Dellorto DHLA problem!

Elan Discussions
1

Help! Sorry, this is going to be long winded, but I think I need to explain properly. In my type 26 Elan I have a recently rebuilt Sprint big valve spec engine, which has been running on the car’s original DCOE 18 carbs. With the Webers the cars has never run sweetly on light throttle openings, not the dreaded off idle stumble, just uneven running. Having tried everything I could think of, I bought a pair or reproduction Dellorto DHLAs (made by FAJS) off Eurocarb who jetted them correctly to Lotus Twin Cam spec for me. I’ve had DHLAs on a fairly highly tuned Ford Crossflow plus my 1974 Alfa and I’ve found them to be more accurately tuneable than the DCOE18s and give better running on part throttle. These FAJS reproductions seem quite well made and use standard Dellorto components. They are repros of the pre-emission 3 progression hole DHLAs.

I’ve now fitted the DHLAs but I’ve hit a problem which I have been unable to solve. At idle number 2 cylinder does not fire at all. I’ve put a Colortune Plug in it and, although one can see a healthy spark, there is no burn at all. Open the throttle a bit and you get a very erratic yellow burn. At higher revs there is sporadic missing and plenty of pops and bangs.

I’ve balanced the carbs with a 4 way manometer (and checked it with a Synchronometer). No 1 & 2 cylinders required no air bleed to balance them, 3 and 4 did a little.

I’ve checked lots of things to no avail, No 2 still not firing at all at idle:

Changed spark plug

Clear strong spark on the Colourtune

Oscilloscope on No 2 spark plug lead at idle shows strong initial spark and longer burn period than the firing cylinders.

I’ve swopped jets and their holders between cylinders 1 and 2 and also the idle mixture screws likewise .

I’ve compression tested no 2 cylinder, over 200 psi.

I’ve verified that the channel between the progression hole inspection well and the mixture screw well is clear and also the hole from the mixture screw well into the inlet tract.

After the engine has run, there is evidence of petrol in the progression hole inspection well, just as there is on No 1 cylinder.

At the best idle I can achieve, the butterflies are just over half way across the first progression hole.

I’ve checked float levels.

Another strange thing is that at the best idle I can get (on the 3 cylinders that work) with a nice blue flame on Colourtune is with the mixture screws out 6 or 7 turns, which is beyond the range given by Des Hammill in his book, suggesting that the current original Lotus spec 50 idle jets are too small, but when I swopped them for some 58s I happened to have, it still required 6 or 7 turns to achieve a decent idle!

Any thoughts anybody I’ve just run out of ideas?

Yours in hope and anticipation.

Vernon

2

I’m not an expert on Dellortos but my only suggestion would be to swap the front and back carbs around and see if the problem moves to #4. You can then definitively eliminate any electrical issues.

Good luck with it, please report back.

3

Vernon
As I understand it, after all the changes and testing, your engine is still not firing on No 2 cylinder is that correct? You say that the spark plug lead seems to be okay and the plug has been changed. Are yiu able to do a compression check, to see if there is a ring or valve issue?

With regard to your Colourtune readings, the blue flame is indicating a weak mixture and you need to perhaps reset so that the flame is just turning from blue to orange as some engines don’t like a weak mixture, this may in turn bring your mixture screws may not be wound out quite so much.
Try, then let us know the results, where are you in Devon?
Tony

4

Hi Richard
Thanks for your idea. Swopping the carbs is a bit of a task as it means changing over the throttle levers which are a bit of a pain to get set right. But what I can do easily to get the same answer is try squirting Easy Start down No 2 intake and if it then fires I know it is not ignition, so thanks, your thought has given me another diagnostic idea.
Regards Vernon

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Hi Tony. Thanks to you also. The main problem I have is No2 completely failing to fire at idle speeds. I really have to solve that first. I have done a compression check and it is fine. There is clearly a spark in no2, but just no burn at all unless you raise the speed onto the progression phase, and then there is sporadic firing.
Re the Colortune colour, I can get an orange flame on the cylinders that do fire by turning the mixture screw even father out than the 6 to 7 turns it takes for best idle, but the idle is then worse. But, as I say, I need to sort no 2 cylinder before anything else.
Thanks Tony, I’m in Teignmouth, are you also Devon based?
Regards
Vernon

6

Just a thought. I had a very similar problem years ago with an S3 S/E. Couldn’t get it to run correctly, tried everything, including idle jet changes, float heights, strip & rebuild of carbs (mine were Weber DCOE 18’s too ). I was completely perplexed, having had Twincams in lots of my previous Elans and an Escort TC. One day a light switched on in my head, and I thought about the one-way vacuum valve for the headlights, that fits into the front of the inlet manifold. I disconnected the vacuum pipe from the one-way fitting, and connected a tube that I could blow & suck through.
Result ? It wasn’t "one-waying " any more…
Fitted a new Norgren one way valve, and hey presto ! Everything back to normal and running like a sewing machine.
I’m not saying that this is your problem, but it’s worth checking. I’d been at it for weeks.

7

Hello Richard
Do you know if you had a similar problem with No 2 cylinder with the Webers fitted.?
Its beginning to sound more like a fuel issue rather than ignition, valves or mechanical issues. However, there is a common denominator in that the problem may have existed to some degree with the Webers. This makes me wonder about valve clearances and whether those on No2 are correct., but you say the compression check did not show anything different between cylinders.

Do you have any Easy Start, if so try this, start the engine and let it tick over as best you can, then try squirting some Easy Start into the throat of the No2 carb whilst observing what is happening at the Colourtune. If you get a positive result that would indicate an issue with the carb on No 2. This may be an easier route than trying to swap carbs across to prove whether the burn moves to No 4 cylinder.
Tony

8

Thanks Tony. The car did not run very well on part throttle with the Webers which were on it before, but it did idle well, with all 4 cylinders firing. I last checked valve clearances only about 1000miles ago, and I have compression tested and pressure is good. I’ve just tried Easy Start and No 2 does fire then, I’ve also tried the choke enrichment, and that also makes it fire. So I am confident it is a carb problem.
Thanks also Elans3. I’ve also had a problem with the vacuum take off for the headlights in the past but this is No 2 cylinder, and the vacuum take of is on No 1, so it can’t be that this time.

Thanks to everybody, but I think the Dellortos will have to come off and go back to Eurocarb to be properly tested, which is a shame - I hate failing to track down the cause of a problem.

Regards
Vernon

9

Hello Richard
Sounds as though you have at last found the source of the problem. Pity you need to send them back, but hopefully Eurocarb can sort them out.
By the way, I forgot to say previously that I am just west of the Tamar, so not too far away.
Tony