Dellorto barrel imbalance

[Donels]

I assume you have this guide?

Carb Tuning 2.pdf

(641.18 KiB) Downloaded 2404 times

[alaric]

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...

[alaric]

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..

Gotta love these cars eh!

Sean.

[Andy8421]

Sean,

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?

Good luck.

[alaric]

Hi Andy.

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.

Regards.

Sean.

[Donels]

Are you sure you dont have the Idle Air Bypass Screws (Used to balance air flow through carb barrels)?

These are pretty fundamental to setting the carbs up correctly. They may be there but have tamperproof screws or similar fitted. Why would they make carbs without them or why would someone blank them off? Check one of the carbs from the inside and see if you can see the air bypass holes. At least it will confirm you do/do not have them.

You say that the butterflies close correctly and that the manometer reading are ok when open, which sort of eliminates the butterflies. As you gradually open or close the butterflies there must be a point at which they become unsynchronised, where is that point?

You say you have hesitation on light throttle which point to idle settings. I suspect your problems are the air bypass settings.

[alaric]

Thanks again for all the support on this.

I believe, from other posts, and those in this thread, that the barrel balancing screws were only fitted to the DHLA40E. My carbs are the DHLA40, with no letter after the 40.

When I look down the holes where the air bleed valves would be fitted, they are empty. I can't see any blanking plugs. I'll go and have a little play but I think those holes are sealed / not drilled.

I do agree that the hesitation is a jetting problem, and that should be sorted out on a rolling road.

I want to swap the carbs over to eliminate the engine as a candidate.

Anyone have a spare set of carbs I could try? I'll give em back, honest - unless you want to sell them.

All the best.

Sean.

[types26/36]

Don't waste your time looking for the air bleeds, they are not fitted on DHLA 40 carbs and I doubt it would be possible to modify the carbs to DHLA 40E spec. which do have the bypass air bleeds.
I think some of the other Dellorto carbs (DHLA40 F-G-H-N-R-S) do have the air bleeds sealed/blanked off BUT these are EMISSION carbs and not really suitably for Twin cams.

[alaric]

Thanks Brian.

I am a little worried that I might have a leak from somewhere in the carb barrels, but don't plan on drilling to fit adjusters.

I just did a compression test and scared myself by doing it with the throttle closed - when I repeated it with the throttle fully open - after checking threads on this forum - I got 200 psi on each cylinder. So that's a relief. The throttle closed values are higher psi on the two cylinders that have the highest vacuum with the manometer. So at least that's consistent. But I'm still not sure if it's the carb or valve clearance - I can't believe it's the clearance, but before pulling the carbs apart again I want to be sure so will swap them over and see if the vacuum pattern goes with the carb. Just having a cuppa, then will be back out to the garage....

Incidentally I just let the engine warm up, and it's ticking over really nicely now with no flat spot on opening the throttle, until it hits 85C. At that temperature and above very light or slow opening of the throttle has a hesitation. Rapid opening does not. So from that I think the mixture isn't right on transition from idle. I guess the colour tune might give an indication of whether it goes lean or rich on light opening.

I just checked with the colourtune. I see a hesitation corresponds to blue flame, but when I open the throttle quickly the yellow flame, presumably due to the fuel from the pump, dominates. At least that's what it looks like.

Regards.

Sean.

[promotor]

Thanks Brian.

I am a little worried that I might have a leak from somewhere in the carb barrels, but don't plan on drilling to fit adjusters.

I just did a compression test and scared myself by doing it with the throttle closed - when I repeated it with the throttle fully open - after checking threads on this forum - I got 200 psi on each cylinder. So that's a relief. The throttle closed values are higher psi on the two cylinders that have the highest vacuum with the manometer. So at least that's consistent. But I'm still not sure if it's the carb or valve clearance - I can't believe it's the clearance, but before pulling the carbs apart again I want to be sure so will swap them over and see if the vacuum pattern goes with the carb. Just having a cuppa, then will be back out to the garage....

Incidentally I just let the engine warm up, and it's ticking over really nicely now with no flat spot on opening the throttle, until it hits 85C. At that temperature and above very light or slow opening of the throttle has a hesitation. Rapid opening does not. So from that I think the mixture isn't right on transition from idle. I guess the colour tune might give an indication of whether it goes lean or rich on light opening.

I just checked with the colourtune. I see a hesitation corresponds to blue flame, but when I open the throttle quickly the yellow flame, presumably due to the fuel from the pump, dominates. At least that's what it looks like.

Regards.

Sean.

Have you confirmed that the throttle discs are uncovering the progression holes at precisely the same time as each other? To make it easier, it's better to use a magnifying glass or a good zoom on a phone camera etc.

[alaric]

Hi. I did remove all the throttle valves again last night, and was very careful to ensure the progression holes are centred on the cutouts and that all are sealing properly I.e. no light through when fully closed and no binding.

I'm heading out this evening to fit the carbs in their swapped round position.

Sean.

[Donels]

Just a thought. Have you checked the face of the intake side of the head and mating face of the carbs to ensure there are no scores, etc that could allow air leaks past the O rings at high vacuum.

[alaric]

Hi. That's a good thought. I have checked and cleaned, and am using new o rings. So I think they are sealing ok. But, the edge of the inlet port of cylinder 3 is not flush with the spacer over about 30 degrees of the bore. I don't think that woukd affect it at idle, although I'd rather it was flush. Apart from that, it all looks good to me.

I've just run the car with the carbs swapped, and after refitting the butterfly valves, and now the balance is a lot better - barrels 2, 3 and 4 almost exactly the same, barrel 1 about 2 cmHg lower, which I think is in tolerance. I'm actually quite impressed by how easily the engine started, and how smoothly it is running. Of course the truth will come out when I let it warm up, but for now I have a G&T with my name on it . I pushed on the offending barrel and saw it's level come up, so I suspect it's scratches on the surface of the flange, or that I've been over cautious and not tightened up the thackeries enough.

All the best.

Sean.

[alaric]

Hi again.

Further to my post above, I just can't get the carb barrel on cylinder 1 to balance. It's clearly got a leak. The leak moved with the carburettor, so it's not the engine - that's actually really good. So it must be the interface onto the manifold or something in the carb. While watching the 4 column manometer I've squirted wd40 around the seals, then thick oil, and pulled up pressed down on the carbs, then tightened the nuts up squashing the Thackery coils together. I also put oil on the throttle shaft where it penetrates the carb. I saw no change in the distribution of vacuum across the manometer columns with any test. So the carbs will be coming off again. Otherwise the car is going really nicely haha.

All the best.

Sean.

[alan.barker]

Do you think the Trumpets and Venturis(choke tubes) are sealing.
On Webers in the Workshop Manuel they say to seal with "Hylomar".
Alan