IDFs are very lean at Idle

[Scrapefest]

Hi I am just trying to set up a pair of 36 Weber IDFs on a fresh build 1904.

The problem I have is at Idle the AFR is in the 20's I have taken the carbs off several times , I have closed the air bypass screws, checked the throttle plates are a close fit, I can get a 0.05mm feeler gauge past the plates set so that the engine idles at 900rpm. I can't see any wear on the carb body. The choke mechanism was missing parts so I blocked off the lower outlet.

I have set up other twin IDFs and the idle is often around 12 afr , and as a quick test I put on a spare 44idf and the idle afr was straight into the 12s.

On a light cruise I was seeing the afr in the 17s but it felt like it had a flat spot .

Any one have any ideas why the idle is so lean.

The current jetting is
52 idles
135 main
180 air
30 Venturi

The engine is
1904cc
Heads AA 043
Engle 110 cam
Bosch SVDA Distributor
Vintage speed sport 130 exhaust

[volksworld]

I'd try 55 or 60 idle jets

[oprn]

What is the ARF at light throttle? Say 10% open still on the transfer ports? 45 - 50 mph steady cruise. Are they the good Italian carbs with 5 transfer ports?

Just some after thoughts, 36's might be a bit small for that engine but that should not effect the idle. Valve overlap might though.
_________________
Our cars get old, we get old but driving an old VW never gets old!

[modok]

How many turns open are the idle mixture screws?
They are fine thread so up to 6 turns out is ok

Idle jets should be 57-62 for 13-1 cruise

Or 50-52 if you want to run lean lean for max mpg

To cruise lean and idle rich you may need to enlarge the hole the needle goes in

[Slow 1200]

If it's idling smoothly at 20 AFR you must have a pretty hefty ignition system or the measurement is wrong

[oprn]

[Scrapefest]

OK, embarrassingly I think I lost my mind with this. I started with 55 idles road testing it was rich in the 11s, that's why I fitted 52 idles which gave high 12s afr even though it drove better with the 55s, that's why I spent time thinking it needed the 52 idles.
It didn't help that it is not my vehicle and the suspension is awful so road testing was difficult.
I put the 55 idles back in and the afr changed to 12 at Idle . I've not had chance to drive it yet.

The carbs are Italian IDFs so hopefully it was just my mistake rather than a problem with them as I have another pair on my Bay which drive very well.

Thanks for the help.

[modok]

Oh, that sounds workable.


I ran several sets of these, and sometimes one or two of the idle mixture screws ended up basically open all the way, and yet they ran great and it was the least of my problems, and I was used to that as a lot of the carbs made in that era were that way........you know... the pessimistic view was the carb engineers were trying to protect mother earth against morons with screwdrivers, but at the same time, some of the most accurate carburetors ever made, once you figure them out and pry the anti-tamper covers off the adjustments and make a few small alterations.


44idfs are just very different, they have enough range built in you can run methanol or whatever, which is good, but touchy adjustment, as in.... the difference between too rich and too lean idle could be just one turn of the screw.