Went lean...

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7milesout

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Guys - I'm confused about what just happened to my AFR. And looking for ideas of what might have happened.

I went and met a guy. He wanted to see my Scamp, and the AVS2 carb. The car ran fine getting there, just cruised only. And a reminder, most don't know or remember. I run an AFR gauge on my car full time, because I'm always tinkering with the tune. LA 360.

So we get there, and during that time I pulled the top off, then pulled the filter and the bottom of the housing off the carb so he could see the carb.

The only thing I can remember is that I fiddled with the breather fitting on the bottom housing. It runs the full closed pcv / breather system. The breather fitting seemed to be sitting in there not quite oriented correctly (from my memory) and noticed the breather filter element was gone.

So I put it all back together and cranked the car and it sat and idled for a while. After a little while I glanced at my AFR gauge and it was idling at about ~20 AFR, versus the ~13.0 or so I normally shoot for.

Then on the ride home, everything was lean. So, what happened? The only thing I can figure is either, the little plastic breather vent thing up inside the filter housing had been sitting at some wrong orientation and negatively affecting airflow into the carb. Or, that when I pulled that stuff off, the breather filter element fell out (why I saw it was missing). But could either of those things (or perhaps both) affect air flow that much???

One last note. I'm running the 1905 AVS2 (with annular boosters). And from day 1 this thing has been a PITA to get tuned. As a matter of fact, I could never get the WOT lean enough. And I have tiny little 0.083" jets in the secondary mains ... and it was still too rich! So it appears that whatever I changed / fixed when I pulled this air filter and housing off has CORRECTED the airflow. Now wide open throttle is a touch leaner than I shoot for. It was ~10.0:1 prior to this (and could never get it leaner, even with 0.083" secondary main jets). But after this event, now it is ~13.5:1.

The problem is not the lean tune. I can easily retune everything. Especially now that I'm getting more air. But ... what could have been the problem? I wasn't suspecting there was a problem. And I wasn't expecting to "fix" anything. I was talking with the guy I met with the whole time, while pulling this filter housing off. So I wasn't looking for problems, not really paying attention. I do remember this breather filter element felt a bit crusty the previous time I messed with it. Could having it fallout out (I didn't see it fall out, but did see it was missing) improve airflow that much? If it were crusty enough, maybe it was not letting the breather breathe much too I guess. Could the vent fitting being misoriented before and fixed after improve airflow that much?

Thanks.


7milesout
 
When you say "pulled the top off", do you mean the top of the carburetor or do you just mean the air filter top? To be clear there was no disassembly of the carburetor and everything your describing happened between the valve cover and air cleaner?

Did you check to see if you unseated or somehow disturbed the PCV valve? That would allow air in but would cause your idle to be high.
 
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Yeah sorry. I tried to be clear, but I'm extra good at assuming everyone can understand what I mean to say.

I just meant the top of the air filter housing. I didn't change anything on the carb. When I got home, I checked to make sure I didn't accidently pull the pcv hose off or the distributor advance vacuum line off. They were still perfectly in place.
 
The O2 sensor doesn’t know if it’s carb’d or EFI. It’s a dumb sensor.

Did the engine behave differently? I always believe the plugs over the O2 sensor. It doesn’t know the difference between a lean miss rich or just rich. You have to be able to differentiate between the two.

If it’s actually lean then you need to look for a vacuum line that got loose or something like that.

If it’s not actually lean then you need to address the sensor and such.
 
Rat - Don't take this wrong. I say that because I have a way of sounding harsh, but it's not meant that way. I don't know what AFR gauges you've been looking at, but you're not seeing what I see. There's no way reading plugs can provide such continuous, instant, accurate info. My car's engine responds just exactly as it would be expected to react based on what the AFR gauge says. There's no issue with the AFR gauge.

If I had been driving and it went lean I might question the gauge. But it happened EXACTLY after pulling off the air filter housing then putting it back on. That exact timing. So, something happened at that event that led to this.

And yes, the engine behaved differently after this. When I open the throttle to pull away from a stop, or begin to accelerate it naturally moves lean. The AFR reflects the same. But all the AFR readings have shifted up / lean after this event. And under lean enough conditions it gets weak and can sort miss some too if it goes lean enough.
 
So did u ever take the air cleaner back off to check it over? Or r u waiting for it to fix itself cause it probably won’t. U probably could of fixed it fairly easily if u just had another look. Then I suppose u drove it back home. Kim
 
7milesout said:
Yeah sorry. I tried to be clear, but I'm extra good at assuming everyone can understand what I mean to say.

I just meant the top of the air filter housing. I didn't change anything on the carb. When I got home, I checked to make sure I didn't accidently pull the pcv hose off or the distributor advance vacuum line off. They were still perfectly in place.
Click to expand...

Really only a few things can make it read lean. Excess air (vacuum leak), lack of fuel (clogged filter, failing pump, pinched line, etc) or an exhaust leak to throw the reading off, then just a failed sensor.

My question would be where did the breather filter element go that is missing?
 
oldkimmer said:
So did u ever take the air cleaner back off to check it over? Or r u waiting for it to fix itself cause it probably won’t. U probably could of fixed it fairly easily if u just had another look. Then I suppose u drove it back home. Kim
Click to expand...

oldkimmer - My post was long enough. But yes, I took it back apart and paid close attention to it all, checked all hoses and placement, and put it back together meticulously. It remained lean. Nothing to fix.

Tuning away the leanness is easy to do. And I will get to that later this week or this weekend. I'm already eyeballing my nerdy tuning spreadsheet.
 
dano said:
Really only a few things can make it read lean. Excess air (vacuum leak), lack of fuel (clogged filter, failing pump, pinched line, etc) or an exhaust leak to throw the reading off, then just a failed sensor.

My question would be where did the breather filter element go that is missing?
Click to expand...
Where did the breather filter go ... now that is a good question. One I've been sweating over. If it got sucked down the carb, well, it didn't do anything detrimental. But because this lean event came on exactly after this event, my mind is stuck on 2 possibilities. 1. It fell out and down on the ground when I pulled the housing off, set it down, and I didn't see it fall out. Or 2. It went down the breather hose. Which it's not really small enough. But like I said, it was kind of crusty the last time I noticed it. Maybe it got worse and broke up, and maybe me picking these parts up lodged it down the tube. But ... would plugging the breather hose lean the AFR? I doubt it, but that's just my assumption, I don't officially know that.

And as far as #1. Maybe it was crusty enough it was blocking up the breather's hose. And when it fell out, now the breather is working correctly. But still, this doesn't seem like it would lean the AFR ... but I don't know that either. Seems like the PCV would pull blowby and air out of the crankcase and into the intake (due to intake vacuum and the PCV valve). Seems like the breather would be allowing some amount of air down into the crankcase when the pcv is drawing it out, creating a balance. In either case, I don't see this affecting AFR that much. But, I could be plain ole wrong.
 
Oh, and I will be replacing the pcv and the breather vent / filter due to this, before I tune. Although I think the tuning will be simple / easy / quick.
 
7milesout said:
Where did the breather filter go ... now that is a good question. One I've been sweating over. If it got sucked down the carb, well, it didn't do anything detrimental. But because this lean event came on exactly after this event, my mind is stuck on 2 possibilities. 1. It fell out and down on the ground when I pulled the housing off, set it down, and I didn't see it fall out. Or 2. It went down the breather hose. Which it's not really small enough. But like I said, it was kind of crusty the last time I noticed it. Maybe it got worse and broke up, and maybe me picking these parts up lodged it down the tube. But ... would plugging the breather hose lean the AFR? I doubt it, but that's just my assumption, I don't officially know that.

And as far as #1. Maybe it was crusty enough it was blocking up the breather's hose. And when it fell out, now the breather is working correctly. But still, this doesn't seem like it would lean the AFR ... but I don't know that either. Seems like the PCV would pull blowby and air out of the crankcase and into the intake (due to intake vacuum and the PCV valve). Seems like the breather would be allowing some amount of air down into the crankcase when the pcv is drawing it out, creating a balance. In either case, I don't see this affecting AFR that much. But, I could be plain ole wrong.
Click to expand...

I think your right on this not being the cause other than the lack of PCV function might actually make it rich but oil contamination might take it lean too.

What breather are you using?

I have a Wagner adjustable PVC and it super sweet, one of the best products I have seen for providing instructions, tools, and replacement parts. You can tell an Engineer put this this kit together.
 
The PCV valve and the breather (on top of the valve cover) looks the same as original equipment.

Overall - It just plain ole seems like I must have had something put together wrong with the air cleaner and housing the last time I put it on. And I got it straight at this event. I say that because, there's just no logical way that I couldn't get the WOT lean enough, unless something was restricting the air flow. I must have snarkled something up somehow when putting on the air cleaner the time before this event.

It doesn't seem possible, but could I have had the choke jammed partially closed with the air cleaner housing? That would certainly restrict airflow. Matter of fact, the choke has been a PITA since this new carb. I'll tear it apart again and see if there could have been some way I was stopping the choke from being returned completely open.
 
7milesout said:
The PCV valve and the breather (on top of the valve cover) looks the same as original equipment.

Overall - It just plain ole seems like I must have had something put together wrong with the air cleaner and housing the last time I put it on. And I got it straight at this event. I say that because, there's just no logical way that I couldn't get the WOT lean enough, unless something was restricting the air flow. I must have snarkled something up somehow when putting on the air cleaner the time before this event.

It doesn't seem possible, but could I have had the choke jammed partially closed with the air cleaner housing? That would certainly restrict airflow. Matter of fact, the choke has been a PITA since this new carb. I'll tear it apart again and see if there could have been some way I was stopping the choke from being returned completely open.
Click to expand...

The 1905 AVS2 should be a mechanical choke? Do you have a cable controlling it? IF you have a drop base air cleaner and too short of an air filter this could prevent the choke from opening all the way. It might also impede the secondary air door? Fro the image below it might not be likely.

upload_2022-4-20_13-17-58.png
 
7milesout said:
Rat - Don't take this wrong. I say that because I have a way of sounding harsh, but it's not meant that way. I don't know what AFR gauges you've been looking at, but you're not seeing what I see. There's no way reading plugs can provide such continuous, instant, accurate info. My car's engine responds just exactly as it would be expected to react based on what the AFR gauge says. There's no issue with the AFR gauge.

If I had been driving and it went lean I might question the gauge. But it happened EXACTLY after pulling off the air filter housing then putting it back on. That exact timing. So, something happened at that event that led to this.

And yes, the engine behaved differently after this. When I open the throttle to pull away from a stop, or begin to accelerate it naturally moves lean. The AFR reflects the same. But all the AFR readings have shifted up / lean after this event. And under lean enough conditions it gets weak and can sort miss some too if it goes lean enough.
Click to expand...


I should have said it this way.

If the O2 and the plugs don’t jive, believe the plugs. Simple as that.

And unless you are data logging the O2 with a MAP and TPS you’re still doing quite a bit of iteration to tune.

The plugs, the chamber and the exhaust port are the “on the ground” parts that show directly what’s happening during the combustion process. The O2 sensor is way late.

If I can find it (maybe @Bewy knows what I’m talking about and where it is) I will post a link to a paper written by a guy who went By “Shrinker” that covers some plug reading verse O2 sensor numbers.

It’s a brilliant piece. I printed a hard copy of it because it’s that good. I just don’t have a copy of it with me.
 
dano said:
The 1905 AVS2 should be a mechanical choke? Do you have a cable controlling it?
Click to expand...
Yes, I do have a cable on it. And use the choke frequently. I make sure I get the cable set properly (no filter housing on at that time) when I have to pull the top of the carb off to swap main jets. But, putting the filter housing back on top after getting the cable back properly, maybe I didn't get the housing on right last time in regards to the choke.
 
So, you drove the car for the guy to look at the carburetor? And the carburetor was on your car and running good? And then you ripped apart a good running carburetor, put it back together and now it's not running right? Have I got that right?
 
RustyRatRod said:
So, you drove the car for the guy to look at the carburetor? And the carburetor was on your car and running good? And then you ripped apart a good running carburetor, put it back together and now it's not running right? Have I got that right?
Click to expand...
No, he only took the air cleaner off and put it back on. Kim
 
7miles,
Responding to the problem of secs too rich. Have you checked that you are getting WOT with your foot on the acc pedal? You would be surprised how many people never check this....& do not get the expected WOT performance.
In your messing around could you have disturbed the throttle linkage?
Secs not fully open can change distribution & give odd O2 readings.
 
Bewy said:
7miles,
Responding to the problem of secs too rich. Have you checked that you are getting WOT with your foot on the acc pedal? You would be surprised how many people never check this....& do not get the expected WOT performance.
In your messing around could you have disturbed the throttle linkage?
Secs not fully open can change distribution & give odd O2 readings.
Click to expand...

I think "secs" must be an auto spell correct issue. If not, I'm not following it. That is a good question. I did check it, when I first saw it was overly rich at WOT. Thinking if the throttles opened all the way it would get more air. I found that the throttles WERE NOT opening all the way. So I changed where the pedal connected to the linkage and the result was a) it opens the throttle all the way, b) the pedal resistance slightly increased ... which is actually pretty decent, no better no worse. The AFR result was very little changed. It was still at that time, too rich.

I tend to think it wasn't due to throttle linkage because if it weren't opening enough, the AFR would still respond mostly linearly to wherever the throttle did open up. It's got to have been the choke partially closed. Best I can guesstimate.
 
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