IDAs, Venturis and intake valves...

[ecksrash]

So I've been battling my IDAs trying to get my jetting right for far too long. It has been suggested that a rule of thumb should be that the venture should be 2mm smaller than the intake valve. So in my case, since my 2276 has 42X37.5 valves I should be running a 40mm venturi in the carbs. While on the dyno which was run by a reputable engine builder, the engine wasn't making what we believed it should. We swapped in 37mm venturis and gained a bunch of power. It was explained to me that the engine wanted more velocity than flow....
I've been trying to jet the carbs with my wideband and just can't seem to nail it down. My engine is a 2276, Web 86B, 1.45 rockers, 1 5/8 sidewinder exhaust and 9.6:1 compression ratio. I'm running a Bosch 010 distributor and blue coil. My current jetting is 37mm venturis, 100 idle jet holder, 60 idle jet, 180 air correction jet, F11 emulsion tubes, and 145 mains. I run lean at the top of the idle circuit and then rich at the bottom of the main circuit. I'm stumped.
I would like to understand more about why the 40mm venturis didn't perform better than the 37mm vents. Could I gain back the power with proper jetting with the 40S?

[cbeck]

Is this why they now offer them with a third progression hole?

[ecksrash]

Mine have been modified with the third progression hole.

[Piledriver]

I don't have any experience with IDAs, but the jet stack setups appear to work ~like IDFs...

Anyway, one thing missing from IDFs at least that IIRC exists on Dells, Zeinits, Solexes and many other carbs is a separate idle fuel air correction jet--- Webers have fixed idle air correction holes built into the idle fuel jets.

With that additional adjustment, you can then tune the bottom vs top end of the idle circuit ~just like the mains, with a bigger air correction leaning out as the load/RPMs increase.

You can fill and redrill the air holes, air isn't as sensitive to tiny variations as fuel is.
I did that on 44 IDF/36s as part of getting them to work on a stock 1700 T4.
Got awesome fuel economy and could easily set AFR and the idle<>mains crossover as desired with the additional adjustment.

[ecksrash]

Thanks Greg, I'm beginning to think that I may also need to try a different emulsion tube to lean out the bottom end of the main circuit as well.

[Ian Godfrey]

I have a 2387 (44 intakes) with an 86B. I got better results with F2 than F11.
I think 40 vents should be good. 40 vents were good for drivability but I was looking for a bit more power so I went to 42's. My best jetting (no aircleaners) with 42 venturies was F2 160M 190A, 110/63 idles 28 deg advance. I spent a lot of time working on the idle tune first, (main jet stacks removed and going for a drive with AFR meter). I tried idle holders of 110, 115 and 130 and idle jets of 55, 60, 63, 65, 70. On IDA's the idle circuit continues to feed fuel when the main circuit cuts in so sort the idle first.
A key thing I found was 15 – 18 deg idle helped the stumbles a lot. (it still starts fine)
it is very satisfying when you get if sorted.

[Piledriver]

So the "idle holder" the idle fuel air correction jet on IDAs?
(its one piece on IDFs)
Good to know, that's a good tuning knob to have.

[ecksrash]

Greg, On the IDAs the idle jet holder is actually an idle air jet. I've tried 100,110,120 and 130. I cant seem to get the idle circuit to run well with anything but the 100 idle air. It just wants to be too lean. So I'm thinking I may need to try a different emulsion tube...??
Ian, when Dave Kawell and I had my engine on his stuska dyno the engine was lazy with the 40mm vents. after we swapped in the 37mm vents it woke up a huge amount. This baffles me....

[Ian Godfrey]

The emulsion tube won't effect anything in the low rev range say up to around 2500.
if it likes the 100 idle holder go with it and and find a good jet thats not too rich and doesn't 'lean spit'
It is pretty touchy to get right. here are my three '110' driving tests, my engine was pretty sensitive to idle jet size
110 holders 65 idles
13:1 cruise at 60 kph pull strong to 3,600 but rich everywhere (interesting to see how high the engine can rev under load with no main stack)

110 holders 63 idles
13/14:1 cruise at 60kph, pulls good, stumble at 2,500 generally GOOD. 27C (temp)/1015B (barometric pressure) Maybe a bit better than 115/65

110 holders 60 idles
14:1 cruise at 60 kph lean spit at 2,200 at very light throttle. Too lean at various small throttle openings

Each time I reset the mixture screws 1/2 turn from a lean cut out.
I was also hoping to get a leaner cruise, but 13/14:1 was the best I could get.
So with the 110/63 then I needed an Emulsion/main air that that would come in early enough to 'cover' the stumble at 2,500. (F2 for me) All this also assumes your timing curve is good too.

[ecksrash]

Good info Ian, may I ask what ignition system you are running? I've been told that with my ignition system (Bosch 010 and blue coil) which is completely mechanical that I should avoid any AFR that is leaner than 13.5:1. When I get the jetting where I am comfortable with the engine I want to go back to my Mallory Unilite which is also mechanical advance.
Right now my idle circuit is set up with a 100 idle air and 60 idle fuel. It runs good at the bottom, but will lean miss at the top then the main circuit comes in and it goes way rich to the tune of 9:1. I'm thinking the F11s are not gonna work for me. I've been battling with them for a while.
Now I do have F2s, F4s and F7s on hand. Figuring out which ones to run will be another adventure.

[Ian Godfrey]

I use a Mallory so I could set up the quick curve from 18 - 28 deg. F7's didn't work well for me (very rich in the mid range), F2 tips in early, which I needed, but they are richer in the mid range than F11. So F2 might be the wrong direction for you. I haven't tried F4 but have read it is like a F2 but leaner at the upper range.

good luck and let us know your experiences.

[buildabiggerboxer]

Good information for the carb guys, big swings back to carbs for racing here in the UK, But Interesting you only run 28* advance Ian?

[Ian Godfrey]

Yep, not a lot, but it made the most power under the curve on the dyno. (i got more peak power with more timing) but that wasn't my aim.

[ecksrash]

So good to be able to share knowledge. Here is where my engine is happy in the idle circuit. It likes the 100 idle air jet with a 65 idle fuel jet. It will be right about 13.7:1 at the top of the idle circuit. I'm ok with that. But with the F11s the main circuit will come on and swing way rich. I can feel it load up the plugs and see it on my wideband. AFRs of 9:1 if I stomp the throttle or 10:1 if I roll into the throttle. The main circuit is a 180 air correction jet(small so as to not turn "on" the main too soon), F11 emulsion and 145 main jet. I have read (Thanks Ian) that the F11s run best with a 200 or greater air correction jet.
Here's what I'm thinking... Use the 100/65 in the idle circuit. I need to figure out which emulsion tube will allow the main circuit to not come on too strong to avoid the rich transition from idle to main circuit. If a large air correction jet will bring the main jet in earlier I have enough air correction jets to adjust that. I have main jets to adjust fuel at the top end as well. Any quick Thoughts are appreciated... DS

[Piledriver]

larger idle air will lean out the idle side of the main transition and should have very little effect at actual idle.
I never noted the main airs having any effect on where the mains kicked in, they more or less just tune the top end richer/leaner. The main size with the emulsions, along with the idle and idle airs determine the transition.
The fuel level in the main jet stack vs. the holes in the emulsions is where the magic happens.

Admittedly, IDAs are not IDFs, but they work essentially the same way.