low AFR at cruise

[71ghiavert]

I've read the long thread but am still a little lost about how to solve my particular problem.

I have Weber IDFs on a 1968cc. At cruise, with 50 idle jets, I'm at 10.8 AFR. If I go down to 47 idles, the AFR improves, but the car stumbles badly off idle (especially if I step on the gas) and up through progression until I get onto the mains.

I tried addressing this with fuel pump, but it doesn't overcome the stumbles.

If I just run 50s, everything is really smooth, and fun, but I'm rich. If I run 47s, my AFR is better, but it's stumble city.

If anyone has thoughts about how to address this, I'm all ears. Thanks.

[Brian_e]

Sidewinder style header by chance?

Brian
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[madmike]

You didn't state what size IDF's ?
What size vents?
Floats set?
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[71ghiavert]

The engine has Panchitos, 1.25 rockers (with Oteva single springs), a Web 218/119 cam, 9:1, and Weber 40 IDFs with 50 idles, 140 mains, 220 airs and Berg 30/34 grooved vents (I had 32s, but got the same behavior regarding 47 vs 50 idles. The Bergs are an attempt to get the mains to come in a bit sooner. They seem to help). Vintage Speed Super Sport 38mm SS143 exhaust. Floats set at 10.

I’m just trying to get the idles into good AFR range so I can work on the transition and the mains.

As is, the engine only seems to drop to 10.8 on cruise. Everywhere else it has much better AFR numbers.

[John Connolly]

clearly at cruise you are fuelling from both progression and main circuits. You need to delay the mains a little. Try 210 airs, or 200s. Deal with the main jets after solving the double fueling issue.

I'd also try a 12 or 14mm float setting. Eventually you will find the magic combo if you persist.
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[74 Thing]

Listen to John.

1. Verify your fuel pressure.
2. Lower the float height some to delay the mains.
3. Put smaller airs in to delay when the mains come in (you will probably have to go to smaller main jets as well if your WOT AFR is already dialed in).

[71ghiavert]

Thanks as always.

John, I'll certainly check, but is there any chance this isn't a main jet issue? If I take the main stacks out, and still get 10.8 at cruise, would that indicate that the mains aren't contributing to this issue?

A couple of months ago, I ran with the stacks out, to find the main "tip in." And I think I was still fat at cruise with 50s idles. But it turned out my distributor was wonky, so all the data from that period is now suspect.

Still, I'd love it if I can determine whether the mains are involved at cruise, and i'm wondering if taking the stacks out would be a way to help determine that. Thoughts?

Fuel pressure is set at 3 lbs with a good quality regulator.

[APPLEGREENVW]

[VdanielW]

AT what mph, rpm, and vacuum are you seeing this low afr? When I was running webers, I could set my freeway steady cruise at 15.2:1 (65mph 3k rpm), but at 45mph it was into the low 12's. This was all on the idle circuit (verified with stacks removed).

[John Connolly]

yeah, remove the stacks and re-test.

I have seen lots of situations where engines had extra fuel; you have to figure out where it's coming from. Spillover from fuel level too high (float, needle/seat, fuel pressure, etc), also accel pump flutter (linkage problem), are a couple things to verify.
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[74 Thing]

Good video recommended above to check fuel level in float bowl with engine running.

Did you set your float height with the hinge vertical like a door hinge:
https://www.youtube.com/watch?v=54uPwTZ3DJw

[71ghiavert]

Just took out the stacks and re-tested. AFR on cruise remained the same, at 10.8 (in fact, seemed to drop even lower, to 10.5).

Will begin checking other things John has suggested. To recap: 47 idles are too lean in transition, 50s too rich at cruise.

One other data point that may or may not be relevant: I think I’ve read that, typically, the snail reading when setting up IDFs should be 5-7. That has never been true for mine, which have always read 3 1/2 to 4 on the three engines I have put them on (1600, 1968, 1968cc with bigger valves and ratio rockers. All jetted differently, of course).

Anyone has thoughts, I’m all ears.

[Alstrup]

I´ll mention it once more and then never again. Float height too high, Idle bleed jet too small. If it is 115 now, you can easily go 130
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[John Connolly]

a lot of folks don't know that snail gauges have a hole in the side of the boot, which you can expose to drop the reading #. Check that it's not accidentally open on yours.
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it's advice. If you don't like it, ignore it. Don't argue.

If you want no-BS technical advice privately, feel free to DM me. Donations appreciated (food gift certificates, etc).

[71ghiavert]

Alstrup—Ha! Def NOT ignoring your expert & appreciated advice. It’s been pouring here so have had little chance to take the cover off the car and try things. Re the bleed jets: trying to eliminate all other possible causes before reaming or drilling them. Never done it before so, yes, it does make me nervous with a part that will be hard to fix if I screw it up. But if that’s where this leads, so be it. I did buy a set of reams . . .

I assume that if I get it wrong with the bleeds, there’s someone in Samba World who can remove my mistakes and press new bleed jets in?

John—will check the snail. As I say, this has been true since I got the IDFs five years ago. It has always puzzled me because I read of other people getting higher numbers.

Thank you both.

[John Connolly]

When something seems off, THINK.

Are all the cyl low, or just one?

I've seen engines read low on snail also because of a leak between the velocity stack and carb top.

A low reading is low airflow, OR air bypassing the snail in some manner.
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If you want no-BS technical advice privately, feel free to DM me. Donations appreciated (food gift certificates, etc).

[71ghiavert]

John: Thanks. I'm THINKING. I've been able to tune the carbs for five years so that my cars have been fun to drive--so, yes, very basic tuning And likely fat. This is my first shot at AFR and working out the interrelationships. A lot to learn, especially when the results are unexpected.

The 3.5 to 4 on the snail has been there with each of three engines, with different cams, heads, rockers, manifolds and piston sizes. All tuned to LBI at maybe 850 rpm. I get the same snail number on both carbs. And all the engines have run well (tho likely fat). The only thing that was weird is that I wasn't getting higher snail numbers. Maybe I just have a wonky snail.

Will think more, maybe try someone else's snail, and check for a hole in the boot. Merci.

[Dale M.]

If you don't like your snail readings (in accurate/unstable) you might want to consider going too a manometer for more precision readings...

https://www.youtube.com/watch?v=Y_WDF6glD5k&t=496s&ab_channel=MikeOD

IF you really what to know how manometer works....

https://www.youtube.com/watch?v=gxrkLkJybnA&ab_channel=RealPars
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