Adjusting the "squirt" on dellortos

[Dave-M]

Martin, Sorry, It appears I misunderstood
Regards
Dave

[elansprint71]

In an attempt to find out why petrol is dribbling out of one of the chokes (into the K&N filter) on one of my Dellortos I am doing the service-kit thing again. As usual I have some litlle "O" rings left over but that's because I don't bother with the choke/starter system. I hope.

One of the things which I have noticed this time (must have missed it back in 2004, or thereabouts) is that the pump jet linkages are both differently adjusted and probably both wrong!

Unhelpfully the Des Hamill Dellorto/Weber book says "Avoid touching these nuts unless there is clearly an adjustment problem" and does not appear to offer any suggestions about where the correct setting lies. The Lotus Workshop Manual merely says "The nuts are used to adjust the pump capacity" again no indication of what the adjustment should be.

Having trawled through various old threads here it seems that the consensus is that there should be about 3mm of thread showing beyond the lock-nut; one of mine was about 7mm and the other 10mm! having adjusted one of them to 3mm to see what the visual effect is, it is clear that there is a lot less movement of the lever. I'm going to set them both at 3mm, fettle the other carb and put it all back together. It does not really look look an adjustment which can be carried out in situ, so if it's no good I'll have to take the carbs off again!

There seem to be many new members on here since the last time the subject was aired- anyone, new or old, care to chip in with advice?

[types26/36]

Pete,
I was recently looking at this subject and collected the following data.....some from here on the forum and some from other sources.
I have not attempted to do anything with as of yet ......have a look as a starting point.

[billwill]

One thing to note, is that your foot-pedal does not "drive" the accellerator pump, it merely moves the stop out of the way so that the spring can drive the pump at it's preset rate.

Hence the threaded rod must move freely in the hole.


I tested mine by doing the 6-foot squirt test, with the carbs off the engine. I think this is described in the book, but I've mislaid my copy so I can't verify that.

[elansprint71]

I can see why you have not started yet Brian. I think you ought to start right away and get back to us by tomorrow at the latest!

Sorry Pete....just a bit busy at the moment trying to solve the Middle East, Northern Ireland and world peace

What about the melting ice? Slacker!

[Esprit2]

I'm late to this conversation, but I'm just going to wade in rather than try to tip-toe around everything else that has been written.

As a first step to getting your head around the Dellorto Accelerator Pump's operation, understand that it's NOT driven by your foot, or any force your foot puts into the linkage. Your foot only releases it.

As an analogy, think of an archer shooting a bow and arrow. He doesn't throw the arrow forward with arm strength. He simply draws the bow string and arrow back to a cocked position and holds it there until ready to shoot. When he releases the string, the spring rate/ potential energy of the bow accelerates the arrow forward at a pre-determined/ engineered rate.

The accel pump is driven by a pre-set spring pressure working on the diaphragm... much like the bowstring working against the arrow. At foot-off, closed-throttle, the center of the pump's diaphragm is drawn down against calibrated pump spring tension. The working diameter of the diaphragm, times how far the center is drawn down, determines how much fuel is drawn into the pump and ready to be discharged.

The length of the draw on the diaphragm is determined by the nut where the pull-rod connects to the free end of the pump lever/ teeter-totter. The more exposed threads below the nut, the higher the pump lever is lifted, the further the diaphragm is pulled back (down), and the more fuel is drawn into the pump, ready to be squirted into the throats.

MORE exposed threads = MORE fuel in the pump to be discharged.
LESS exposed threads = LESS fuel in the pump to be discharged.

As a test, if you grasp the pump lever/ teeter-totter and prevent it from moving, then fully open the throttle, you'll see that the pull-rod just slips down through the hole in the pump lever. The nut underneath can pull up on the pump lever, but there's no feature that allows the pull-rod to push down on it. The pump?s driving force is built into it as a pre-determined, calibrated spring rate. When you step on the pedal and floor the throttle, the pull-rod and nut move fully down, getting out of the pump lever?s way, and releasing it to do whatever it must do... but they don't drive it down.

You release the cocked accel pump mechanism, and a spring applies a pre-determined force against a diaphragm, pressing it against a pre-determined volume of fuel (you pre-adjusted the nut/ exposed threads), forcing that volume out through a pre-determined orifice/ pump jet.

(Snip)?
1) The jet size controls the duration of the squirt, not the amount
2) The pump stroke controls the amount of fuel
Dave

I may be splitting hairs here, but I?d reverse the order to better clarify (in my mind) what?s happening.
1) ?The pump stroke/ exposed threads controls the amount of fuel?? drawn into the pump. If you put your foot to the floor, releasing the pre-tensioned mechanism, and hold it there, then spring pressure on the pump diaphragm will eventually discharge the full amount of fuel.

2) ?The jet size controls the duration of the squirt, not the amount.? Yes? well? with a pre-determined spring tension/ fuel pressure, the fixed orifice determines the flow-rate. With a fixed volume available, the resulting flow rate secondarily controls the duration. The accelerating engine requires FIRST enough fuel to keep up with demand, and THEN requires that amount to be delivered over sufficient time. With the spring tension/ fuel pressure constant, the pump jet size will determine both delivery rate and duration.

Rate is important FIRST. A pump jet must be selected which delivers the proper fuel delivery rate to keep up with demand. Too little and the engine will stumble. Too much and the engine will flood and bog down.

Duration comes SECOND. If the jet keeps up with demand initially, but the pump shot runs out too early and the engine stumbles later, then more fuel is required? not a greater flow rate. It?s shot size that?s deficient (stroke length/ exposed threads), not the flow rate (pump jet size). Adjust the pull rod nut to expose more threads.

If the engine responds well initially, but then floods and bogs down, then the flow rate was right but the shot size was too large and lasted too long. Adjust the pull rod nut to expose fewer threads.

*~*~*
The Idle Jet and Idle Air Corrector (aka Idle Jet Holder) combine to control proper mixture up to about 3200 rpm, at which point the Main Circuit takes over. The Accelerator Pump is only supplemental and transitory, and should not be given priority. The Idle Circuit mixture must be dialed in first! Only once it?s right should you attempt to adjust the Accelerator Pump.

Set the idle speed as slow as possible consistent with smooth running. Then open the throttle slightly. Not a big blip of the throttle, and slowly enough that the Accelerator Pump is pretty much a non-issue. If the engine hesitates off-idle, then the Idle Air Corrector is too lean. Go a step or two richer until the hesitation just disappears. If there is no hesitation, then go a step or two leaner until the hesitation just appears, then a step richer again until it just disappears (ie, as rich as required, but no richer).

Then take the car for a drive on a wide-open stretch of road. Accelerate at full throttle in a mid-range gear, like third, looking for a stumble at approximately 3200 rpm. That?s the transition point between the Idle and Main circuits. If the engine stumbles, go a step or two richer with the Idle Jet until the stumble just goes away. If the engine does not stumble, then go a step or two leaner until a stumble just develops. Then go one step richer until the stumble just disappears.

Now start messing with your Accelerator Pump circuit.

All else being equal, a heavier car requires a richer Idle Circuit and a bigger shot from the Accelerator Pump to pull away from a dead stop. Also, larger chokes result in a weaker vacuum and leaner overall condition, and therefore require a richer Idle Air Corrector and Accelerator Pump. A gentle, civilized start shouldn?t require much of any input from the Accelerator Pump, but a boy-racer start will depend heavily on the shot from the Accelerator Pump.

........................................... Sprint &
........................... Elan Std ... Elan +2 130 ... Europa TC
Choke (mm) .......... 30 ............ 33 ............... 30
Idle Jet ................ 50 ............ 50 ............... 50
Idle Air Corrector ... 7850-1 ...... 7850-2 ......... 7850-2
Accel Pump Jet ...... 33 ............ 45 ............... 40
Pump Delivery ........ 7.5 cc ?.... 8 cc ............. 8 cc
? in 20 Strokes

Idle Air Corrector Jet Sizes:
7850.5 .. Leaner
7850.10
7850.9
7850.4
7850.1 .. Normal
7850.3 .. Normal
7850.6 .. .6 & .7 are very similar, almost interchangeable
7850.7
7850.2
7850.8 .. Richer

Lotus knew what they were doing jetting the engines for performance, but they were struggling (like everyone back in the day) with tuning for emissions. The Idle Jets were usually about right from the factory, but the Idle Air Correctors tend to be one or two steps lean? especially into the early 1970?s when they were groping around trying to meet new, more strict emissions standards. Then, the correctors were typically emissions-lean and could benefit from being one or two steps richer.

[bast0n]

Dellortoes - Wassiss they............................?

[Esprit2]

(Snipping a bunch of attitude)... What I am struggling with is the basic adjustment point; I can find no learned opinion on the web which tells me where to set the adjustment nuts, apart from well-respected folks on here who suggest 3mm of exposed thread. I shall proceed on that basis.

You're looking for what isn't there. Dellorto made no attempt to calibrate the length of the Accelerator Pump's pull rod linkage, thickness of the nut, exposed thread, etc. Yes, all the bits have dimensions and tolerances, but the overall assembly is not calibrated.

The Accelerator Pump's discharge volume is set by adjusting the nut to give a specified discharge in 20 full strokes of the throttle. That's supposed to be done during assembly prior to installation on the manifold. That may not be what you want to hear, but it is what it is.

The factory specified way to measure that is with a graduated cylinder from a chemistry/medical supply, drugstore, JAE, etc. Yes, the carbs must be off the engine for that procedure, which I understand you wish to avoid.

The generic set-up for a new-in-the-box DHLA is 8cc in 20 strokes, but settings for specific engines may vary from that value as required. Lotus' spec for the standard Elan, for instance, is 7.5 cc.

If someone else has made the effort to perfectly adjust their pump's discharge volume, then measured the length of exposed threads at 3mm, transferring that dimension to your carbs won't necessarily give the same result. It might get you into the loose ball park, but it's unlikely to be "right" for your engine's needs.

If you're more motivated by convenience than correctness, then 3mm is a good number. Let us know how it works for you.

Regards,
Tim Engel
also a Mechanical Engineer

[elansprint71]

Tim,
Thanks for that; seeing as most carburettors are worked on by garagistes rather than lab technicians I can see where the "leave well-enough alone" stance of the written tomes is coming from. It seems a peculiarly difficult adjustment to make and, as the design and construction of the diaphragm has changed (the new ones I have are rather different from the ones I just took out) one that can be easily messed up.

Enough midnight oil has been burned for one session, I'll rejoin the battle later.

[Esprit2]

Thanks for that; seeing as most carburettors are worked on by garagistes rather than lab technicians I can see where the "leave well-enough alone" stance of the written tomes is coming from.(Snip)...

I don't think that Dellorto cared if an owner played with carb settings. The cc/ 20 strokes is not meant as a roadblock, it's just a more accurate, repeatable way of pre-setting the accelerator pump volume.

However, car manufacturers are obligated to make cars meet mandated emissions standards; and once certified, no manufacturer can facilitate or promote making changes to the standardized setting without repercussions. Hence the anti-tamper screws and caps that came in later carbs. Lotus has/had no choice other than to take an offical stance against modifications... it's the only politically correct thing they can do.

(Snip)... It seems a peculiarly difficult adjustment to make and, as the design and construction of the diaphragm has changed (the new ones I have are rather different from the ones I just took out) one that can be easily messed up.

As to the difficulty of the standard method for gauging the accelerator pump settings... yeah, it's less than convenient, but it's not locked-out and the prescribed method is more accurate than spinning a screw or nut. The carbs could have been R&R'd many times in the duration of this thread to date, so doing the task correctly is really not a big deal once you resign yourself to doing it.

Given a base line setting that's repeatable between multiple carbs, tuning will always still come down to trial and error equal adjustments on both/all carbs followed by evaluation on the road or a dyno.

Having found a setting that works, it would be helpful if the cc displacement in 20 strokes were reported here. I'm not generally a big fan of "It worked for me" solutions, but a statistically meaningful database of settings for different engine configs could be a useful tool. But short of a database that might develop here, I'm not aware of such "standard" grassroots settings... even as less than repeatable millimeters of exposed thread.

Best of luck with whichever way you go.

Regards,
Tim Engel

[elansprint71]

Tim,
Got it all together this afternoon, finally got rid of the other distraction in the early hours (bearing-scraping for a steam locomotive) so now I can concentrate on "simple" motive-power units!
The engine fired up as soon as fuel arrived, warmed it up to temp on fast tick-over and took it for a drive around the block. Ran like a bag of shit; sounds like the timing is miles out, the head gasket is blown and paraffin had been accidentally put in the tank! So; one thing at a time and we'll get there.

I think that a compression test might be the best use of my first ten minutes tomorrow, just in case the gasket has gone!

One thing I'm sure of is the settings on the accelerator pumps were wildly out, it was literally chucking a bucket of petrol in there on one of the carbs.

Throttle bodies, anyone?

[Esprit2]

This is an old thread, so I have no clue why I received an e-mail notice of message activity here today. But as long as I've logged in and drafted the following before realizing it's an old thread and I've been here before, I'll post the following redundant message anyway.

The Dellorto accelerator pump discharge is specified as so many cc of fuel dispensed in 20 full strokes. To test that, graduated cylinders (chem lab) are used to catch the fuel as you fully stroke the throttle 20 times. Adjust the pump linkage as required to achieve the specified discharge volume.

The linkage spring is compressed when you stroke the throttle, then the spring drives the pump. The linkage setting determines how much fuel is dispensed per stroke, and the pump jet size determines how long the full discharge can take. With a small jet, the stream may flow for some time. When counting 20 strokes, don't just stroke through it as fast as you can. Open the throttle hard against the stop, and hold it until the stream stops flowing. Then to back to the closed stop, and repeat.

If you measure the resulting linkage set-up, and transfer the setting to another carb, you'll be in the ball park, but not perfect. There's enough variation between carbs that it's still best to tune each carb for the specified discharge in 20 strokes.

Dellorto DHLA40 Accelerator Pump Delivery on 1558 cc Lotus-Ford Twin Cam:
7.5cc / 20 strokes ... Lotus Elan - Standard
8.0cc / 20 strokes ... Lotus Elan Sprint and Elan +2 / +2S / +2S-130
8.0cc / 20 strokes ... Lotus Europa Twin Cam & TC Special

Not related, but while I'm at it:
Clearance between coils of Thackeray Washers: ... 0.040" (1.016 mm)
Clearance each side of carb soft mount plate: ........ 0.030" (0.726 mm)

Regards,
Tim Engel
Lotus Owners Oftha North (LOON)