Lean condition under light acceleration

[zakimodo]

Hi All
I am having this issue with a Thermoquad but assume that the method for addressing it is similar for any carb using metering rods. I have done some searching (maybe just not hitting the right keywords) and have not seen much on this topic. If there is a thread I have missed, please point me to it.

The lean condition is encountered under light acceleration or while engine load increases at constant throttle (climbing a hill).I have a wideband in the car which is aiding in tunning the carb. In addition to the gauge indicating a lean condition (16 to 17 AFR), I can feel it in the car as well.

Currently, the AFR is good while at cruise (13.7- 14.5, I could probably richen this up a bit) or under WOT (high 12's at the moment). I also have the secondary transition pretty much dialed in.

 

[Mattax]

Hi All
I am having this issue with a Thermoquad but assume that the method for addressing it is similar for any carb using metering rods. I have done some searching (maybe just not hitting the right keywords) and have not seen much on this topic. If there is a thread I have missed, please point me to it.

The lean condition is encountered under light acceleration or while engine load increases at constant throttle (climbing a hill).I have a wideband in the car which is aiding in tunning the carb. In addition to the gauge indicating a lean condition (16 to 17 AFR), I can feel it in the car as well.

Currently, the AFR is good while at cruise (13.7- 14.5, I could probably richen this up a bit) or under WOT (high 12's at the moment). I also have the secondary transition pretty much dialed in.

It should get leaner with more throttle.
WHAT!
Yup.
Don't feel bad.
You, me, and most every hot rodder and wanna be hot rodder get the idea that the mix should get richer with more throttle.
It's only partially true. At idle (warmed up) engines generally want a somewhat rich mixture. And at wide open throttle with full load, engines require a somewhat rich mixture.
In between engines run best when provided leaner and leaner mixtures.
'Best' here being defined as engine running most efficiently in terms of torque and power.

If your Thermosquad is from a factory application fuel mixes at idle were targeted for leaner AFR's than would be used be used for most mechanical efficiency. The first targets in pollution reduction was idle and decelleration. Running leaner idle required slightly higher idle speeds and loss of fuel efficiency in return for big reductions in CO and leftover Hydrocarbons. Leaner again being relative. Idle ratios of 14 to 14.2 were leaner than non-emissions tunes which typically were in the 13:1 range.

 

[dano]

Just remember the wide band is reporting what just happened (lagging indicator) and not what could or is happening (unless you hold a steady state position to maintain the issue). So you need to look at what is going on just before it goes lean. I don't know anything about Thermoquads but with the light movement you may need to adjust what brings fuel in to account for the change.

 

[Mattax]

My suggestion: Tune for best performance. In other words don't worry about the AFR number, much less aim for any AFR number. Use the AFR to see the effect of changes to carb under various conditions.

This is a cut and past but worth reposting -

If you're just going for engine efficiency the general trend of AFR can be described as follows:

"The [relationship] is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or" somewhat more. Cruising, especially at lower speeds, requires little throttle and puts relatively little load on the engine. Half throttle or mid-load would be going up hills or part throttle acceleration. "The thing to understand is most engines respond to being leaner...at part-throttle."

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 below, or as high as 90%. Load relates to manifold vacuum, and therefore vacuum is used to signal when enrichment is needed. This enrichment is created by the "step up" lifting the rods on a Carter type carburetor and the 'power valve' opening on a Holley type.

F is Fuel/Air ratio for gasoline. Invert the numbers to convert to Air/Fuel ratio.
0.08 = 12.5 AFR
0.667 = 14.7 AFR
0.06 = 16.6 AFR
Constant power is any steady throttle condition.
Steady 15% might be something like driving 40 mph on a flat.
Steady 25% might be cruising 60 mph on flat or steady 40 mph up a long grade.
Steady 100 % might be something like towing or dragging max load up up a really steep long grade, foot to the floor, without losing or gaining speed.

The acceleration loop shows that maximum acceleration (full throttle) has about the the same fuel-air needs as constant 100 % power.

"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.
This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle."

quotes and graph from Tuner on Innovate Motorsports Forum and full text now reposted at RFS
note. If you follow the link to RFS and tapatalk requests you join, just click the x, and ignore the request. You do not have to join to read the forum.

 

[AJ/FormS]

The metering rods are adjustable.I think the cruise step is tapered;just crank them up or down as may be required. You may need a slightly richer cruise-step, or a slightly higher wet fuel level. I noticed years ago, mine ran better with a lil higher than stock spec.

 

[zakimodo]

The metering rods are adjustable.I think the cruise step is tapered;just crank them up or down as may be required. You may need a slightly richer cruise-step, or a slightly higher wet fuel level. I noticed years ago, mine ran better with a lil higher than stock spec.

View attachment 1715603074View attachment 1715603075

Yeah, I think richening the cruise up will be a good place to start. From some past fiddling, I remember a failed attempt at using Edelbrock metering rods that yielded awesome part throttle power but choked at WOT. It would have been delivering more fuel than my current setup at part throttle.

 

[Mattax]

It's too lean at "cruise" when the engine surges (revs and dies) driving steady at 60 mph or higher.
Nothing magic about 60 mph on a flat road, its just a ballpark where the main system takes over and is providing most of the fuel under a steady condition.

 

[zakimodo]

It's too lean at "cruise" when the engine surges (revs and dies) driving steady at 60 mph or higher.
Nothing magic about 60 mph on a flat road, its just a ballpark where the main system takes over and is providing most of the fuel under a steady condition.

I wasn't really clear about this in my original post. I am on the edge of that surge with the current setup. It's getting just enough fuel that it's not completely falling on its face, but it wants to.

 

[AJ/FormS]

The lean condition is encountered under light acceleration or while engine load increases at constant throttle (climbing a hill).I have a wideband in the car which is aiding in tunning the carb. In addition to the gauge indicating a lean condition (16 to 17 AFR), I can feel it in the car as well.

If you think about this, it will become apparent what is happening, and is actually an excellent test, because you can watch it unfold.
on the flat;
The metering rods are sucked down and everything is fine.
Your butterflies are somewhere up on the transfers.
As the load increases, vacuum drops, and the transfers should be supplying more fuel. But with the power-piston sucked down,sooner or later, the the circuit stalls and cannot keep up with the demand. So it goes lean.

You can sometimes prove this by examining where the mixture screws are set. A) At idle,if your rods are too far down, then you will have to go richer on the trims in compensation. B) If the rods are too far up at idle, the trimmers will have to be set leaner. Interestingly, set this way, the engine goes lean off-idle. Because the trimmers are always alive, and they are set lean..

What I sometimes do is;
Set the mixture screws in the center of their adjustment range. Then, put the throttle up on the fast idle cam at about 2000 rpm; exact rpm not important. Now, since you have an AFR sensor, using the powerpiston height adjuster, crank the rods up or down to get a good number.
Then see what happens back at idle.
You might have to go back and forth a couple of times. And you may figure out, that you can only get so and so close.
When this happens it is usually because your butterflies are not in the right place up on the transfer slot, at idle. So ,adjust the idle speed up or down, up should go rich, and down should go lean. When you think you are getting close, but the idle speed is outta whack, use timing to reel it in.
Then start over at 2000 on the fast idle cam.

If you haven't thrown a bunch of timing at it, you should get really close on the first go-round. Most guys try to run way too much timing, and the engine likes it, and there is nothing inherently wrong with lotsa timing...... it just makes setting the carb up a lil tougher .

I'll help you;
Make sure the secondaries are closed up tight but not sticking. And you gotta run; a PCV plumbed to the proper port on the carb, and if you have a brake booster, I run it off the common plenum, not a single runner. Make sure the Vcan is on the sparkport. Then;
with a stock 318/360 set your Idle-timing to 10/12 and see how close the first go-round gets you.

 

[Mattax]

As the load increases, vacuum drops, and the transfers should be supplying more fuel.

A/J I think you know better 'cause you do know this stuff.
Assuming the carb is pulling from the 'idle circuit', as vacuum drops, there's going to be less fuel moved from the bowl to the slots.

Also, assuming the carb is on the idle circuit, the power piston isn't going to matter much if at all. The restrictions that will matter will be the IFR etc.

I think, based on what's been posted, that these tests may be when the carb mostly onto the main circuits.
The question then is whether the rod jet combo is too lean, or whether with this engine the power piston needs to lift the rods at a higher vacuum.
Generally the more radical the engine, the sooner it will need to be supplied with enrichment. But it depends on efficiencies and fuel distribution so this is just very general.
If this is happening with manifold vacuum well above 11"Hg, then more likely that the rod jet combo needs to be richer.
If its around 11 or lower, then would try changing the springs so it steps it up sooner.

 

[AJ/FormS]

As the load increases, vacuum drops, and the transfers should be supplying more fuel. But with the power-piston sucked down,sooner or later, the the circuit stalls and cannot keep up with the demand. So it goes lean.

@Mattax
I thought you had me there for a second.

What I meant was that the transfers ought to be supplying more fuel, but they can't because the metering rod cruise step is the limiting factor.
I'm assuming that when OP hits the bottom of the hill, the carb is still on the transfers, cuz Op talks about "light acceleration" in the same sentence. So this is transitioning from a mid-point on the transfers to a higher point on the transfers with the power valve still closed, and not yet up on the booster.
I guess it could happen that with a less powerful engine than mine (low-rpm) Op's engine could be up on the mains; that thought just never occurred to me,lol.
But doing the test as I outlined IMO would prove it,one way or the other.
In any case, I get what you are saying. and
I could be wrong,lol.

The more I think about it, the more I think that I am wrong,lol.
But then the test would be invalid as well. Daymn, that hurts..

 

[Mattax]

You very well may be right that at the beginning of the incline its still on the transfers. 30 mph could be, 60 almost certainly not. In between I don't know - especially with a spreadbore which oguht to be on the mains sooner. ????

As far as the rod being the restriction, IF its on the transfer/idle circuit I think its not.
I've not measured Thermoquad restrictions but generally it seems that Carters, Webers, Holleys the IFR restriction is signifcantly smaller than the main restriction. The rule of thumb for two restrictions in series is if one has four times the area of the other, the large one has essentially no effect on the flow.

This isn't the best comparison but the only spreadbore I have pinned, which is a Holley.
Primary IFR .028 " dia => .0006 sq inches
Original Jet #62 .061" dia => .0029 sq inches

The Carter has to be in the same ballpark. The engine is consuming about the same air regardless of carb. The restriction for main system has to be sized so the same amount of fuel. The difference in main systems with a Holley 6213 spreadbore versus the Carter T-quad shouldn't be that much as both use small primary venturis and throttles. The T-quad's booster design should create more signal and I don't know about the air bleeds (or one year a solid fuel circuit was used?) - so its not quite apples to apples but its not going to be orders of magnitude different either.

Now there is one more little monkey wrench in the restriction relationship. Carter's 'low speed circuit' has a two air bleeds and two fuel restrictions.
Discussed a little on page 4 here Carter AFB-AVS Service Manual
And Tuner gives his take on them in a sidetrack here 3310 and Fuel mileage

 

[Mattax]

The more I think about it, the more I think that I am wrong,lol.
But then the test would be invalid as well. Daymn, that hurts..

If it hurts, I hope it just hurts a little. That's what good discussion is about, checking and challenging what we wrote and our analysis or reasoning. We sure ain't perfect or all knowing. I make plenty of mistakes and know my understanding of many of these things varies from pretty good to very basic, and sometimes still just wrong.

 

[zakimodo]

Update: I have a spare parts carb that I was using to chase the air bleed circuits (just to get a better idea of where the circuitry originated and terminated).
The primary bleeds on my parts carb where smaller than the one in the car so I decided to swap them. The result a slight enriching of the primay circuit overall but more importantly my lean spot clearing up.

I didn't get a picture of the work but these are the bleeds that sit up against the choke horn.

 

[RustyRatRod]

I just scanned through so forgive me if I am repeating anything. Here is your "base adjustment" for the metering rod tower.

Take the metering rod bracket and with your fingers lightly bottom it in its travel. Now, adjust the small screw in the center until the metering rod assembly is down as far as it will go. Next, turn the screw until you JUST see the metering rod assembly begin to rise. From there, raise the metering rod assembly by one full turn of the center screw. This is your base adjustment. From here, you can easily tune by raising (enriching) or lowering (leaning) the metering assembly. I would keep a log of how many turns you adjust each time (I would go either half a turn or a full turn) so that you have a record you can refer back to so that you do not have to make that base adjustment every time.

Again, I apologize because I cannot remember "which way" to turn the screw to richen or lean it. I THINK clockwise raises (richens) and counter clockwise lowers (leans) but I am not 100%. Just pay attention to that when you make your base adjustment.