I’ve just re-fitted my Dellorto DHLA40, after a full strip down with new throttle shafts, have set up the mixture with my colourtune, set the ignition timing, and the carb to carb balance with my carbtune 4 column manometer.
I have a significant difference in the vacuum between the barrels in each carburettor. Barrel 1 is much bigger than barrel 2, and barrel 4 is much bigger than barrel 3 - so 1 and 4 are about the same, and 2 and 3 are about the same. The difference in vacuum between the barrels is about 2:1, so not slight.
Has anyone seen this before? I can try swapping the carbs over to see if the pattern changes but I don’t think it will, as I think the difference must be due to something in the cylinder head. If someone could help me diagnose this that would be great.
If you are worried that there is something wrong with the head (ie valves not sealing etc etc) then a good way to check would be to do a compression check - this should be carried out with the carb butterflies fully open to negate the effect of the plates. The engine should be warm ideally to do this. If the compressions are low and you see a pattern relative to your checks, then it’s likely the head and not carbs.
If no problem is evident on the compression test, it would suggest carb issues.
Check throttle spindle twist - make sure plates are opening the same amount at idle - check progression hole covers to see where the plates are in relation to the progression holes. They all need to be uncovering at the same point relative to throttle input. Then you need to see if all other screw adjustments are similar, or if not, are they compensating for a possible issue like bent throttle shaft etc.
Dellortos have balance screws that allow the airflow at idle to be balanced between the two barrels in the same carburettor (which I believe is your problem). The doc included above describes how to adjust the balance between two carburettors, which isn’t the same thing.
on my 1972 Lotus Elan Sprint fhc dhla40 i’ve never found an air balance.
The only ones i have seen on Big Valves with a balance have been replacement new Carbs ref Weber 151.
Maybe i need to have a close look i’ve never seen them in 23 years.
Alan
No, My 72 Sprint also does not have the bypass screws, that is because they are DHLA 40 and not the DHLA 40E.
Look closely at the end of the model number, some have letters …some are emission and some non emission, DHLA 40E are non emission with the bypass screws.
If you have fitted new throttle shafts, you may find that you have not centralised the throttle discs correctly. I have found this a really fiddly and difficult job to get correct.
My carbs are the non emission type with no letter after the dhla, and have blanked off holes where the idle bypass screws could have been.
I spent a long time centralising the butterfly plates, and am sure that’s not the issue that I have; there was no light coming through the edges around them when closed once I had them screwed in. The pattern I’m seeing is actually the same as before I took the carbs off, and one of the reasons I stripped them. The manometer is measuring the pressure on the engine side of the butterfly plate, and it’s the two outside cylinders (1 & 4) that are giving the higher vacuum. That suggests to me that the barrels are flowing less air or that the demand for air, for the same throttle opening across all barrels, is greater than the two centre cylinders (2 & 3).
If it’s the engine then I can think of the following reasons for it:
The exhaust manifold is not symmetrical, and the outside cylinders are venting more efficiently than the inside two - but at idle I’d be surprised.
the valve clearances may be less on the outside two cylinders, so have wider open duration. If it were the shims, might that explain the flat spot that I had on starting to accelerate when the engine is hot?
the camshafts are worn more on the inside cylinders
Low compression on the centre cylinders.
So, actually, the compression test may give me a quick indication of the answer: the rate at which the pressure builds should be different if there’s a difference between the cylinders, although that may be hidden by differences in maximum compression.
Am I being completely barking for thinking it might be the engine? If so I’ll swap the carbs and see whether that changes the pressure pattern.
Alternatively I can ignore the barrel imbalance and drive the bl**dy car, and see if it’s ok haha.
After a compression test, I think swapping the carburettors would be instructive. You mention that the carburettors have a casting that could be drilled for idle bypass, this might be worth considering.
I have just remembered something I did having reassemble my Webber Carburettors. I made a plug that connected my garage vacuum cleaner to the engine side of the carburettors. My reasoning was that the vacuum cleaner is a simple device that should be able to apply the same vacuum to each carburettor throat, thereby reducing my doubts that any variation in vacuum could be due to the engine.
Hi again. Thanks again for all your helpful replies.
I love the idea of using the vacuum cleaner - wish I had thought of that… I have an old Dyson in the garage… not sure I can resist giving it a try now haha.
My Dellortos don’t have the adjustment for air balance between barrels. Here’s a pic of one of them. I wonder if it might be possible to drill them, but my old twinc didn’t have them either, and that was perfectly balanced, even though I say so myself.
The pdf suggests that the vacuum measured on the manometer will change with the mixture setting. Ok, I haven’t checked that, so there’s hope - thanks for posting the pdf Donels. I set the mixtures with the colortune, and then set the balance between carbs with the carbtune 4 column manometer… could it be as simple as a tweak to the mixture screws? I think I’ll head out to the garage…
Hi again. I tried Adjusting the mixtures with the engine hot. It made no difference at all to the manometer reading. So thats a shame. I also blanked off the servo and headlight vacuum hoses. No difference. I swapped the hoses on the manometer around. The measurements moved too. Unfortunately I still have the hesitation on very light throttle from idle. Only when it’s hot enough for the fan to come on. So I will of course try it at that temperature with fan disconnected in case it’s the ignition, but I don’t think it is.
So, it’s out with the vacuum cleaner!
I did notice that the vacuums were even when I had the choke open. So that does give me some hope that it’s the carbs still…
At the risk of bringing up Occam’s razor, you changed the throttle spindles, refitted the butterflies, and now the carbs won’t balance between the barrels. The most likely answer is that there is something adrift with the way the spindles / butterflies went back together. I know this is an issue on Webers without balance screws, and can involve twisting the spindle to get the barrels to match.
Is it worth revisiting the fit of the spindles/ butterflies to see if this is the cause?
I do think it’s a problem with the carbs, but the pattern I’m seeing is the same as before I took them off. I don’t therefore think it likely it’s a problem with the way I put them together. But it might be. Next activity will be removal of the carbs to apply the vacuum cleaner, and swapping them to see if the imbalance moves with the carb.
