was very easy for a slightly richer mixture could overwhelm the spark of some plugs causing a misfire. The gas analyser showing large hydrocarbon increases as a plug failed to ignite the charge. The stumble on small throttle openings was shown very clearly on the screen too. So, first step to take out the idle jets and as he hadn't got the exact 50 F8 ream out to 50 rather than the 45 using the proper Weber tools. This made a very noticeable difference, but still there the issue of the odd plug being overwhelmed by fuel. All plugs out and checked, clear signs of over rich in cylinder 3 and 4 (not firing)! Testing revealed that plug 3 was definitely faulty despite being new NGK BP6ES, replaced these with the nearest equivalent that was handy and another improvement. Suspicion then moved clearly to the coil, the Accuspark one provided to match the ignition module, which was not providing a strong enough spark to fire the mixture at higher revs. The engine was set up as close as could be achieved in the circumstances and we called it a day. The short trip home showed the car to be much much better, but I had been tasked with replacing the coil and the plugs. So this morning I bought and fitted a new Bosch Blue coil and 4 new NGK BP6ES plugs. Initial start up and revving showed that there was NONE of the stumble when gradually increasing the revs from idle to 3000 and the engine sounded smooth, every cylinder appearing to make its proper contribution. (Hooray )
There might still be a need to chuck it onto the analysers again as the stronger spark from the Bosch coil would probably mean idle mixture screws might need further adjustment, and I might yet get some proper 50 F8s. Haven't tried it out on the road yet, the weather here is atrocious with high winds and driving rain Overall not a bad result though Many thanks for the helpful suggestions on here

[simonknee]

Good news. Another fuel problem to chalk up to electrics. Many years have taught me that when it comes to electrical (and electronic) items it's the 10-20 year old components and the brand new ones that will fail. Old ones are worn out and new ones were defective from the start. You can only trust them when they become middle-aged!

[Esprit2]

Sorry about the bandwidth... it gets long.

The first step to tuning carbs is always a full ignition tune-up to eliminate all stray problems. Proper valve clearances, no intake air leaks, balanced carbs, and a full ignition tune-up. If you don't, then all your carb tuning is wasted effort.

In "50F9", the 50 is the Idle Jet, and the F9 is the Idle Air Corrector/ Emulsion Tube combination. The 50 and F9 are separate parts that can be changed individually. If you have a 45F9 set-up, you don't need to buy a 50F9. Just buy the part you need, the 50 Idle Jet.

With the engine fully warmed up, the usual tuning order is to set the idle speed as slow as possible consistent with smooth running without missing. Then, from a stable idle, open the throttle slowly enough that the accelerator pump isn't much of a factor.

If the engine hesitates just as the throttle moves off idle (ie, "off idle hesitation"), then the Idle Air Corrector is too lean. Go a step or two richer until the hesitation just disappears.

If the engine doesn't hesitate, then go a step or two leaner with the Air Corrector until a hesitation just shows up, then go back a step richer until it just disappears. The goal is just enough richness in the Idle Air Corrector, but no more than required.

Reality Check -- What works with the engine idling in the driveway may prove to be a step too lean when under load in the real world. If the engine hesitates when you let out the clutch to pull away, then go a step or two richer, as required. A heavier car will require a little richer air corrector than a lighter one (Elan +2 Vs Elan). In my experience, that's so frequently the case that I normally complete the tuning procedure, then go a step richer before even test driving the car... I know it will need it anyway.

*~*~*
With the Idle Air Corrector 'initially' sorted out, go for a drive on a country road. Short-shift into an intermediate gear, then accelerate at full throttle to beyond the transition rpm between the Idle Circuit and Main Circuit. With a Dellorto DHLA, that's about 3200 rpm. With a Weber DCOE, it's closer to 4000 rpm. The gear you use for the test will depend upon the speed the road can support. You'll rip through 1st too quickly, so it doesn't work well. 4000 rpm in 2nd and 3rd will produce higher speeds, so be prudent... don't get a ticket. If you can do it, 3rd gear is probably best.

If the engine stumbles before reaching the transition rpm, then the Idle Jet is too small. The idle circuit feeds the engine up to the transition rpm, and if it runs out of capacity before the main circuit takes over, the engine stumbles. If that happens, a larger Idle Jet (more capacity) is required. Go a step or two richer until the stumble just goes away, but no more.

If the engine doesn't stumble, then go leaner until a stumble just occurs, then back a step or two richer until it just goes away... no more.

The Idle Jet and the Idle Air Corrector both affect one another. So any time you change one, you must go back and re-evaluate the other. So repeat the above procedures until the last iteration produces no further change.

The jet and air corrector both affect the Air Fuel Ratio. The jet adds more fuel, and the air corrector adds more air. You can adjust the A/F richer by going to a larger jet, or a smaller air corrector. The trick is to end up with enough combined volume of correct A/F to feed the engine up to the transition rpm, but not beyond. So if you need a richer mixture, going to a larger jet isn't always the correct choice. If the circuit has enough capacity to reach the transition rpm, then a bigger jet will be too much fuel. The answer is less air.

If the Idle Circuit has too much capacity (Idle Jet is too big), then it will continue to feed the engine beyond the transition point. In that case, both the Idle and Main circuits will be feeding the engine at the same time, giving it much more fuel than it needs, and the result will be a way too rich mixture, black exhaust, and fouled plugs. So, as much capacity (Idle Jet) as it needs, but no more. Then as much air as it needs to get the A/F right. Sneak up on it in the order described above.

Transition Stumble -- A stumble BEFORE the transition rpm means the Idle Jet is running out of capacity before the Main Circuit takes over... increase the Idle Jet size. Stumble AFTER the transition rpm means the Main Air Corrector is too lean... go richer on the Main Air Corrector.

If the engine gives up before reaching redline, then the Main circuit has run out of capacity before the engine peaked... increase the size of the Main Jet... then go back and re-check the Main Air Corrector.

Again, jet and air corrector affect one another, so if you change one, double check the other.

The Main circuit tuning procedure is similar to the Idle procedure above, but the required road speeds make it impractical on public roads. Main circuit tuning is best done on a dyno... or a race course.

*~*~*
Jan 26, 2016 2:50PM, Goeff, Geoffers71 wrote:
(Snip)... 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... (Snip)... The carbs (DCOE 18s) were very gummed up... (Snip)...

Jan 27, 2016 12:10AM, Simon Knee wrote:
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. (Snip)...

Jan 27, 2016 7:33AM, Simon Knee wrote:
(Snip)... If you have SE spec cam then you need SE spec jetting in the carbs. (Snip)...

Simon and I disagree a bit. He points out that if you go with the SE spec cam, then you need to go with SE size chokes and jetting. That's an option, but you don't need/ have to make the choke size change if top end power isn't 'your' goal.

Lotus was trying to keep sales up in the face of stiff competition, especially since their little, but expensive 1.5 liter car was often compared to bigger, more powerful cars closer to it's price range. Lotus knew horsepower sold cars, and they wanted more horsepower. So they went with twin hotter C-cams, which made more power, but moved the power band up higher in the rev range. Then, putting all their tuning marbles in one box, they installed larger choke carbs, which also made more power higher in the rev range. Overall, they got more horsepower, but it came at the expense of off-idle/ low-end and mid-range torque and drivability. But they had more horsepower, and that sold cars.

The Sprint with twin D-cams was more of the same story... plus big valves which also pushed peak power higher in the rev range.

Lotus' priority was more horsepower over all else. If that's not your priority, then adding one C-cam to an otherwise stock '67 Standard Twin Cam is not a big change. Yes, that one C-cam intake will make more power higher in the rev range, but not as much as twin C-cams. And you have the option of putting all your tuning marbles in that same box like Lotus did, or tuning other aspects of the engine, like the carbs, to take better care of the low end of the power range.

Keeping your carbs in 'Standard' TC spec, with the smaller 30mm chokes is an option for you. All the TC variations have about the same redline, so even the small chokes can feed it to redline. Granted, the smaller 30mm chokes will result in the engine not revving as freely approaching redline, but they will also result in better throttle response off-idle, and a stronger mid-range. What's that worth to you for the way you intend to drive the car? Are you a berzerker boy-racer, or a gentleman looking for a nice driving Elan?

Also, if there's a budget involved, it's extra cost to rebuild the carbs to S/E spec. Extra cost that's not necessary when 'one' C-cam isn't exactly taking the engine to full S/E spec anyway, or when you don't share Lotus' horsepower priority.

You're allowed if you wish, to keep the small 'Standard' 30mm chokes, and adjust the jetting a bit to optimize running. Then, if you still want larger chokes, you can defer re-inventing the carbs for another day. The smaller chokes helping the low-end of the rev range in the meantime. Later, you can make the power peak higher, more narrow, and crammed higher in the rev range. Your option, but you don't 'need/ have to do either.

Back in the day, emissions standards were becoming more of a reality. USA Federal standards were the first to become really restrictive, but even UK/ Euro restrictions were being felt. Each successive DCOE set up (Std, S/E, Sprint) not only went bigger-choke and higher in the rev range, they also became more influenced by emissions standards.

App ...... Std, B-cam... S/E, c-Cam... S/E, Sprint, D-cam, Big Valve
Carb ..... DCOE-18 ... DCOE-18 ... DCOE-31
Throat ......... 40mm ....... 40mm ........ 40mm
Choke ......... 30mm ....... 32mm ........ 33mm
Main Jet ...... 115 .......... 115 ............ 120
Air Corr ....... 200 .......... 150 ............ 155
Emul Tube.... F11 .......... F11 ............ F11
Idle Jet ......... 50 ............ 50 .............. 50
Idle Air Corr.. F8 ............ F8 .............. F8

The choke is the reduced diameter portion of the main venturi (the waist), and a more pronounced reduction produces a stronger vacuum for the same airflow. Going fatter, from 30mm Std to 32mm S/E weakened the overall vacuum condition in the carb. Since vacuum is what pulls the fuel over into the throat, the bigger chokes also created an overall leaner condition. To keep the mixture similar to before, larger jets should have been used.

But, instead, the S/E main jets remained the same and the overall mixture condition went leaner. The Air Correctors were made smaller (less air) to keep the A/F ratio rich enough, but the main circuit was effectively reduced in capacity.

The Idle Circuit was unchanged, so by virture of the weaker vacuum, became leaner and with reduced capacity overall. Yes, the larger chokes flowed air more freely, and, along with the twin C-cams, pushed the peak power higher in the rev range; but there were also emissions drivability issues starting to show up.

The Sprint set-up was more of the same. The 33mm chokes further weakend the overall vacuum condition, and leaned the A/F ratio even more, such that this time the Main Jets could not be left as before. Lotus had to increase them to 120 in order to maintain enough circuit capacity to avoid a transition stumble, but they also increased the Main Air Corrector a bit to keep the A/F lean with the bigger jets.

Once again, the Idle jetting remained unchanged, so with the weaker vacuum condition it effectively became leaner yet. As a result, the off-idle response and mid-range drivabilty became weaker, with even more emissions drivability issues showing up.

Given a free hand (no emissions restrictions), the S/E and Sprint carb set-ups would have been different, more rich.

So no, you don't need or have to go to S/E carb jetting just because you have one S/E cam in your otherwise Standard Twin Cam. But if you do decide your priorities are for more top end power, and elect to go with larger chokes, then don't restrict the new jetting to the old by-the-book S/E or Sprint factory spec. Get on the dyno with an exhaust gas analyzer, do some road testing, and set the jetting to what best supports the engine... and that won't be the vintage emissions-lean settings.

Make jetting changes according to what's needed, not limited by what your shop guy has laying around.

Regards,
Tim Engel

[Geoffers71]

Wow, Tim that was some comprehensive reply ! What's more I understood all that you were saying it was so well explained As Simon was saying you don't expect new components to be part of the issue, especially as they are working after a fashion, just not up to the required standard. Anyway I will be trying the diagnostics you recommend to see if further improvements can be made. BTW you did not mention the bigger inlet valves the engine now has compared with stock. Those, together with the C cam on the inlet might well require more fuel surely, so that 50 F8 jetting would perhaps be required. Anyway this would become clear if I employ the process that you recommend. Very many thanks for your (and Simon's) helpful responses.

[simonknee]

Amazing post Tim and a great explanation of tuning the Webers by feel.

(can you sense a but)

There is more than one way to skin a cat. You can also tune Webers by measurement. It does require fitting an O2 sensor so is not for everyone (probably not for many people it would appear). This is what I use:

http://www.innovatemotorsports.com/products/lm2.php

To get real useful information you also need to record RPM. If you then add a throttle position sensor (TPS) you get even more data. Then every road is a rolling road! You can spot all sorts of conditions and see the effect adjusting the jetting has on them. You can also be sure that changes you are making are taking you in the correct direction. I found it easy to mistake a too rich condition for a too lean condition and vice-versa when going by feel. I bet I am not alone.

I understand this is not for everyone but trust me if you like getting a bit more empirical about things and are fed up paying for rolling roads that leave you having to accept compromises then this route is very addictive and yields great results. On the down side it also leaves you dissatisfied with the jetting options that Weber offer but there's a solution for that...

It allowed me to tune the C-cams so I can be a gentleman and a boy racer. I think the differences between B, C and D have always been rather over stated and none of these are particularly wild cams. If you are suffering from poor off-idle or low down power then you need to get your tuning sorted whatever your cams.

As an aside how do you split the idle jet into two parts (I have just been trying!!)? Are you thinking of a different carb or have different parts to me... (or have hypojets?)

[Chancer]

What fantastic explanation Tim, it may have been 15 years since I was actively tuning Webers but I completely "got it!" .

In 5 minutes of reading a well written and informative article you have given me far more knowledge than I gained in years of iterative tuning and reading every book on Webers that I could lay my hands on.

My only sadness is that it could well turn out that I never play with Webers again which will be a real shame, I wish that I had a time machine, back in the day I was pretty knowledgeable and very good at getting the best out of Webers with the knowledge that I had, if i could go back now with the above info I would have done an even better job and cured some of the stuttering that I was forced to live with on a couple of vehicles in various states of tune, usually I would rebuild the engine for more power before I actually was 100% happy with the old set up and the new engine brought new jetting challenges.

I can see that I only really knew part of the story and the science behind it, yet back then I never found anyone that knew more than me, in fact chatting to many or watching them work just proved how little that they did know, clearly successfull Rolling road tuners did know more than me but they never wanted to share the knowledge, those that did made me realise that they may have known what to do but did not understand the science behind it.

I commend you

[Esprit2]

Amazing post Tim and a great explanation of tuning
the Webers by feel.

(can you sense a but)

There is more than one way to skin a cat. You can also tune Webers by
measurement.

Simon, I have no quarrel with that. I understand and appreciate the modern computerized diagnostic equipment approach to tuning carbs... new tools for old tech. That's cool. I have the luxury of a skilled dyno shop near me, DynoTech, which has a full load of diagnostic equipment, plus bins and bins of Weber and Dellorto jets, air correctors, chokes, etc. It's a one-stop tune-up without waiting for the next size jet to arrive in the mail. And you get to name your poison... competition, street performance, drivability, or economy. My Europa/ DCOE is running on a DynoTech 'autocross' set-up, but I take pleasure in playing with tuning my other cars by a "laying on of the hands".

> There is more than one way to skin a cat.

It wasn't me discounting the existence of other options... it was you who kept jumping in, putting Geoff back on your straight-n-narrow with, "you need to...". If you had then allowed that there were, "other ways to skin a cat", we wouldn't be having this conversation now. No, Geoff didn't "need to", he had options. I simply expanded upon one option, and he doesn't "need to" follow it if he'd prefer to take the car back to the shop for another session. As long as he gets the car right, sooner than later, then enjoys driving it.

I'm not the Troglodyte you imply. Yes, I did learn about carbs many decades ago. But I also crawled out from under the rock where I was born, discovered a new-fangled thing called school (what a concept !), and became an engineer. By golly and gee-whiz, I do grasp the concepts of some of these new, high-highfalutin technologies.

(Can you sense a but)

But despite that, I've also managed to cling to an appreciation that one of the very interesting aspects of playing with vintage cars, is the "vintage" part. You apparently have no room in your thinking for that concept.

Tim
A throttle, clutch & gear lever soothes the soul better than a flappy paddle auto-shift any day...

[Esprit2]

What fantastic explanation Tim
(Snip)...
My only sadness is that it could well turn out that I never play with Webers again which will be a real shame,
(Snip)...

Thanks for your kind words, Chancer.

If you enjoyed tinkering with Webers in the past, then it would indeed be a shame if you never did it again. Even if you don't own a Weber-carbed engine again, hook up with a local club and help out those who do. Our local Lotus club is a mixed bag of wrenchers, polishers, exhibitors and drivers, but we have enough tech-savy in the club that those who wish to learn don't have much trouble finding someone who will tutor them. That's part of the fun. Especially when you run into a relative youngster who is genuinely interested in learning about vintage cars. Besides, you get to spend someone else's money while you have fun... not a bad deal.

Sorry about my little rant, above. Hope that doesn't put you off from reading something I might write down the road.

Regards,
Tim Engel

[simonknee]

Hey sorry Tim, I thought I put enough disclaimers in my post to try and let you and other people know that I was presenting another way and that I realized it's not for all (if indeed anybody else, since nobody ever popped up asking me questions about fitting the O2 sensor).

I am also guilty as charged of saying things like "you need to" when perhaps I could say "an option is".

Finally I wasn't damning you with faint praise when I said "Amazing post Tim and a great explanation of tuning the Webers by feel." No accusation of Troglodytism intended but I see how you can read it like that with my "sense a but" comment. Sorry that it provoked a rant.

[Geoffers71]

Just to put this topic to bed. The new Bosch coil, new plugs and increased idle jet size seem to have done the trick. Once warmed up there is no hesitation/stumble when opening the throttle a little or at 3-4K revs. The car feels responsive and picks up well from 1500 to 5000 in top gear. However when starting from stone cold there is spitting back through the trumpets on at least 3 cylinders. When hot though this spitting back disappears. I've enriched the mixture screws by a 1/4 turn and it's better, but the idle is a touch lumpier. Not bad though and is acceptable at the moment. Thanks again for all the help.

[Esprit2]

Geoff,

I'm glad to hear the engine is running better now.

The optimal setting for the Idle Mixture Screws is the one that produces peak manifold vacuum at the slowest idle possible consistent with smooth running (no missing or back firing). If you're using a manometer or vacuum gage, that's a direct reading. If you use an airflow meter, like a Synchrometer or Uni-Syn, then peak air flow corresponds to peak vacuum.

Peak vacuum also corresponds to peak rpm, so you can use a tach. The one in the car isn't sensitive/ responsive enough, so use a diagnostic tach... like a hand-held tach & dwell meter.

Failing all of that, you can simply listen to the engine and tune for the fastest idle. If the rpm increases as a result of an mixture screw adjustment, then adjust the idle speed screw to take it back down to the slowest idle possible consistent with smooth running. Then move on to the next mixture screw. So several rounds until the last adjustments produce no further change.

Finally (as far as my bag of tricks goes), use a length of small diameter fuel line or vacuum hose as a stethoscope. Hold one end near one carb inlet, and the other near (but not in !!) your ear. Adjust the mixture screws for loudest hiss.

Regardless of the tools used, adjust the mixture screws for highest, greatest, fastest, loudest. In the end the throats may not be equal, but that's a balance problem, not an idle mixture problem. Do not use the mixture screws for setting balance... adjust them to set each throat to it's peak, only. Then use proper balance procedures (which does not involve the mixture screws) to deal with any peak differences between the throats.

When you're all done, set the idle speed back up to spec. The early manuals call for 800 rpm, and a late TSB 1974/1 called for 900-950 rpm. My personal preference is for a fast idle.

All this takes longer to explain than it does to do. It's an easy adjustment that makes a difference.

Later,
Tim Engel

[Geoffers71]

Thanks for that Tim, I'll do my best