Possible carb problem? Possibly not…

[pabbage]

Hello … long post coming up but any help in the right direction would be welcome as I’m a little at a loss.

Car is a 1972 sprint with dellorto carbs. It had an engine rebuild before I took ownership with a unknown cam upgrade. Carbs have been rebuilt and it uses an electronic ignition.

It starts and runs well… bar the odd backfire (more so on lifting off the throttle). However, pulling the leads on 4,3,2 gives a noticeable drop in engine running/ note but not on 1. If you pull the lead on 1 nothing changes. I’ve changed the ht leads and the plugs but still no change. Plugs look unremarkable except 4 which is black (soot not oil).

Compression test was completed and looked ok.

Carbs were balanced - progression holes checked and appear in line - with colour tune and a carb synchronizer. Interestingly, the 2 carbs balanced well but when comparing the first carbs air flow, cylinder 1 was half that of cylinder 2.

Is there any link to why cylinder 1 would behave like this? In terms of removing the ht lead and also the reduced airflow on that first carb? Could there be an air leak?

As I mentioned the car appears to be running quite well and I stumbled across this issue rather than thinking there’s a problem I need to find.


Car goes in for its mot this week so I can pick this up with a trusted lotus specialist but it would be better if I can make some headway or develop my understanding further before I hand it over!

As always - your thoughts and interpretations welcome! Thanks.

[mark030358]

Blocked idle jet probably, or rather the passage.

Cheers

[pabbage]

Thanks Mark - I’ll take a look.

[mbell]

I believe dhla have air balance screws, so might need to balance the flow across the carb for 1&2 cylinders not just between the two carbs.

[pabbage]

I believe dhla have air balance screws, so might need to balance the flow across the carb for 1&2 cylinders not just between the two carbs.

I thought that but for the life of me I can’t see any!

[elans3]

If you have an imbalance between two barrels of the front carb, then you probably have a slightly twisted throttle shaft . This is easy to remedy on the later Weber 151's as they have additional airscrews where you can even up the "bad" barrel by letting more air in / out with that screw.
Unfortunately with in period Weber DCOE's, and Dellorto's, it isn't possible, because neither have this facility.You have to twist the shaft back into position so that the butterflies are opening exactly together.
It is possible if you're used to spannering. Really needs the carb removing to do it properly though.

The other possibility is the non-return valve that screws into the front of the inlet manifold, (adjacent to number 1 cylinder), that creates the vacuum for the headlights to move.
I had this problem on a previous Elan, and it took ages to fathom.
Turned out that the non-return valve was goofed, and I could blow though it both ways ! Replaced it with a Norgren valve and all was good again. My car had the similar symptoms.

[SENC]

it's worth doing a leakdown test too

[miked]

From the factory some of the butterflies are not sat right in their orifice. I recently went right through a set of Dellortos changing the shaft bearing etc. Upon building back up and looking up through the barrels I could not seat one buttefly with its screws in. By removing the offending screw it seated and dropped in nicely. The old shaft withess marks on the butterfly now taking up a new position.
So.... the things had never been balanced. Remedy was to remove the butterfly and file a tiny bit out of the hole area fouling on the screw. A small fettle!
After doing this I found said remedy in Des Hamill book. DOH.
It is not the first time i have seen this

Mike

[mbell]

I thought that but for the life of me I can’t see any!

The first photo might help:
viewtopic.php?f=40&t=17456&start=

Mike's one way valve suggestion is also a good suggestion and probably worth checking before messing with any carb settings.

[George S L]

This might help

[pabbage]

Thanks for the replies - really useful stuff. I’ll hopefully get to have a look mid week.

[alaric]

Hi. I'm just rebuilding my dellortos now and have a similar problem with one of the butterflies. In my case the offending one is difficult to fit through the bore. I have improved it slightly by polishing the edges of the valve plate, but it just seems slightly oversize. The other three plates shut completely, this one will only shut so far and sticks if I push it closed. So I was wondering if it's just tolerance on the valve plates. I have tried it in other barrels and the problem does move with the offending valve. I've ordered a new one in the hope that fits.

Sean.

[alaric]

Hi again. By way of a small update, I used a bit of wet and dry to take more off the sides of the butterfly plate and now it pops into the bore as easily as the others, and it shuts fully without sticking. I'll still fit the new one when it arrives though.

Regards.

Sean.

[pabbage]

Hi again. By way of a small update, I used a bit of wet and dry to take more off the sides of the butterfly plate and now it pops into the bore as easily as the others, and it shuts fully without sticking. I'll still fit the new one when it arrives though.

Regards.

Sean.

Great stuff! I’ll check this too… looks like I’ll not get away without taking the carbs off.

[2cams70]

The only way you can twist throttle shafts is by abuse. Either during assembly/disassembly or incorrect adjustment of the throttle cable where the butterflies are fully open well before the accelerator pedal reaches the end of it’s stroke. As for filing the edge of the butterfly that should be a big no-no. The angle of the edge is a calibrated item and the correct way of setting the plates is to loosen the screws and settle the plates in the correct position whilst they are gently closed