Reading up on the jet transition for Weber ICT’s it appears that most of the idle and cruise throttle opening is through the idle circuit. They perform well on my 2.0 type 4. The question is however is why the air fuel ratio needs to be on the rich side to get a steady idle. Anything weaker than 12.5:1 and the idle becomes very erratic. I have got a balance hose to even up manifold pressures so I don’t get why it needs the richer mixture. The ICT’s are advertised as lean best idle but this doesn’t seem to work for me. Is it because of the firing order pulling on the same carb in succession? Cruise a/f is fine round about 14:1 with WOT at about 11:1. l know it’s not really a big issue but would like to know thoughts on why.
Firstly ignore the afr at idle, or rather - don't tune to it. Set best idle for best running. You are finding what everyone does - a/c flat four idle a bit rich - it's of no matter. This is (almost) totally unconnected to running afr and totally unconnected to idle jet size. Your cruise is a bit lean for a bus if we're talking over 35-40mph....but...what do you call cruising? If you are faster than 30-35mph you are on the mains. Your WOT is too rich. What's your jetting to get those numbers?
55 idles 150 mains & 170 airs. F6 emulsion. I think the WOT a/f is actually nearer 12 than 11. Cruise speed i am talking about 50-55 mph. As you have said previously Zedders you can get carried away with jetting when you fit an a/f gauge . I am going to revert back to 155 mains 175 airs which it has been running fine on all year. As you say rich idle setting is no big deal. I was just wondering why this was the case to get a smooth idle. I would be interested to know what the a/f values are on a standard set up with the pdsit twin Solex carbs.
So would I! OK, 55mph "constant speed" is not the same as the "cruising" that is associated with idle jets. Perhaps idle jets would be better named as very low throttle cruising jets. I've concluded personally that lean cruise jetting a bus is virually pointless due to the tiny amount of running on the flat at 30mph or below with just 2-3mm of throttle. More throttle than that you're on the mains. Ignore the idle afr, you could set idle exactly the same with ANY size idle jet. Idle jet size only effects driving. Perhaps go for a very sedate drive under 30mph and watch the afr at say 20- 25mph with tiny throttle. Best to back the pumps right off if in doubt at what you see. I can't over emphasise the tiny, tiny throttle opening for this. If it doesn't pop and fart your idles are big enough for THAT purpose. That doesn't mean it wouldn't drive better with 57s though which might also help your lean higher constant speed afr. So, onto the mains. Your problem is too rich WOT combined with too lean constant speed. So many things can cause that from float height to emulsions to air jet size. I'd start by double checking your float heights are correct. WOT 11 afr. All through? Lean then rich at high revs? Rich from pump followed by lean then riching up with revs? It's not uncommon to go rich right at the top of the rev range which I wouldn't worry about as (if like me) you never actually do that apart from this testing.
Some great getting info on here if you haven't read it already. https://www.wayoutwestie.com/aircooled-tech/running-weber-icts-on-a-vw-aircooled-engine/ Sent from my Pixel 5 using Tapatalk
That uneven idle from two single barrel carburettors caused by the firing order is probably why stock duals have a third idle circuit carburettor feeding all four cylinders through a loong tube . . Its quite fun googling around "CFD carburettor simulation" (Computational Fluid Dynamics) , I found there is a university in India where one of the favourite subjects seems to be carburettors - this paper I found is quite good as the candidate gives a good explanation of how it works before presenting pretty pictures ... http://ethesis.nitrkl.ac.in/2296/1/final_report.pdf
