Lotus Elan

Using Leaner Idle Jet Holder on Dellortos

PostPost by: William2 » Tue Sep 26, 2017 3:00 pm

Having got my car to run really sweetly now with a very steady idle, no hesitation throughout the complete rev range whilst driving and getting about 30mpg driving with restraint I am very pleased. The engine has been rebuilt to standard SE spec with Dellorto DHLA 40s', 30mm chokes and standard air filter setup. My question is that all the jets, emulsion tubes, etc are exactly as per the manual and Miles Wilkins book except for the idle jet holders. It wood seem that the engine runs best with a 7840.1 idle jet holder with 50 size jet as opposed to the suggested richer 7850.2 holder. I was interested to know if any one else had this combination? I don't want the engine to run too lean but the plugs appear to be a greyish colour on the electrode.
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PostPost by: Esprit2 » Tue Sep 26, 2017 10:28 pm

William2,
Set the idle speed as slow as possible consistent with smooth running (no missing or spitting). Then crack the throttle open slowly enough that the accelerator pump isn’t much of a factor. Does the engine hesitate just off idle?

If it does hesitate, then the Idle Air Corrector (aka, Idle Jet Holder) is too lean. Set the Idle Air Corrector richer one step at a time until the hesitation just goes away.

If it does not hesitate, then you can’t be certain where it is relative to correct. In that case, set the Idle Air Corrector one step leaner until the engine just starts to hesitate off idle. Then go back the other way, and set the Idle Air Corrector richer one step at a time until the hesitation just goes away.

That’s the correct Idle Air Corrector setting if you’re just sitting in the driveway, idling. However, it’s very likely that the engine will pick up a slight off-idle hesitation when driven in the real world with that setting (ie, trying to get the weight of the car moving off the line). If that’s the case (it picks up an off-idle hesitation when driven), then go one step richer with the Idle Air Corrector until the off idle hesitation in real world traffic ‘just’ goes away.

*~*~*~*
With the Idle Air Corrector set, warm-up the engine and go for a drive. On a stretch of open road, short-shift in to a lower gear at a lower than normal rpm, the accelerate at full throttle through about 4000 rpm. Not 1st gear, because you’ll rip through it too quickly. Not 4th gear because you’ll risk a speeding ticket. Something socially reasonable for the road you’re on.

If the engine stumbles at or before 3200 rpm, then the Idle Jet is too lean. The Idle Circuit feeds the engine up until the Main Circuit takes over at around 3200 rpm. A stumble indicates the Idle Circuit is too lean, and ran out of capacity before the Main Circuit took over. Adjust the Idle Jets a step richer until the stumble just goes away.

If it does not stumble, then you can’t be certain where it is relative to correct. As you did with the Idle Air Corrector, set the Idle Jet one step leaner until the engine just starts to stumble at around 3200 rpm. Then go back the other way, and set the Idle Jet richer one step at a time until the hesitation just goes away. No richer.

The Idle Jet and Idle Air Corrector are closely related, and changing one affects the other. So, after completing one cycle of both procedures, go back and start over again with the off-idle hesitation and the Idle Air Corrector. Then the Idle Jet and 3200 rpm stumble. Repeat as necessary until the last round produces no further change. Yes, it's putzy / fiddly work.

If the end result agrees with the Lotus jetting specs for your engine, cool, it’s great when everything works as it should. If it doesn’t agree, don’t sweat it, it's working. Every engine and carburetor has it’s own set of tolerances, wear patterns, and needs. They’re old school, and they don’t have feedback sensors and ECU’s to keep them in spec. You can’t simply stomp your foot and throw a fit, insisting that the carb perform to spec. If you jet your engine and carb per the above procedure, and it runs well, be happy and don’t sweat the spec.

Regards,
Tim Engel
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PostPost by: jono » Wed Sep 27, 2017 11:55 am

Hope you don't mind me butting in but I'm going through exactly the same thing with DHLA40's trying to resolve a stumble between 2,750 to 3,200 rpm

I've played around with every combination of jets and have recently found a position where I've improved the stumble but not eliminated it. I did this by going to a leaner idle jet holder which got me wondering - is a stumble in this region always symptomatic of a lean condition or will the converse, an overly rich condition, also cause it?

I could try an even leaner jet holder but the problem is I've ran out of them - an idle jet loan library would be a good facility!
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PostPost by: jimj » Wed Sep 27, 2017 3:08 pm

Coincidentally, just last Tuesday I had our Sprint on a rolling road at Alfreton, Peter Burgess, highly recommended. Ever since we`ve had the car there`s been just a slight stumble, nothing much, just a bit noticeable (only to me) when, say, just feathering the throttle maintaining a steady low speed. Various professional attempts have failed to improve matters.
I was happy to live with it, you just adjust your driving and it`s not significant. A month or so ago I was collecting a pal from dropping off his car with Peter Banham. He`s a real, old style guru who supports significant rally cars and done some serious stuff himself. He immediately noticed the "fault" as I reversed from his drive, recommending Peter Burgess.
It`s a long story but he fiddled with this, then that, removed some jets (?) soldered and re-drilled them, fiddled some more, then some more, determined to make the car perfect. The car was making a respectable 106bhp at the wheels at 5500 rpm and he didn`t improve that but he did improve the torque especially lower down. It took three and a half hours and he still wasn`t satisfied. Really, he said, the car would benefit from smaller chokes, maybe 30mm rather than 31mm, but would then need setting up again
Driving the car home, and since, the difference is remarkable, much quicker and smoother all round. I really am satisfied. The price? it wouldn`t be fair to say but a quarter of what I expected. Maybe he has a fixed price irrelevant of time taken.

So, a question: if a smaller choke is better than one too big then how can a bigger airbox, discussed elsewhere, make a difference. I regret I didn`t think to have runs on the dyno with and without the airbox. Facts are so much better than opinions.
Jim
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PostPost by: William2 » Wed Sep 27, 2017 4:10 pm

I believe that 30mm chokes are the recommended size for an SE spec engine. The deeper air box means that the carbs can suck in air more efficiently and evenly. I think the choke size is probably not relevant in that context.
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PostPost by: Esprit2 » Wed Sep 27, 2017 5:37 pm

The small valve Twin Cams with Dellortos used 30mm chokes. The Big Valve engines used 33mm chokes. (Lotus Technical Service Bulletin number 1974/1, dated Feb 12, 1974.)

Flow Velocity through the venturis is important. At any given rpm/ cfm, the flow velocity will be faster through a smaller venturi, and slower through a larger venturi. Naturally, the flow rate is lower at slower engine speeds, and higher at faster engine speeds. On an absolute scale, the slow flow rates through the venturis won't create sufficient vacuum to pull fuel into the throats with 'authority'. Responsiveness to throttle changes will become sluggish. The engine can stumble. In general, a variety of slow running drivability issues show up as the Flow Velocity drops off.

For that reason, engines/ carbs that are tuned for street driving use smaller chokes to keep the Flow Velocity higher further down into the rev range. But small chokes are more restrictive to maximum flow potential... ie, peak power potential. So engines that put more emphasis on peak power and performance use larger chokes so the carbs can flow 'more'. The two goals are in direct opposition to one another. Peak power comes at the expense of low speed drivability, and vice versa. You can't have both at once, so choose one as your priority, and live with what happens to the other.

Anything that restricts air flow into the carbs limits peak power potential. The smaller a filter is, and the more dense and restrictive it's filter element is, the more it will restrict airflow and peak power potential. A small paper element air filter will be more restrictive than a large K&N free-flow filter... and the engine can produce more power with the K&N... all else being equal. Or it can produce even more power without any filter at all... but that's a bad idea due to the dirt & grit that can be ingested by the engine.

If you switch to a freer flowing filter, like from generic paper to a K&N linen, then the additional air admitted to the engine will cause an overall lean-shift in the Air-Fuel ratio. The additional air must be balanced with an appropriate additional amount of fuel. In other words, whenever you do anything to the air filter or the airbox configuration to improve air flow into the engine, you also need to follow-up by re-jetting the carb richer. Simply throwing on a larger free-flowing filter in a larger airbox may result in an overall leaner condition that costs performance and drivability, rather than enhancing them.

An airbox must be large enough not to 'shroud' the carb inlet. There should be a gap between the end of the air horn and the opposite air box wall equal to the inside diameter of the air horn/ throat. For 40mm throats, there should be an open gap of 40mm between the end of the air horn and the opposite wall of the air box. Anything less is shrouding the inlet and restricting air flow. In truth, it's a little more complicated that that. But gap = throat ID is an easy rule of thumb if you don't wish to make a study out of Fluid Dynamics. Heck, I'm an engineer, and even I hated Fluid Dynamics. Just note that the stock Elan air box is quite slender.

The engine will run better with a well designed air box breathing cool outside air ducted in. An open, free-flowing K&N air filter may be less restrictive, and potentially flow more air, but it also breathes hot 'under-hood' air. The loss of power due to breathing hot air more than offsets and power gain due to less restriction/ freer breathing. The best option would be a free flowing K&N inside a proper air box that doesn't shroud the inlets, and with cool air ducted in.

Carbs are like a spider web. Touch it over here, and it tingles over there. Every change you make affects every other setting. If you're running otherwise stock jetting, but have changed the chokes from 30mm to 31mm, then all the jetting is now wrong.

Carbs don't have sensors, feedback loops and an ECU to control results. And production tolerances in both engines and carburetors mean that each specific combination needs it's own jetting/ tuning in order to be 'right'. The factory carb specs are a good 'average' setting that will give broadly acceptable results on engines that are correctly built to spec as they come off the end of the line... within an range of acceptability. Getting one off the end of the line that is a dead-nuts perfect combination would be a huge fluke. If you want perfect mixture and optimal running, then each and every engine requires one-on-one tuning on a rolling road. Expect the perfect end result to vary slightly from the book-spec jetting.

Carbs are also sensitive to environmental changes, like elevation, ambient temperature, and barometric pressure. So get your engine tuned to perfection at sea level, then drive it home into the foothills, and the mixture will be two rich at altitude. An engine balanced on the edge for peak power will be more sensitive to minor changes. A set-up that has been 'dumbed-down' for low speed drivability will be more tolerant. But both will be sensitive, just to different degrees.

Regards,
Tim Engel
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PostPost by: Esprit2 » Wed Sep 27, 2017 6:17 pm

jono wrote:Hope you don't mind me butting in but I'm going through exactly the same thing with DHLA40's trying to resolve a stumble between 2,750 to 3,200 rpm

I've played around with every combination of jets and have recently found a position where I've improved the stumble but not eliminated it. I did this by going to a leaner idle jet holder which got me wondering - is a stumble in this region always symptomatic of a lean condition or will the converse, an overly rich condition, also cause it?

I could try an even leaner jet holder but the problem is I've ran out of them - an idle jet loan library would be a good facility!
Factory jetting presumes a standard set of conditions. As those conditions change, the spec Idle jet that was a little on the lean side, can become a little too rich. And yes, the stumble you then experience might be because the transition mixture is too rich. Elevation and Barometric Pressure can cause that. The higher you go and the lower the Barometric Presssure, the richer the mixture becomes.

That's why during the full throttle road test, you adjust the Idle Jet leaner and leaner until the engine just picks up a lean stumble. Regardless of the jet required to get there, you have now found a recognizeable condition... the lean stumble... so you now know where you are. Now go back the other way, a step richer at a time, until the lean stumble just goes away... go no further.

If you do ignore instructions and go richer than necessary, then the Idle Circuit continues to fuel the engine beyond the transition rpm; and when the Main Circuit kicks in, the engine is double-fueled. It loads up, and you get a rich stumble at the very least... or it may just bog down, blow black smoke, and loose power. Too rich is not good either.

Regards,
Tim Engel
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PostPost by: William2 » Thu Sep 28, 2017 4:38 pm

Hi Tim, once again thank you for your very comprehensive and explanatory reply. One other thing I'm not quite sure I understand is the idle adjuster screw workings. Does this only have an effect on the fuel/air mix at idle (i.e. below 1000rpm) or does it make a difference to the mixture all the way up to when the main jet circuit takes over? I took the car out today and did all the accel tests as you suggested. It looks like everything is pretty close to being correct as there is only a very slight hesitation when I accelerate slowly or floor it - hardly noticeable. Regards, William
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PostPost by: Esprit2 » Thu Sep 28, 2017 7:43 pm

William2 wrote:One other thing I'm not quite sure I understand is the idle adjuster screw workings. Does this only have an effect on the fuel/air mix at idle (i.e. below 1000rpm) or does it make a difference to the mixture all the way up to when the main jet circuit takes over?
I was tired when I posted that last message, and it got a little garbled in a few places. Sorry about that... I've edited it. Hopefully it makes more sense now.

In the overall Idle Circuit, the Idle Jet and the Idle Air Corrector (aka, Idle Jet Holder) mix fuel and air into a froth with a specific A/F ratio. Shake your beer... or soft drink... and the foam 'head' that results is about the same consistency as the froth. That froth is fed into two drillings that run along the top sides of the two throats (looks like a mole tunnel), toward the carb's mounting flange.

Midway along that drilling/ tunnel, there are several holes drilled through the floor, into the carb throat... ie, the Progression Holes. Remove the Progression Hole Cover, and look in while operating the throttle lever. The edge of the butterfly can clearly be seen sweeping past the Progression Holes. When the throttle is close, most of the Progression Holes are on the air filter/ atmosphere side of the butterfly. As such, they're not subject to significant vacuum, so Air-fuel froth is not being drawn into the throat through them.

At closed throttle, the edge of the butterfly may partially cut across the first Progression Hole, in which case, yes, a Progression Hole 'can' be active during closed throttle idle. Depending upon a lot of inter-related settings in the carb, it may or may not be active at closed throttle.

The drilling/ tunnel continues past the Progression Holes toward the carb mounting flange. One last drilled hole is located there, with the tapered needle of the Idle Mixture Screw centered in it. The screw & needle make the hole's open area variable, like a mini-throttle. It's located well down stream of the throttle butterfly, so it is always subject to full manifold vacuum whenever the throttle is closed.

This last drilling is 'the' primary hole for feeding the engine at closed throttle. The first Progression Hole may or may not be active, but this one, last Idle Mixture Screw hole is 'always' fully subject to manifold vacuum when the throttle is closed/ partially closed/ in transition. It and its companion Idle Mixture Screw function as a mini-throttle... one throat at a time.

The Air-Fuel ratio of the froth has already been determined by the Jet and Air Corrector. The Progression Holes and the Idle Hole/ Mixture Screw simply control how much of that froth is dispensed into the throat.

The Idle Mixture Screw is the tool used to fine-tune the Idle Circuit for optimal 'closed throttle' mixture. It's effect is most noticed at idle and during off-idle transition.

The Idle Mixture Screw's adjustment is accomplished by turning the screw in and out while observing the manifold vacuum. Tune for peak vacuum, which is an indicator of strongest running. From that peak position, turning the screw either in or out will result in lower vacuum. Once you have found the peak, turn the screw out/ counter-clockwise a half turn or so, then reverse and approach the peak one last time in a clockwise direction, stopping at the peak without backing up... ie, take up all the slack/ slop in a CW direction.

The Dellorto Idle Circuit feeds the engine up to 3200 rpm (4000 rpm in a Weber). The Idle Mixture Screw's effect is all at the far opposite end of that range, in the idle/ early transition end. Any stumble at 3200 rpm is way beyond the Idle Mixture Screw's reach, and it's a total waste of effort to attempt to fix a 3200 rpm stumble by cranking on the Idle Mixture Screws. You're not fixing anything that way, but you are screwing-up something. The Idle JET is the primary tool for fixing the 3200 stumble, the Idle Air Corrector fixes an off-idle hesitation, and the Idle Mixture Screw optimizes closed throttle running... with a secondary assist to off-idle hesitation (focus on getting the Air Corrector right first).

*~*~*~*
The Idle Jet and Idle Air Corrector achieve a correct Air/ Fuel ratio. But each of them is available in incremental size steps. In the case of Webers, the Idle Jets are in steps of '5'... 45, 50, 55, etc. With Dellortos it's steps of '1'... 50, 51, 52, etc. Dellortos offer much finer resolution for sneaking up on the perfect mixture, but it's still in incremental steps. At large throttle openings, that step-resolution may not be noticed as much; but it can be significant at close throttle and off-idle transitions. The Idle Mixture Screws are that last, step-less fine-tuning tool that allows you to fill in the gaps between the jetting steps.

A lot of owners treat the Idle Mixture Screws like a main tuning tool, and are then disappointed when they don't get the big-effect result they wanted. Do all the front line idle mixture control with the Idle Jet and Idle Air Corrector. Get them right first! Then use the Idle Mixture Screws to smooth out the gaps between jet sizes. Don't expect more than that. If the Jets aren't right, the screws won't save you.

When you tune for peak manifold vacuum, the best tool is a 4-tube manometer. Failing that, a distant second best is a Synchrometer. The Uni-Syn is best used as wall art. The big problem with the Synchrometer and Uni-Syn is that they disturb the very airflow they're trying to measure.

Another second-best alternative is to tune for peak idle rpm. The idle rpm will vary as the screws are adjusted, and will rise and fall with the vacuum. The manifold vacuum and idle rpm will both peak at the same time. However, the tach on the car's instrument panel is woefully inadequate. It doesn't have anywhere near the resolution that's needed. If that's the only tach you have, then don't consider using idle rpm as a tuning tool. A 'diagnostic' tach is required, like a quality bench-top Tach & Dwell meter... emphasis on 'quality'.

Having said that, a lot of people tune by ear, listening to the engine's idle speed rise and fall. That's even lower resolution than tuning by the car's dash tach.

Another disadvantage of tuning by rpm is that the tach (or your ear) gives an overall reading for the engine. A 4-tube manometer (or one of the airflow meters) reads each throat individually, so you can specifically focus on the impact of the one setting you're making. The tach/ ear approach just gives you a homogenized, overall engine response, and one throat can get lost in the clutter. That method can be useful if that's all you've got, but it's far from ideal.

First set the idle rpm as slow as possible consistent wtih smooth running... no missing or spitting back. The more closed the throttle is, the stronger the vacuum signal you've got to work with. Then adjust each Idle Mixture Screw for peak Manifold Vacuum (rpm). Adjust all four, then go back and repeat, adjusting all four again. Repeat until the last round produces no further change. Finally, set the idle speed back up to 900-1000 rpm.

I tend to ramble. I hope all that makes sense.

Regards,
Tim Engel
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PostPost by: Chancer » Thu Sep 28, 2017 10:09 pm

I for one love reading your explanations Tim, it has been many years since I was regularly tuning Webers but for me its a bit like riding a bike, most of what I learned was just by trial and error and intuition, readiing your posts made me smile and I was saying to myself "thats exactly how I did it" but do you know what, I didn't really know why I got to those techniques other than trial and error and I did not fully or in some cases even partially understand the physics behind it until reading your postings over the years.

I am self taught in most things and sometimes on a job site I may see a pro doing something he has done all his working life, it may be something as simple as how he wobbles a drill (and his hips!) when using a hole saw and again it makes me smile that i do the same, I have seen some realold hands tuning webers just as I do and they would probably not be able to explain to an apprentice what to do other than to do it like them, a retirement age mechanic friend here in France prepares and races historic Panhards, i watched him balancing twin Solex 40's and doing a dizzy swing by ear, exactly as i do. On the other hand I have seen some people pretending they know all about tuning webers twiddling screws and I could see clearly they had no idea of what they were doing.

You have not only the knowledge of what to do but the ability to explain it which is vary rare.

You may have already mentioned it but my first starting point before twiddling with anything is to make sure the ignition system is spot on, the plugs cleaned and gapped and the timing set, then I make sure tha carbs are correctly balanced and repeatedly recheck/adjust the balance as I am tuning the idle.

You have not mentioned listening to the induction hiss using a rubber tube, I have never had a 4 venturi vacuum guage set up and agree that my carb flow meter restricts the airflow and richens the mixture, I have to pop it in place and take the reading quickly, I find that I get the best results with a tube in my ear and only use the flow meter for the final fine adjustments, i will buy a set of vacuum guages when I next do a carb set-up.
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PostPost by: Esprit2 » Fri Sep 29, 2017 12:13 am

Chancer wrote:You may have already mentioned it but my first starting point before twiddling with anything is to make sure the ignition system is spot on, the plugs cleaned and gapped and the timing set,
Hi Chancer,
Agreed, the first step to tuning the carbs is always a thorough ignition tune up.

Chancer wrote:You have not mentioned listening to the induction hiss using a rubber tube
I tend to forget about that since my own hearing is not very good any more. That, plus the fact that there's a lot of stray noise when the engine is running means that listening to the hiss is no longer 'my' best tool.

Also, it takes one hand to hold the rubber tube stethoscope near but not in your ear, a second hose to hold the other end near the carb throat, and a third to adjust the Idle Mixture Screws, Air Bleed screws, and the Adjustable Linkage Coupler's screw. I usually come up short one hand.

I use a Morgan CarbTune II 4-tube manometer. I've used just about all the other tools and methods, but once I started using a manometer, I lost interest in other methods. That's just my preference, nothing more. Simply hook-up all four throats at once, then it's like watching an audiophile graphic equalizer while turning screws.

Thanks & Regards,
Tim Engel
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PostPost by: Robbie693 » Sat Sep 30, 2017 10:35 am

Tim you should write a book; your explanations on the workings of carburettors and tuning are the only ones I can get my head around!

Contributions like yours are what makes these forums great.

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PostPost by: Chancer » Sat Sep 30, 2017 11:21 am

Agreed on both counts.
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PostPost by: jono » Mon Oct 09, 2017 11:24 am

Tim,

Thank you also for your invaluable wise words.

In relation to my transition stumble described earlier in the post, I attach a dyno plot from my car with the jetting as it currently is. The engine is mildly modded, including QED 360 cams and some light porting, and the jetting is as recommended by QED on teh other attachment (they make no mention of the idle air correctors)

Can you discern anything from this in terms of whether i'm to lean or too rich in the idle circuit/transition?

Many thanks again

Jon
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PostPost by: Esprit2 » Tue Oct 10, 2017 10:39 pm

jono wrote:In relation to my transition stumble described earlier in the post, I attach a dyno plot from my car with the jetting as it currently is. The engine is mildly modded, including QED 360 cams and some light porting, and the jetting is as recommended by QED on the other attachment (they make no mention of the idle air correctors)

Can you discern anything from this in terms of whether i'm to lean or too rich in the idle circuit/transition


Yup, a communal jet inventory would be very helpful. Given the far-flung nature of the internet, that's probably best done at a local club level.

As far as your question, my simple answer is, "No, I can't". To begin with, you really haven't clearly stated what jets you are using. Especially your Idle Air Corrector... you just talk about going 'leaner'.

As far as the QED Q360 cam in a Twin Cam is concerned, I have no experience with it.

Is your jetting currently exactly what QED recommends? Except for the Idle Air Corrector... which they don't mention.

What Idle Air Corrector are you using? You've talked about leaner, but haven't been specific.

Adjustments made to the Idle Jet and Idle Air Corrector do influence one another. That's why after changing one, you must re-check the other... and repeat the check & re-check cycle until no further change is required. So yes, it's conceivable that you could make the Idle Air Corrector so rich that it 'cures' the usual 3200 rpm lean-stumble. It's also possible for an overly rich mixture to cause the engine to load-up and stumble... a rich-stumble. And in that case, an overly lean Idle Air Corrector 'might' improve that condition. However, by taking the Idle Air Corrector out of it's normal range, how badly have you screwed-up overall Air-Fuel Ratio for the Idle Circuit, and/ or the off-idle performance? It's best to address a problem at it's source, rather than take something else way out of it's range to force a... a what?... another compensating error?

As mentioned in an earlier post, the correct approach is to first adjust the Idle Air Corrector to eliminate an Off Idle Hesitation. Then go for a drive and use a full throttle run to diagnose and adjust the Idle Jet to eliminate a typical 3200 rpm lean-stumble. Even if it's a more uncommon rich-stumble, address it with the Idle Jet. Adjust the Idle Jet so that the Idle Circuit is running out of flow capacity just as the Main Circuit is kicking in.

*~*~*~*~*~*
You already have a baseline recommendation from QED, and it's normal to do some tuning from there (from the starting point). I do find it curious that QED omits the Idle Air Corrector from their jetting recommendation. If you're following their spec and having trouble, I strongly recommend that you call QED and ask for more details.

I've never messed with the Q360 cam myself, so I'd have nothing to offer that's any 'smarter' than QED's recommendation. That being the case, the following is just a ramble on the broad subject.

*~*~*~*~*~*
Looking at QED's dyno chart, the O2R x 10 plot is pretty lean at low rpm... ie, at Idle Circuit speeds. Their data sheet on the Q360 cam does recommend a 50 Idle Jet (which is fairly rich), yet the dyno chart shows a lean condition at low rpm, so I'm 'guessing' that the Idle Air Corrector is on the lean side.

Which Idle Air Corrector are you using?

*~*~*~*~*~*
Is the float height (preferably the fuel level) correctly set? That's a foundational setting, and everything else is affected by it. Make sure it's right before trying to adjust any other jets.

Emulsion Tubes are chosen according to cylinder displacement. The TC is 1558cc / 4 = 389.5cc per cylinder. Dellorto Emulsion Tubes go rich to lean in part number order. In other words, 7772.3 is richer than 7772.4. There is some overlapping of ranges since a very efficient, high performance engine will draw in more air than a less efficient grocery-getter engine of the same displacement.

Emulsion Tubes
Richer / Larger Cylinder Displacement
7772.3 for 675cc and above
7772.4 for 550cc - 675cc
7772.5 for 450cc - 575cc <<< Lotus & QED both specify richer yet - it's a performance engine.
7772.6 for 350cc - 475cc <<< Dellorto generic for 1558 / 4 = 389.5cc - Richer
7772.7 for 275cc - 400cc <<< Dellorto generic for 1558 / 4 = 389.5cc - Leaner
7772.8 for 250cc - 325cc
Leaner / Smaller Cylinder Displacement

Idle Jets
40 for 250cc - 350cc per cylinder.
45 for 350cc - 420cc <<< Dellorto generic for 389.5cc/ cyl. Also Lotus for emissions small valve.
50 for 420cc - 490cc <<< Lotus & QED both specify richer, except Lotus for emissions small valve.
55 for 490cc - 560cc
60 for 560cc - 630cc
65 for 630cc - 700cc

The Twin Cam's displacement is 1558 cc, and QED claims the Q360 cam with 10.5:1 compression plus their jetting set-up can produce 135 bhp @ 6500 / 117 lbs-ft @ 5500. If you take that displacement and power output, and work backwards through the standard Dellorto formulas, you get the following 'Performance' and 'Flexibility' recommendations:

QED Vs Generic Dellorto Jetting Recommendations:
............................................. Dellorto Generic... Lotus Big Valve
Spec Source .... QED ......... Performance ........ UK Dom
Carb ................. Dellorto ... Dellorto ................. Dellorto
Model .............. DHLA40 ... DHLA40 ................. DHLA40
Choke (mm) .... 33 ............. 33 ........................... 33
Main Jet ........... 135 ........... 135 ......................... 120
Main Air Corr... 150 ........... 180 ......................... 130
Emul Tube ....... 7772.5 ..... 7772.6 or .7 ........... 7772.5
Idle Jet .............. 50 ............. 45 ........................... 50
Idle Air Corr ..... 7850._ ...... 7850.2 .................... 7850.2
Pump Jet ............................. 35 / 45 .................... 45
Aux Venturi ......................... 7848.3 .................... 7848.3

Idle Air Corrector Jet Sizes:
7850.5 .. Leaner, going down the list gets richer.
7850.10
7850.9
7850.4 .. 5-10-9-4 are too lean to be good for anything other than emissions
7850.1 .. Normal
7850.3 .. Normal
7850.6
7850.7
7850.2 .. Rich <<<<<<<<< Lotus & Dellorto generic jetting, above.
7850.8 .. Richest

The QED and generic Performance columns start out in agreement on the choice of Choke and Main Jet. But QED's Main Air Corrector and Emulsion Tube are richer than standard. On the other hand, Lotus' UK Domestic spec for the Big Valve agrees with QED on the richer than standard Emulsion Tube. And QED's & Lotus' Idle Jets are richer than generic.

Just for comparison, here are Lotus' Dellorto jetting set-ups. They're from a 1974 Technical Service Bulletin, so may vary slightly from any earlier manual publications. Also note that all small valve Twin Cams use 30mm chokes, while the Big Valve TCs use 33mm chokes for the UK domestic market, and 32mm for the Euro ECE-15 emissions standard markets.

Lotus Technical Service Bulletin -- Number 1974/1 -- Date: 12 Feb 1974
(1974 - ie, looking back with the benefit of hindsight.)
Amend Dellorto carburetors settings to:
................................ ------ Small Valve TC ------ ... -------- Big Valve TC--------
................................ UK ............. Euro ECE-15 ... UK ............. Euro ECE-15
................................ Domestic... Emissions ....... Domestic... Emissions
Throat.. (mm) ........ 40 .............. 40E .................. 40 .............. 40E
Choke (mm) .......... 30 .............. 30 ..................... 33 .............. 32
Main Jet ................. 115 ............ 120 ................... 120 ............ 120
Main Air Corr ........ 160 ............ 200 ................... 130 ............ 160
Main Emul. Tube... 7772-l.28 ... 7772-5 ............. 7772-5 ...... 7772-5
Idle Jet .................... 50.02 (?) .... 45L .................... 50.02 (?) ... 50L
Idle Jet Holder ....... 7850-2 ....... 7850-4 .............. 7850-2 ..... 7850-1
Pump Jet ................ 40 ............... 33 ..................... 45* ............ 33
....................................................................... * Use 35 if necessary to improve driving
Starter Jet ............... 70 ............... 70 ..................... 70 .............. 70
Starter Emul Tube.. 7482.1.28 .. 7482-1.28 ....... 7482-1.28 .. 7482-1.28
Needle Valve .......... 150.33 ....... 150.33 ............. 150.33 ....... 150.33
Float Ass'y (P/N) ..... 7298-01 .... 7298-02 .......... 7298-01 ...... 7298-02
Float Wgt (grams)... 10 ............... 8.5 ................... 10 ............... 8.5
Float Hgt. (mm) ...... 14.5-15.0 .. 16.5-17.0 ......... 14.5-15.0 ... 16.5-17.0
Air Horn Lgth (mm).. 40 .............. 40 .................... 40 ............... 40
Idle Speed (rpm) ..... 900-950 .... 900-950 ........... 900-950 ..... 900-950
Camshaft ................. C-Cam ...... C-Cam .............. D-Cam ....... D-Cam

Sorry, just rambling... no direct, simple answer for you.
Tim Engel
Last edited by Esprit2 on Thu Oct 12, 2017 7:33 pm, edited 1 time in total.
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