2276cc 48ida idle jetting issues

[Buggin81]

I have a street bug (weekend warrior) with a 2276cc, 48ida's, (40mm vents), FK89 Cam, S/E Heads, 1 3/4 sidewinder exhaust. Current jetting now is: F2 tubes, 185 mains, 210 air 60/120 idles. Carbs currently have two progressions holes (Italian).

So, the issue i'm having is around 2,000-3,000rpm (half throttle) around town it stumbles through the rpm range in 2nd and in 3rd till after 3,000rpms or so then it goes away. If I get into the throttle from a dead stop heavily then there's no stumbling issues whatsoever.
I was running the carbs "as is" with F7 tubes, 160 mains and 175 airs with 37 vents before re-jetting it. same issue as before. I haven't messed with the idle jets yet and was wondering if that could be the issue for the stumble/chugging?

What idle jets (assuming that's the issue) would be recommended for this engine set-up for a weekend driver?

[PPRMicke]

If I say so
  It depends on which country you live for. The fuel differs from country to country today
Here in Sweden with 10% alcohol in the fuel, you have to go up 3% on the fuel compared to it was for two years
Your low speed holders (120) can try to test with less In my 2176 I drive today with 110
When driving with widebands lambda it showed that it was the best at low speed In my it sits Air 200 F7 M 185 idle 110/57
40mm vents And then we have ignition If you have 009, they are bad at idle, they work best between 3800>
In my case, it's MSD
I can drive down to 40kmt(25 mph) on 3rd gear
My valve lift is 15.67 mm and dyno no is 155whp
The best aid you can get is a Wideband lambda so many question marks are solved
/// Micke

[j-dub]

Buggin81,
Yes the jet we commonly call the idle jet is the one that governs the low speed operation of the carburetor. Stumbling and popping is often caused by a lean mixture. You will likely need to experiment with either a smaller idle jet air bleed or a larget idle jet size.
The idle jet is controlled by vacuum through the carb when the throttle position is low, the high speed circuit comes on when the amount of air coming through the carb is enough to create a low pressure zone in the venturi and act on the booster. The actual idle is controlled by the mixture screws.

As Micke said, a wideband gauge can illustrate your problem on many drive-ability problems and takes much of the guesswork out of it.

Have you verified the float heights are set properly? A low float height can delay the onset of the main circuit which might be contributing.


If you read the first handful of pages you can get some great understanding here of practical tuning.
https://www.thesamba.com/vw/forum/viewtopic.php?t=293837

[flightofphx]

Why not have the third progression hole drilled?  I am running 65 idles at sea level with nearly the same motor specs.

[Bruce]

Your low speed holders (120) can try to test with less In my 2176 I drive today with 110

I recall talking with Doug many years ago about the idle cct jets. He said the stock 120 hole is the "air corrector" jet for the idle cct. In other words, it has more effect at the top end of the idle cct, or in the 2500-3000 rpm range.
Art welds up the 120 hole and drills them to 110-115 to give a bit less air at the top of the idle cct to fatten up the mixture.
Before trying that, I had a set of factory Weber 100s I tried (IIRC, Weber only made them in 60, 100 and 120). I drove the car south from Canada to LA for Classic week on the 100s. At about 2800 it was quite rich. Then after climbing the grapevine and cruising at altitude, it would load up really bad at that rpm. It was so bad, I had to de-clutch and free rev the engine to clear it. I used the stock 120s after that.

[Jim Ratto]

I have a street bug (weekend warrior) with a 2276cc, 48ida's, (40mm vents), FK89 Cam, S/E Heads, 1 3/4 sidewinder exhaust. Current jetting now is: F2 tubes, 185 mains, 210 air 60/120 idles. Carbs currently have two progressions holes (Italian).

So, the issue i'm having is around 2,000-3,000rpm (half throttle) around town it stumbles through the rpm range in 2nd and in 3rd till after 3,000rpms or so then it goes away. If I get into the throttle from a dead stop heavily then there's no stumbling issues whatsoever.
I was running the carbs "as is" with F7 tubes, 160 mains and 175 airs with 37 vents before re-jetting it. same issue as before. I haven't messed with the idle jets yet and was wondering if that could be the issue for the stumble/chugging?

What idle jets (assuming that's the issue) would be recommended for this engine set-up for a weekend driver?

While it is impossible to prescribe exact jetting fixes over the internet (as two identical engines, cc, state of tune, etc can react differently to mirrored carb settings) My opinion is you have a main jet which is overlapping (too rich) your idle circuit at the rpm range you're experiencing this issue. I would also advise you make sure your float levels and needle valve heights are "dead on" to what Weber dictates. The emulsion tube, regardless of which one you run, it designed to "time" the introduction of the main circuit into the aux vent, based on correct float level. In other words, if it's off, you'll chase your tail throwing jets at it and it never being correct.

Jim