Dellorto barrel imbalance

[alaric]

Hi Alan.

I'll look into that. It sounds like the sort of thing I'm looking for. I don't think I need to disturb the butterfly valve to get them out.

Regards.

Sean.

[Donels]

I have just removed my intake trumpets and found that the mating face on all 4 is bowed such that when bolted up there is a gap between the trumpet and carb inlet face. I have polished all 4 flat using a flat surface and sand paper.

I'm not sure that it would cause the problem you're seeing as it's upstream of the butterfly, but if it's sucking air through this gap it will be disturbing the flow down the trumpet. It's easy to check and correct before removing the carbs.

[alaric]

Thanks Donels.

There's no gasket between trumpets and air box on my car, but I think its all flat. I'll check though. The leak is there with or without air box and trumpets.

Sean.

[Craven]

Not at all familiar with Dellorto choke arrangement but on Weber non- seating cold start pistons would leak air directly downstream of butterflies. I see there is a choke operating cam have you looked for it’s correct position.

[alaric]

You are correct re the seating of the cold start valves. I took one of them apart so that the top cover could be repaired, but the other has not been disturbed. The seals on both are original. The air is split between both barrels, but it may be that there's a break in the seal on the side of the seal that goes to the offending barrel; there's a dividing wall that leaves an indent in the seal. Good news is I can test that by swapping the top covers. Great. I have a theory, and will test it and report back.

Thanks for the suggestion.

Sean.

[alaric]

Hi again. I swapped the top covers over. I couldn't see anything amiss with the seals. Sadly the barrel balance didn't change. So at least that counts that out.

I think next is polishing the flanges and maybe swapping round the o ring plates to see if the pattern moves.

Regards.

Sean.

[Donels]

Sean,
Going back to the accelerator pump. I went back and checked the volume being pumped and found that the 5 mm stroke setting was only giving about 7 cc per 20 strokes. To get 8 cc I had to increase the stroke to 6.5 mm on both carbs.

[alaric]

Hi. Thanks for letting me know. I've not progressed much with the carbs since the last post. I've been fitting a modern dab radio CD player instead. It's looking very neat, but I need to get the dab arial fed through to the back of the radio now, so that will be interesting.

Sean.

[alaric]

Hi.

I want to replace the lead plugs that are in the face on the dellorto that mates with the inlet manifold. From YouTube it appears this is done by drilling out the old plugs and hammering a 5mm diameter lead ball into the hole with a flat faced punch. Looks easy on YouTube haha.

I've struggled to find lead balls. Does anyone know of a supplier?

Thanks.

Sean.

[types26/36]

Sean, I’ve heard of .22 air rifle pellets being used and .22 equates to 5.5mm.
I have a few and could send you a couple ......how many do you need? pm me address if you want to try them.
Failing that have you tried fishing tackle shops? they used to supply small lead weights but not sure if lead is still used?

[alaric]

Brian, thanks for the reply. I do have some fishing tackle weights as it happens. I think the idea is to have a solid lump of lead with no joints in it so it seals; probably a short cylinder of lead of just larger diameter than the gallery with flat ends so it can be drifted into the hole. I've found a furnace for about £20, but not a cheap ball mould. I can get one for about £75. I have ordered a bag of shot too.... that might be easier.

I've just found another YouTube video where the chap just flattens lead shot slightly before hammering it into the hole. It looks really easy, but no doubt it'll take a bit of practice haha. Here's the vid:

https://www.youtube.com/watch?v=jEvmk8VziPQ

Here's a photo of the mounting face of one of my dellortos.

Hopefully you can see my reason for wanting to replace the plugs. There are two in each face, both of which may be leading to leaks past the O rings, and also which may be leaking in their own right back into the gallery. One of them is for the mixture setting, so would explain the issues I'm having with one barrel have a leak and not responding to mixture adjustment. An easy fix if I just need to hammer in a slug of lead.

All the best.

Sean.

[Donels]

Why not try a little sealant first. It’s much lower risk.

[Craven]

Don’t really want to butt in but the holes are not a plain hole, two diameters large on the outside with a step down to a smaller one a couple of mm in. So the seal is against the change in holes size. Almost any shape of pellet will do, I just melt into a rounded blob a small piece of lead.

[alaric]

Hi Donels. Yes good idea I have a bunch of sealants to try too. That was my original plan and I may try that first, but I thought I'd look in to fixing it properly too.

Hi Craven. Thanks for letting me know - I didn't know that there's a step for a lead plug. The plugs next to the mixture screw are quite recessed on my carbs and look brittle now so I'm hopeful that's the source of the leak. If I can find some lead in the garage I'll have a go at melting it into a blob. I did have some old roof flashing but am not sure where that is now - I remember thinking that might be useful. Actually, I have a ladder, and a roof... haha.

Sean.

[alaric]

Hi all.

I've put a new set of carbs on the car. Unbelievably I have found the same barrel imbalance as at the start of this thread. I'm really lost with this now. The new ones have the air bleed screws to balance the barrels in one carb. I adjusted them and got all barrels the same. Tickover was smooth and I was happy - I even got video. As I was videoing the manometer it was as though one of the bleed screws suddenly blocked and the imbalance came back on one barrel. I couldn't adjust that barrel again after that.

So, with two sets of carbs that both look in perfect order, I have to conclude it can't be the carbs. I don't think it's the engine as the problem moved with the carbs when I swapped the originals over.

I hope to see some of you at the lotus track day. My car, unfortunately, will remain sitting in shame in the garage.

Regards.

Sean.