Going lean when shifting...

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7milesout

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Guys,

I have an AFR gauge on my car. LA 360, with Edelbrock 600 CFM, manual choke (1405).

When I'm gently accelerating up to speed, the AFRs go way lean from 1st to 2nd, and 2nd to 3rd.

I've been searching the internet to try to UNDERSTAND WHY this happens. And to try to determine how to prevent it, or read some advice on how to prevent it.

Generally it doesn't stumble when it does this, except before the engine is fully warmed up.

I haven't fully finished tuning it in, as I'm currently working on making my own metering jets. Once I have the Power Mode and Cruise Mode AFR's where I want them, then I'll get serious about this issue.

Does anyone understand this? Or have you read about this somewhere you could link me to? What I find sort of kind of just skims the surface of this.

I don't have much of a theory, but I wonder if the metering rods may be dropping into their jets during a shift (from the power step to the cruise step) and starves it of gas for a second. Dunno. I don't know what happens to the vacuum during a shift. If the vacuum spikes during a shift, I guess it would suck the metering rods down before kind of returning to where they were. Kind of makes me want to increase the length of my custom power step further up (toward the L) on my metering rods. Maybe then it won't switch from power to cruise (if this is what is happening).


7milesout
 
So you're on the primaries,right?And the PCV is hooked up and working,right?And this is an automatic,right? and headers right? And your gas pedal is remaining fixed at this time,right? And this is only during the shift,right?
What intake manifold and at what rpm is this happening? What's your Idle-timing, and at what rpm is the centrifugal timing all in by? And cam-specs if you gottem?

Ok so, Here's the situation as I'm reading you. My diagnoses only applies to this situation;
the secondaries are closed and the metering rods are down all the way, so they are at full-lean. So there are just two circuits working; the low-speed circuit and the mains. And airflow thru the carb is falling from the outshift rpm to the inshift rpm.

so question #1, can you feel it nose over? as in a momentary loss of acceleration, or an actual slow-down? If yes,a slow-down, then make sure that this is not caused by shift overlap.
But if just a mild loss of acceleration;
A) Make sure your vacuum advance is not vacillating.Either disconnect it and retest or; Put a vacuum gauge on the VA line and watch the gauge,during the shifts. If at the outshift the vacuum is say 20 and it falls to 15 after the shift. Stop the car and check the idle-timing PLUS VA timing,at those two vacuum settings. If it changes in that range, readjust the screw inside until it doesn't.Then retest the shift AFR.But if the throttle is open far enough to disable the VA, then
B) Next check the ignition timing (no VA) at the outgoing rpm, and at the incoming rpm. Say you outshifted at 3200, and it inshifts at 2000. Say the timing was 36* at 3000 and 20* at 2000. One would expect a significant torque loss during the shift. The torque loss should be accompanied by a vacuum loss, and therefore the PV system should come on line, causing the AFR to go rich or at least smooth out.
If it doesn't, then you'll need swap out the metering rod springs for a pair that pops 'em up a lil sooner.
C) But if you're running a 3200 TC, well then B) would not apply to you.
And if you're one of those guys that likes to run full-timing by 2000,lol, then B) also MAY not apply to you.
D) But if the car does not nose over, and the AFR recovers right after the shift, Then I wouldn't worry about it. I'm just lazy that way,lol.

E) But if you are running a lot of ignition timing,and this is happening at small throttle openings,then, I'll bet you're T-port sync is off, and that's probably the biggest problem.

That's all I got
 
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  • On the primaries: Yes.
  • PCV hooked up: Yes.
  • PCV working: Dunno.
  • Automatic: Yes.
  • Headers: Yes.
  • Holding the gas pedal steady: Yes.
  • Only during the shift: No, but I was going to mention it later. It also does this same lean spike about 1.5 seconds after easing into the throttle from a dead stop. Completely related is my theory. So fixing one, should fix both.
  • Intake manifold: I believe is the stock LA 360 manifold
  • At what rpm: It shifts at about 2,200 - 2,400 rpm. As I mention, I'm just easing it around when it does this.
  • Idle timing: Static 11°~12°
  • Timing Advance all in: ~2,600 rpm or so. I'm guessing it's the original distributor.
  • Cam Specs: Dunno. Slightly aggressive, not a big loper.
Your reading of my situation ... here's where I can identify possible differences.
  • Secondaries closed: Yes.
  • Metering rods down: I doubt it. I've got the biggest spring in there available from Edelbrock. At least I think it's the biggest spring (I'm starting to 2nd guess myself). The friggin colors are difficult to distinguish but I spent a lot of time staring all the barely different colors in an effort to try to choose the stiffest ones. And squeezed them too. That's not to say I didn't flub it up. But I was careful.

  • A1: When the engine is not fully warmed up, it tends to nose over. Especially 1.5 seconds after gently accelerating from a full stop. Full throttle or just aggressive starts, this is not an issue. But the poe-lease around here don't like it when I do that. It noses over a bit at 1.5 seconds from leaving a dead stop, even when fully warmed up.

A) I sure hope my VA is not vacillating. Hey uhhh ... what's vacillating mean?
My vacuum gauge doesn't have a long enough hose to watch it while I'm driving. Guess I could just swing by Home Depot and correct that though. I'm just learning all this stuff, so, the screw inside (the distributor?) sounds difficult, because I've not done that or learned about that screw before. What does this screw do?​

B) I'm assuming the PV system is the Power Valve? Does an Edebrock have this? I've heard of this on Holleys, but not on Edelbrocks. Or maybe you're referring to the Power Mode portion of the metering rods. This is my leading theory, the rods are not pulled up out of the mains, or fall below the step when it shifts. And the AFR's go lean. I can feel the power loss, when just toodling along, slowly accelerating. I'm going to double check the springs I've got in there, make sure I have the stiffest.

C) Not running a 3200 TC. Not full timing by 2,000 rpm.

D) I would not worry about it either, if I weren't feeling it.

E) Not following the T-port. What is a T-port? Throttle port? Is this related to the screw you mentioned previously?

Very helpful. Thanks for the info.


7milesout
 
Comments in the quote; click to expand
7milesout said:
  • On the primaries: Yes.
  • PCV hooked up: Yes.
  • PCV working: Dunno.
  • Automatic: Yes.
  • Headers: Yes.
  • Holding the gas pedal steady: Yes.
  • Only during the shift: No, but I was going to mention it later. It also does this same lean spike about 1.5 seconds after easing into the throttle from a dead stop. This could be a different issue. This could be a an accelerator pump duration issue. But if indeed the low throttle position is lean, then it may just be that the AFR is returning to it's pre-pump leaness.
  • Completely related is my theory. So fixing one, should fix both. Maybe
  • Intake manifold: I believe is the stock LA 360 manifold
  • At what rpm: It shifts at about 2,200 - 2,400 rpm. As I mention, I'm just easing it around when it does this.
  • Idle timing: Static 11°~12°
  • Timing Advance all in: ~2,600 rpm or so. I'm guessing it's the original distributor.
  • Cam Specs: Dunno. Slightly aggressive, not a big loper. Ok, got it, that's all I need
Your reading of my situation ... here's where I can identify possible differences.
  • Secondaries closed: Yes.
  • Metering rods down: I doubt it. I've got the biggest spring in there available from Edelbrock. At least I think it's the biggest spring (I'm starting to 2nd guess myself). The friggin colors are difficult to distinguish but I spent a lot of time staring all the barely different colors in an effort to try to choose the stiffest ones. And squeezed them too. That's not to say I didn't flub it up. But I was careful.

  • A1: When the engine is not fully warmed up, it tends to nose over. That's lean for sure Especially 1.5 seconds after gently accelerating from a full stop. Full throttle or just aggressive starts, this is not an issue. But the poe-lease around here don't like it when I do that. It noses over a bit at 1.5 seconds from leaving a dead stop, even when fully warmed up.this could be pump-shot

A) I sure hope my VA is not vacillating. Hey uhhh ... what's vacillating mean?
vacillate; To sway unsteadily from one side to the other; oscillate.

My vacuum gauge doesn't have a long enough hose to watch it while I'm driving. Guess I could just swing by Home Depot and correct that though. I'm just learning all this stuff, so, the screw inside (the distributor?) inside the hose nipple of the VA can. sounds difficult, because I've not done that or learned about that screw before. What does this screw do? it controls the amount of vacuum required to drive the VA to it's maximum value thus allowing full VA earlier to later.​

B) I'm assuming the PV system is the Power Valve? Does an Edebrock have this? The metering rods are your PV system. I've heard of this on Holleys, but not on Edelbrocks. Or maybe you're referring to the Power Mode portion of the metering rods. This is my leading theory, the rods are not pulled up out of the mains, or fall below the step when it shifts. And the AFR's go lean. I can feel the power loss, when just toodling along, slowly accelerating. I'm going to double check the springs I've got in there, make sure I have the stiffest. This may not be the best course of action. When you do this A) more fuel than necessary only costs you money at the pumps, and B)tends to wear out the engine faster. You want to run it as lean as possible yet be driveable,lol.

C) Not running a 3200 TC. Not full timing by 2,000 rpm.good/good

D) I would not worry about it either, if I weren't feeling it.

E) Not following the T-port. What is a T-port? Throttle port? Is this related to the screw you mentioned previously? Read about it here; AJ's guide to Transfer Port Synchronization; oops linkey no workey. Click on the link at the bottom of the post, with the blue M

Very helpful. Thanks for the info.


7milesout
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Ok well in your case
What is your idle vacuum, and at about what rpm does the vacuum peak? Stop if over 3600
I'm gonna guess it idles at 16 or more in Neutral, and peaks at about 2200ish
I usually start with the T-port sync first, but you're probably pretty close with 11/12 degrees of idle- timing. But Personally with the stock TC, I would slow the all in to somewhere just over 3000Rpm, But I don't think this is a contributing factor.
Outshifting this early, you'll probably be deeper into the throttle, so the PV system will probably be on line, AND the VA will probably be off-line.
What rear gear are you running?
The metering rods are your PV system. So at this point I would set the mixture screws to 1.5 to 2 turns out, and reduce the in-gear idle rpm by about 50 if required.Then get a longer vacuum hose and take her for a spin with the VA disabled, and the carb-port plugged.I usually use a wiper blade to hold the gauge in place at these low speeds. Notice the max vacuum just before the shift, and what it drops to.
Assuming no change,and with vacuum readings above 12 inches,I would remove the PV springs completely, and retest. This might make the car somewhat difficult to accelerate to the upshift speed so just take it easy until you get close, then level off at the previous vacuum reading as the shift approaches, and let her do her thing. Ok so no change means the mains are lean. You've proved it. Now it's just a simple matter swapping in rods with a smaller cruise step or a bigger MJ.
But before you do, I would verify that the fuel level is correct and that the fuel is fresh if it's oxygenated.
The fuel level is very important to low rpm/low throttle settings.If the level is even just 1/16 low, fuel pull-over from the low-speed circuit will be negatively affected, especially when the engine is cold. To thatend all tuning should be done with the engine fully warmed up and the engine should be running a minimum of 180*F, with 195* preferred. If your cooling system has problems at 195; fix it. The warmer she runs the leaner you can run it and the longer she'll last.Well that's a bit misleading, due to it being a run-on sentence,lol. Warmer engines can be run leaner, And then the engine will last longer than if it was running too rich;how's that?
 
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What "values" are you speaking of. What is the A/F on cruise and after the RPM drop?
 
Stoichiometry at its finest.
But this is waaay over my head.
Interesting reading though.
 
AJ/FormS said:
Comments in the quote; click to expand


Ok well in your case
What is your idle vacuum, and at about what rpm does the vacuum peak? Stop if over 3600
I'm gonna guess it idles at 16 or more in Neutral, and peaks at about 2200ish
I usually start with the T-port sync first, but you're probably pretty close with 11/12 degrees of idle- timing. But Personally with the stock TC, I would slow the all in to somewhere just over 3000Rpm, But I don't think this is a contributing factor.
Outshifting this early, you'll probably be deeper into the throttle, so the PV system will probably be on line, AND the VA will probably be off-line.
What rear gear are you running?
The metering rods are your PV system. So at this point I would set the mixture screws to 1.5 to 2 turns out, and reduce the in-gear idle rpm by about 50 if required.Then get a longer vacuum hose and take her for a spin with the VA disabled, and the carb-port plugged.I usually use a wiper blade to hold the gauge in place at these low speeds. Notice the max vacuum just before the shift, and what it drops to.
Assuming no change,and with vacuum readings above 12 inches,I would remove the PV springs completely, and retest. This might make the car somewhat difficult to accelerate to the upshift speed so just take it easy until you get close, then level off at the previous vacuum reading as the shift approaches, and let her do her thing. Ok so no change means the mains are lean. You've proved it. Now it's just a simple matter swapping in rods with a smaller cruise step or a bigger MJ.
But before you do, I would verify that the fuel level is correct and that the fuel is fresh if it's oxygenated.
The fuel level is very important to low rpm/low throttle settings.If the level is even just 1/16 low, fuel pull-over from the low-speed circuit will be negatively affected, especially when the engine is cold. To thatend all tuning should be done with the engine fully warmed up and the engine should be running a minimum of 180*F, with 195* preferred. If your cooling system has problems at 195; fix it. The warmer she runs the leaner you can run it and the longer she'll last.Well that's a bit misleading, due to it being a run-on sentence,lol. Warmer engines can be run leaner, And then the engine will last longer than if it was running too rich;how's that?
Click to expand...

AJ - I'll have to open a lab and get my phD before I will understand all that. I'll be working on what you say above, when time permits.

crackedback said:
What "values" are you speaking of. What is the A/F on cruise and after the RPM drop?
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As I tend to stare at the AFR gauge a lot, the average of where it hovers in and around, at cruise from 40 mph to 50 mph is about 14.7 AFR. Cruise that is. As I'm accelerating, it is a bit leaner than that (not what I want), and then it shifts and hits the upper 16's to 17.0 or so for 1/4 to 1/2 a second.

Roughly, what I would consider my current Cruise AFR is 14.7
Roughly, what I would consider my current Power AFR is 15.7

The custom metering rods I'm trying to make will be designed to target the below:
Cruise AFR: 15.5
Power AFR: 14.0
Will I achieve that? I hope so. The mathematics are solid ... but here in the real world, things are complex. I'm hoping I'll get to do some work on the metering rods tonight (while my son rides on the bmx track and I'm hanging out there). I've got brass rods and will put them on a drill and hand sand down to my target diameter. It's bending the brass rod is where it will be difficult. The distance from the (main jet side) rod tip to the elbow is critical.


7milesout
 
What is happening is you are pushing the throttle so slight that you get no pump shot, or it is just barely dribbling so it goes a touch lean until the booster catches up. I suspect you could fix this with a very small diameter squirter, but that may induce a huge stumble when mashing the throttle.

You will end up in a rubber room sucking your toes if you live and die by the A/F meter. The lean spike is so short, for a short duration with very little load it is incosequential.
 
yellow rose said:
What is happening is you are pushing the throttle so slight that you get no pump shot, or it is just barely dribbling so it goes a touch lean until the booster catches up. I suspect you could fix this with a very small diameter squirter, but that may induce a huge stumble when mashing the throttle.

You will end up in a rubber room sucking your toes if you live and die by the A/F meter. The lean spike is so short, for a short duration with very little load it is incosequential.
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Hahaha, aint that the truth?:D
I think you hit it on the nose about the pump shot, as I drive pretty lightly most of the time, and my 1406 does the same thing unless I step on it enough to get the pump shot.
I notice zero difference in the motor running and power when it does it, but I did quit looking at the A/F unless something feels different.
(Like when the car hesitated one day for no apparent reason and the A/F was up around 19)
I got an extra fuel pump on my way home that day and the one in the car died the next day.:D (It was a cheapie electric) and the new one is a Carter rotary.
 
I like what YR said.
I would just put the next bigger MJ in it and start over; you're too lean on the powerstep anyway. Them maybe you can back up on the springs
 
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I'll put bigger main jets in it when I have my custom metering rods ready.
 
It's one or the other not both.

You need to calculate the net jet size with the current combo, add say 5% and recalculate the jet size to use the same rod. OR recalc the new rod for the existing jet-size.
Yes you can do both if you know what your doing and also recalc all the other steps to estimate what is going to happen there.
Since you seem to be lean everywhere, I would just skip all that math and slam the next bigger MJs in there and roadtest it. Fine tuning afterwards with rods only takes a few minutes per change.
 
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AJ/FormS said:
It's one or the other not both.

You need to calculate the net jet size with the current combo, add say 5% and recalculate the jet size to use the same rod. OR recalc the new rod for the existing jet-size.
Yes you can do both if you know what your doing and also recalc all the other steps to estimate what is going to happen there.
Since you seem to be lean everywhere, I would just skip all that math and slam the next bigger MJs in there and roadtest it. Fine tuning afterwards with rods only takes a few minutes per change.
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I'm beyond your method. I've developed my own tuning spreadsheets. I know what every available rod and jet combo will do. I've changed rods and jets enough to confirm that I know what I'm doing as far as idle, power mode, cruise mode, and WOT. And I have the AFR gauge to confirm. That's why I'm making my own custom metering rods. I'm actually going to get a pair of my custom rods 3D printed next week. And run them long enough to confirm. Then figure out how to get a permanent set made. So yes, I can change rods and jets at the same time.

And I'm too rich on cruise (barely) and too lean on power. My custom metering rods fixes both along with a bigger main jet.

My problem is the lean spike. The AFRs I have are solvable for me. I'm just in a holding pattern waiting for my metering rods.


7milesout
 
.. the average .. at cruise from 40 mph to 50 mph is about 14.7 AFR. Cruise that is. As I'm accelerating, it is a bit leaner than that (not what I want),
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Hard to beleive, but what it is currently doing is exactly what the engine wants for best efficiency. That is, if you can trust testing by the folks who developed engines for a living. :LOL:

and then it shifts and hits the upper 16's to 17.0 or so for 1/4 to 1/2 a second.
Click to expand...
This is typical when the load on the engine gets very light.

Roughly, what I would consider my current Cruise AFR is 14.7
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That is low speed cruise. The carb, its still partially on the "idle circuit", although some of the Carter designs may be getting further onto the mains at those cruise speeds. Get up around 60-75 mph to be sure its fully on the mains.
Roughly, what I would consider my current Power AFR is 15.7
Click to expand...
That's confusing terminology, call it part-throttle power. Power is when throttle close to 100% open for full acceleration or very heavy load. The importance of this differene will be seen in the graphic.

Attaches is a graph showing the relationship of AFR to LOAD for a gasoline engine.
LOAD in our situation is revealed by throttle position and vacuum. Low vacuum indicates throttle is open wide and the engine pulling hard (it can spin up fast because of the load).
AFR is gasoline, but in these tests it is was recorded aast Fuel Air Ratio. The math is simpply inverse the number. AF = 1/FA

Here's the explantion for why the graph is shaped like a hook. If its too much right now, then just focus on the shape of the graph. Later you can come back to the combustion chemestry if you are so inclined.
"The thing to understand is most engines respond to being leaner than stoichiometric at part-throttle because the lean exhaust gas has hot unburned oxygen, and hot oxygen improves combustion"

In summary "at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT." The load where richer is needed varies with engine, gearing and vehicle. It may be 60-70% as shown here, or as high as 90%. That's where the step up or power valve vacuum comes into play.

"This ... graph is from Walter B. Larew, Carburetors and Carburetion At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967. ​

He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications."
quotes and graph from Tuner on Innovate Motorsports Forum. ​

blob.jpg
 
Mattax, I've seen that graph before. In the information I've read, and what I've experienced with this vehicle's engine, is that best efficiency will be at leaner than stoich. And this engine acclerates good until somewhere near 16.0. Don't get me wrong. I don't want to accelerate at that AFR. But since it will acclerate at that AFR, then it will make good cruise hp at 15.5 and at that light load (cruising), there should be no high temp danger cruising at 15.5.

I want to accelerate at 14.0. As for my definition of Power Mode, I don't know if I'm correct, but this is my understanding from reading the Edelbrock carb manual. Power Mode is accelerating briskly without opening the secondaries. And simultaneously when the rods are high pulled up in the jets. What you describe is WOT to me, because you mentioned 100% throttle.. I can see on the AFR gauge when the secondaries open. So I can judge Power Mode AFRs by acclerating just under where the secondaries open. When my secondaries open, it goes very safely rich and accelerates very nicely.

Actually WOT is easy to set. Once the cruise and power mode are set, the secondary main jet is changed to lock in the AFRs or base it on trap speed. AFRs are easier. But my WOT AFRs right now, while they're ok, they could be better. But my main jet and metering rods are about to change and that will affect my WOT so I haven't finalized my WOT tuning.

AJ/FormS - My original post could have been clearer. My goal is to UNDERSTAND the lean spike. The AFRs probably got started talking about because I'm not great at being clear. But AFRs are an interesting topic.


7milesout
 
Secondaries should open on airflow demand, not power demand.
In other words - high rpm, part throttle would start the secondaries with no change in afr from the two barrel at the same circumstances. For example 3000 rpm on an interstate going over the Alleghenies or some similar steep hills.
 
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