Lotus Elan

DHLA - hesitation

PostPost by: europatek » Sat Mar 23, 2013 8:34 am

My Elan runs great while warming up and needs no choke. Once at full operating temp the carbs hesitate. It's at low engine speed when opening and or closing the throttle. Particularly noticeable in second gear whilst negotiating side streets. Could it be the accelerator pump? Adjustment insitu looks impossible. Removing the airbox cover helps which indicates it's too rich when warm but...??? Any ideas?
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PostPost by: worzel » Sat Mar 23, 2013 10:52 am

Hi

Might be worth trying a simple test- when it starts to hesitate pull out the choke a bit- if problem worsens you're too rich- if problem reduces well you're too weak.

Good luck

John
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PostPost by: types26/36 » Sat Mar 23, 2013 11:29 am

With the carbs in place I think you are pretty much limited to cleaning out the pump jets and setting the float level, to adjust the pump delivery you really need the carbs off. I would try cleaning the pump jets/setting the float level first.
Here is a couple of threads on the carbs.
http://sideways-technologies.co.uk/foru ... uel-level/
elan-archive-f16/dellorto-pump-jets-t10494.html
fuel-system-carbs-f40/adjusting-the-squirt-dellortos-t13117.html
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PostPost by: billwill » Sat Mar 23, 2013 12:51 pm

May be a problem with the progression holes.

Get a copy of Des Hammill's book "How to build and power tune Weber and Dellorto DCOE, DCO/SP and DHLA carburettors."

He deals with just about everything, though not much on accellerator pumps.
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PostPost by: ricarbo » Sat Mar 23, 2013 4:10 pm

This is from the Dellorto Manual 35.1, if I've managed to copy it ok.
regards
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PostPost by: ricarbo » Sat Mar 23, 2013 4:51 pm

I should perhaps have added it's how Dellorto say to test and adjust the accelerator pump, but I didn't want to waste too much time typing when I was unsure if the attachment would work. What it doesn't say is what the amount should be. Others have said 7.5 cc, but I don't know if that is right or wrong, or if it depends on which version your engine is. I don't see the information in the workshop manual. Perhaps it would be good to set it up for the 7.5 cc and then carry on by altering and measuring again, so you can make a record of how much you need to alter the adjustment to achieve, say, 6, 7, 8, 9, 10 cc etc. That way, you could maybe have some confidence in the effect of future changes you might make, without having to remeasure.
As you can guess, I have never done this myself, so don't know how it would work out,
regards
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PostPost by: Esprit2 » Sat Mar 23, 2013 8:28 pm

Webers or Dellortos? The basics are the same, but the Webers are more dependent upon the accelerator pump than the Dellortos are.

EDIT: Oops, I should have read the subject line... Dellortos. Got it.

Does the hesitation occur when you make a gentle throttle movement off-idle? Gentle enough that the accelerator pump isn't much of a factor? If so, then the Idle Air Corrector is too lean. Go a step or two richer until the hesitation just goes away... no further.

Idle Air Corrector Jets ("Idle Jet Holders" in Dellortos) are not numbered sequentially, but totally randomly. Don't get caught making any assumptions about which one is the next size richer or leaner. Go by the following chart:

Weaker ...................................................... Richer
7850.5,__.10,__.9,__.4,__.1,__.3,__.6,__.7,__.2,__.8
.1 & .3 are "normal"
.6 & .7 are very similar, only a minor change between them.

Then make a full throttle run in an upper gear, like 3rd. Does the engine pull well, but stumble at around 4000 rpm (Weber) or 3200 rpm (Dellorto)? If so, the Idle Jet is too small. Go up a step or two until the stumble just disappears, but no further.

The Idle Jet and Idle Air Corrector affect one another, so if any change is made to the Idle Jet, then go back and repeat the check on the Idle Air Corrector. Repeat back and forth until subsequent trials provoke no further change.

Finally, only after the Idle Jet and Idle Air Corrector are properly set, evaluate the Accelerator Pump and Pump Jet. At low speed, if you quickly open the throttle, does the engine fall flat on it's face right away, or does it get off to a good start, and then fall flat on it's face?

If it gets off to a good start, then falters, the Pump Jet is about right (flow rate is good), but the pump stroke (total shot volume capacity) is too short and the shot ends too quickly. Increase the pump stroke so the shot lasts longer.

If the engine falls flat right away, then the Pump Jet is too small (flow rate is insufficient), so increase the jet size.

As with the Idle circuit, the Pump Jet size and Pump Stroke impact each other to some degree, so any time you change one, re-evaluate the other. Go back and forth until the last change in one provokes no further change to the other.

In both cases, Idle Circuit and Accelerator Pump, more is not automatically better. Go richer/ or add more gas only to the extent required to eliminate the hesitation or stumble. Excess fuel causes an entirely different set of problems without making the initial problem better. Use just enough, no more.

Have fun,
Tim Engel
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Last edited by Esprit2 on Sat Mar 23, 2013 9:44 pm, edited 1 time in total.
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PostPost by: Esprit2 » Sat Mar 23, 2013 9:38 pm

ricarbo wrote:(Snip)... What it doesn't say is what the amount should be. Others have said 7.5 cc, but I don't know if that is right or wrong, or if it depends on which version your engine is. I don't see the information in the workshop manual. Perhaps it would be good to set it up for the 7.5 cc and then carry on by altering and measuring again, so you can make a record of how much you need to alter the adjustment to achieve, say, 6, 7, 8, 9, 10 cc etc. (Snip)...
The spec I have for an Elan with a Standard Twin Cam on Dellortos is 7.5cc/ 20 strokes. Similarly, the spec for the Europa Twin Cam with a Big Valve engine on Dellortos is 8.0cc/ 20 strokes.

Both of those values are pretty generic for out of the box Dellortos. If you take the time to measure and set the fuel delivery by the book, the odds are that you'll still end up having to tune it to suit your engine. Carbs are not as precise and repeatable as modern electronic fuel injection systems, and a certain amount of tuning is a normal part of set-up.

Regards,
Tim Engel
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PostPost by: europatek » Sun Mar 24, 2013 10:57 am

Thanks for the feedback gents. I had a quick look at the idle jet/corrector. It is the standard fitment of 7850.2.
The hesitation - if you crack the throttle open quickly then no hesitation. Open it more gently and it's like va, voom. Perhaps its better described as a stumble.
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PostPost by: europatek » Sat Apr 06, 2013 11:14 pm

OK. I spent some time this week messing with the carbs. I haven't had a chance to take them off and put kits through them as most of my garage time has been spent working on the twink in the Cortina.
So, I took the tops off and checked the float level. Elan shop manual states 14.5-15mm whilst lid held vertical. Mine were at about 11. So, adjusting what I determined was the correct needle valve tab (the fork around the valve I assume - shop manual not very clear) I moved my floats down to 14mm. Reassembled and went for a drive. As usual pretty good when warming up but once up to temp the horrible stumble begins. So, basically resetting the float made no difference.
The carbs perform perfectly in every other driving situation. No hesitation, no stumbling etc...

To better describe the stumble;
The engine has a horrible stumble when opening and closing the throttle. If you open the throttle really slowly or really quickly you can avoid the stumble.
Opening the throttle with anything like a normal speed the engine stumbles. It's horrible and makes driving the car slowly around side streets unpleasant. You have to constantly try and drive around it and it's tiring.

It's been suggested to me that the diaphragms could have gone hard thus affecting the pump jet system. Does anyone concur?

So, I think the best way forward now is to remove and rebuild the carbs with the kits I have. I'll also have the pump jet calibration setup properly, something the Elan Factory in Melbourne can do for me. Then refit and see how it performs. I am very reluctant to start buying jets and changing things without first rebuilding the carbs to start with a good foundation.
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PostPost by: UAB807F » Sun Apr 07, 2013 5:45 am

I think your logic is sound, before swopping out any jet sizes you should make sure the carbs are operating correctly. If the car has been sitting for any length of time and fuel vaporized from the carbs then my first thoughts would be a complete strip down & clean to make sure all the jets are clear.

Another vote here for Des Hammil's book. I bought it last year when I started work on the Europa's carbs and found it the best reference I've seen so far. The revised edition has lots of colour pictures, and I'm the sort of guy who likes pictures :)

I had a split diaphragm on one carb, but that just let fuel leak out and it was fairly obvious where the problem lay.

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PostPost by: types26/36 » Sun Apr 07, 2013 9:14 am

While I am not saying the carbs are not the problem is everything OK with the electrical system, could you be getting a high resistance in the ignition system, a plug lead,suppressor, coil,dist cap or connection or even a plug breaking down when hot?
A high resistance often occurs as heat builds up yet all seems fine when cold.
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PostPost by: bobm3142 » Sun Apr 07, 2013 1:55 pm

I agree (for what my opinion is worth) with Brian (types 26/36) that you should start with the spark. I recon that well over half the fuel problems I have been asked to fix (on 1960's/70's cars) have been electrical, which is easier and often cheaper to fix, and has to be done anyway (teaching you to suck eggs - make sure that it's a good spark all round, that the points gap [if you have points] is correct and that the timing is correct and advances correctly. I have Des Hammill's book (its great, affordable and available) I also have one on Weber's by John Passini (even cheaper). In my opinion, John's is a little on the flamboyant side but does describe the engineering behind carburettors, including issuses such as spark and valve timings. I'm always a little dissapointed when I read about getting the correct mixture (and associated aspects) without reference to the state of advance; after all the two are intrinsically linked.

The subject of carburettors is complicated and these two books are excellent at describing it and solving it. However, I have the following question for those out there that really know:-

Des Hammill's book states that the float setting for weber carb's is 7.5/15mm where as I have always believed it to be 8.5/15mm for the 40DCOE 18 & 31 (as fitted to the Elan).

Bob
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PostPost by: billwill » Sun Apr 07, 2013 6:53 pm

When cleaning your carbs, get an air blast which you can use to blow through any of the passages. You can buy it in a 'spray' can if you have no other method such as a compressor.

I'm still betting that some of your progression holes are blocked. See Des Hammill's book.
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PostPost by: terryp » Mon Apr 08, 2013 6:35 am

Its funny I had exactly the same problem on both of my +2's with Dellortos.
The way I cured it was to get horizontal outlet pump jets of the reduced size (mentioned in the manual for drivability) and also to ensure the car ran as cool as possible.
I was running mine with a 71 degree thermostat with the new type Kenlowe fan coming on at 78 degrees. make sure all foam and side and bottom closures around the radiator are intact

Good luck

Terry
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