Removing the Webers

[nmauduit]

I did try to twist the throttle shafts in order to balance the vacuum in the throats on the same carb.

if the engine was running ok prior to that I would check that carb closely, more harm than good is easily achieved when forcing on shafts (shaft bent out of true then butterfly not sealing identically, leaks at bearings ...) usually undoing and relocating the butterfly is what I would attempt on a poorly assembled carb, checking for fit with a light, and changing a shaft for a fresh one if damaged. A little difference of flow at full idle is not the end of the world if the butterflies are all centered and sealing well, what matters is that the flow should pick up simultaneously and homogeneously among all four as soon as you're getting off idle.
Also if you did indeed twist a shaft or butterfly, this may affect the synchronization of the opening of the transition holes, which would in turn alter richness in a single barrel and create all sorts of issues. Checking flow with a snail synchronizer and checking spark plugs may help locate the cylinder with an issue.

good luck !

[SENC]

Re: the load question - after rebuild, I was able to get a nice idle and smooth neutral revving in the driveway, but hesitation and popping under load in the 1800 to 2800 rpm range suggested a lean condition when more power was required to move the car. That it was ok revving in neutral gave me some level of confidence that I wasn't experiencing mounting air leaks as I think that would show whether or not under load - so just eliminated a variable for me. That neutral revving had also allowed me to check that distributor timing was advancing as expected.

Good luck, and hoping that the shafts aren't twisted so you can keep testing without having to stop for a carb rebuild.

[Fourducs]

I put a couple of flashlights in the carb throats and could see light in two of the three progression holes in each throat. When fully closed the innermost or manifold side hole was fully blocked by the butterfly in all four throats. The straightedge of the butterfly was just barely, barely noticeable on the middle progression hole. All four butterflies seemed to be synched, both within and between the carbs. I am not sure if this is where the butterflies are supposed to be when fully closed? I also realize I have much more to check.

[Craven]

If your carb’s are 31’s then that the way they are, progression is way too far advanced, never be right. All sorts of fudges tried but they are simply wrong for a Lotus 1558cc Twincam.

[Fourducs]

I read one article (which was not 31 specific) that stated the butterfly valve should block 1/2 of the first hole or less.
Is this correct?

[mbell]

Yes basically. None of the progression bolts should be open to the engine at idle.

[stugilmour]

If your carb’s are 31’s then that the way they are, progression is way too far advanced, never be right. All sorts of fudges tried but they are simply wrong for a Lotus 1558cc Twincam.

Fouducs, if the issue with the 31’s is the position of the progression holes, I think Keith Franck can help you out. No direct experience, but he has made up a small jig to allow drilling additional progression holes, which is apparently a common issue with the 151’s.

I don’t know details, but he has pictures on his forum of the various progression hole configuration, including your 31(if I am reading things correctly). If you just need to add a hole, it seems pretty easy. However, If you need to ‘move’ a hole, I believe he can make up new brass inserts with the holes placed where you need them. Not sure on this, but this is how he has conducted experiments with P hole placement. Might be worth an ask on the sidedraft forum. This may be a simpler way to correct your carbs rather than complete replacement.

My understanding is ideally the P holes shouldn’t need to be exposed at idle, the selected idle jet size and mixture screw setting should be able to keep things working at idle without ’cheating’ with the P hole. That said, if the 31 P holes are just wrong for your needs, I understand you will have difficulty tuning ‘around’ them. If I take Craven’s post correctly, this sounds like what is going on.

Just a thought from someone happy with Keith’s W idle jets and VP tubes. I am not an expert, and try not to play one on the interweb.

HTH