Cut-outs causing super lean mixture????

Canadian1968

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I added some cut-outs or dumps, whatever you want to call them, to my 440. I am running an AEM wideband 02 sensor on the driver side bank. The cut-out is approx 10 inches from the opening of the cut-out.

When I run the car with the cut out closed I idle at around 14.0 AFR . When I open the cut outs my gauge goes off the chart lean . Even driving the car, a bit the gauge will not start to read until rpms are up around 3000.

Is it possible that the cut outs are leaning out the AFR of the engine that much ???
 

qkcuda

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Air back feeding to the O2 sensor and giving you a false reading until there is enough exhaust flow to stop it?
 

BSB67

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I added some cut-outs or dumps, whatever you want to call them, to my 440. I am running an AEM wideband 02 sensor on the driver side bank. The cut-out is approx 10 inches from the opening of the cut-out.

When I run the car with the cut out closed I idle at around 14.0 AFR . When I open the cut outs my gauge goes off the chart lean . Even driving the car, a bit the gauge will not start to read until rpms are up around 3000.

Is it possible that the cut outs are leaning out the AFR of the engine that much ???
The sensor is too close to the cut-out.
 

Canadian1968

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Here is a picture I took while welding things up . I run the sensor on the passenger side.. you think that's still to close to the cut out ?

20220822_201312.jpg
 

Photon440

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Changing the back pressure also changes the signal at the carb. I once bought a 455 GTO with lots of holes in the headers, it ran lean and terrible - hesitation, backfiring, no power. Just putting on new exhaust and no other changes made it run like it should have all along.
 

qkcuda

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Yes I believe that is too close. Your mixture is probably fine, you are just throwing the gauge off.
 

Doorkicker

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I had a similar issue. Turns out the cutout (when open) pulls air in from the exhaust contaminating air farther up the pipe. Also, you have a very low pressure situation at that juncture. As far as the O2 is concerned, it's setting about 2" from your tailpipe. I suspect a better spot would be just behind or at the neckdown. But, I don't know what headers you're running... standard log manifold, shorties, etc. (I'm assuming shorties?)
 

Canadian1968

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Changing the back pressure also changes the signal at the carb. I once bought a 455 GTO with lots of holes in the headers, it ran lean and terrible - hesitation, backfiring, no power. Just putting on new exhaust and no other changes made it run like it should have all along.
I'm going to have to check again but I have an In car vacuum gauage and I'm pretty sure that was reading the same . If the carb signal had been effected I would think it show on my gauge a change in vacuum and idle speed .

I'm running Hedman full length headers 13/4 primaries
 

Paul_G

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Really should have the sensor in the header now. And make sure the header collector gasket is sealing.
 

Doorkicker

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I'm going to have to check again but I have an In car vacuum gauage and I'm pretty sure that was reading the same . If the carb signal had been effected I would think it show on my gauge a change in vacuum and idle speed .

I'm running Hedman full length headers 13/4 primaries
I struggle with concept the carb is getting a bad signal, albeit possible. If you're dropping vac radically it's unlikely you'd make it to 3000 RPM without issue. Conversely, a spike in vac is unlikely with open throttle. But in both cases, you "could" have carb bypass issue (idle circuit, etc.), but that typically causes it to go rich, and not huge extremes. If the reading is going over 20 (22-24), the engine would be freaking out up to 3000 RPM. Back pressure is important for signal, but what you're experiencing is pretty radical.

Based on what you're sharing, I think you're dealing with a O2 placement issue. Also, those headers are plenty long enough, meaning the bung could be moved pretty far up. The "easy" fix is pop another bung in there and plug the current one. There's a min distance you want from the head exhaust port, but most people just say 6-8" past the collector. The really important part is the min amount of pipe you have after the sensor, which is 18-24". In a perfect world.. you'd have the sensor located 18-24" from the engine and 18-24" from the end of the pipe. Once you open the cutout, air is getting to the sensor from the long pipe, negating the length of the cutout extension.

I should have asked this first, but you're running a carb, not an EFI, right?

One last point, the more the sensor is exposed to the atmosphere when running (electrified) the more it shortens it's life.

OK.. one more comment. I would swap the sensor to the other side. It's an easy, quick check. it's unlikely that you'll see a difference, but at least that will exclude any valve issues in the right head. (it looks like there's a bung on the driver's side).
 

70bigblockdodge

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A friend of mine has a full multiport EFI on his Pontiac. When he pulls the exhaust and adds collector's he has to do some trickery to make it idle. I don't know what, I'm a carburetor guy.
 

Canadian1968

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Yes I am running a carb. I think I agree with what a few others have said, which is that I am simply getting air in to the system when I open up the cut out. The car still runs fine , with them open. If it really was leaning out as much as the gauge said, I would for sure be noticing some other issues.

Like i said the car idles perfectly with them open , and vacuum readings are fine. Although i have only driven it a short distance with them open it did seem to run perfectly fine.

I am playing with the idea of totally rebuilding the entire exhaust this winter....
 

dvw

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The O2 is reading false. Reversion is pulling fresh air back into the pipe. Idle readings on open headers with the sensor close to the end are useless. I've experienced the same issue.
Doug
 

bm02tj

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To put your mind at ease open cut outs and hold a tissue over outlet and see it blow and suck
Move sensor or add pipe to cut out
 
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