air flow question

[JPSmit]

well engineers here is tonight's question

Recently we had the discussion about my car running rich. One suggestion was to mask of part of the air cleaner. I have done this and it has improved matters to the point that I now actually have to use the choke to start it cold.

Was thinking today. Theoretically we could figure out the flow rate of an OEM aircleaner. For the sake of simplicity, (without worrying about different vacuum rates) the two main variables are diameter of air cleaner opening and flow rate of filter. Likewise we should be able to calculate flow rate of Longflo foam and opening size (large) On the basis of that it should be possible to calculate how much of the Longflo to block off to compare to the OEM.

Any ideas how to actually do this?

 

[bthompson]

Yep...

trial and error.

(Easier math.)

 

[aeronca65t]

One suggestion was to mask of part of the air cleaner. I have done this and it has improved matters to the point that I now actually have to use the choke to start it cold.

See now, John-Peter, that doesn't make any sense. If you blocked off the air filter and made things richer, you should need the choke even less. The choke makes things richer, but that what you've already done.
The only reason that the choke might help starting after you've done this is because it might "crack" the throttle a bit. You could do the same thing by slightly stepping on the gas.

Anyway, the idea of mathematically calculating how much of the filter to block off would be real hard to resolve (Fluid Dynamics, anyone?). And keep in mind that the standard air filter isn't designed to restrict.....just filter. Assuming all was as it should be, your engine shouldn't really run much different with a clean air filter as opposed to having <span style="text-decoration: underline">no</span> air filter.

First: Be real, real sure that your float level is correct. A low float level will make the car run lean.

Then....
A simple way to richen things is to make sure the needle on the damper piston is "pushed into" the piston a little extra.
See ~THIS~ diagram (upper right side) and note that the shoulder of the needle is supposed to be even with the flat area of the piston. Loosen the lock screw and push it in a bit (maybe a 1/16" or more) and see if that helps things.
Another way to richen things is to put the needle in an electric drill and "file it" thinner with a file. But this is not reversible if you mess the needle up (unless you have a spare needle).

 

[JPSmit]

One suggestion was to mask of part of the air cleaner. I have done this and it has improved matters to the point that I now actually have to use the choke to start it cold.

See now, John-Peter, that doesn't make any sense. If you blocked off the air filter and made things richer, you should need the choke even less. The choke makes things richer, but that what you've already done.

Here is where I'm reminded why I became a Pastor and not an engineer like my dad, and uncle, and cousins etc. etc. I was actually trying to get the car leaner not richer. Apparently (and this was beaten to death in this thread - https://tinyurl.com/bvpes7 ) - general consensus is that restricting the air richens the mixture - I think it's angels or fairies that do it, but everyone else seems to think that if you restrict fuel it will draw more air - lean/ and if you restrict air it will draw more fuel. - thus far the field research bears out the theory - about carbs that is not fairies and angels. :banana:

BTW did you send that receipt?

 

[aeronca65t]

Yes, we all have our strong points.

There's a reason I never became a musician
(you wouldn't want to hear me sing the Doxologies in church! :wink: )

I sent the stuff last week. I imagine you'll get it any day. I included some extra info you probably don't need and even photos just in case.
Belts and suspenders.

 

[bthompson]

Aeronica, you are right...<span style="font-style: italic">if </span>you're talking about a Weber, or any carb with a fixed venturi. You can push as much air as you want past the jet, but the needle won't permit excess gas. So, it'll run leaner with freer air.

<span style="font-style: italic">But</span>...

A carb with a variable venturi doesn't have a direct link between throttle position and needle opening. It's all done with air pressure. Decreasing the pressure from the air cleaner, will cause the piston to sit higher in the bore, which increases the area of the venturi, which increases the pressure across it, so it pulls less gas. I don't know why a Zenith is more susceptible to this than an SU, though. These carbs have to work a bit to pull the air to make them efficient.

 

[Morris]

It seems that a more logical (though more difficult) approach would be to either change the needle or reduce the spring pressure that holds the piston down (by cutting out pieces of spring). Thus, less vacuum would be required to lift the piston... thus leaner mix.

 

[JPSmit]

I don't know why a Zenith is more susceptible to this than an SU, though. These carbs have to work a bit to pull the air to make them efficient.

I suspect that the answer is, in part that we're all talking about carbs designed to have pollution controls on them - at a time when the whole concept wasn't very well understood. And we've pretty much all pulled off the pollution stuff.

 

[bigjones]

Bill,

I still don't understand this.
Could you start again with the following scenario:

1. The driver puts his foot on the gas.
2. The butterfly in the carb is therefore opened.
3. Vacuum (from the intake valve in the engine) is introduced into the carb.
4. The vacuum is directly proportional to the position of the butterfly.
5. This vacuum is what lifts the carb piston.
6. Now, if I have the above correct, what happens to the air/fuel mix, at a steady car speed, if JP has duct-taped half of the aircleaner.

Cheers,
Adrian
PS. It's not that I doubt your words but I was just looking for some clarification. Cheers!

 

[blkcorvair]

Im with everyone on this. Ive a car run better when A carb is clogged up that when you hold your hand over the air cleaner to restrict some flow it will in turn pull more gas. With a wide band sensor on my car I can pull the filter off. put it on, it really has very little effect on a properly tuned carb. Sound like your problem is else where.

In fact common practice is that when you install a high flow K&N filter you need Increase the Jet size to take full advantage of the increase in airflow. Thus aiming for a proper fuel ratio. Otherwise a higher flow filter will cause a leaner conditon.

 

[bthompson]

Hi, Adrian...let's see if I can scour my brains to help put the pieces together.

First, some basics...

The vacuum in the manifold, on the "other side" of the throttle butterfly, doesn't have a lot to do with the carb operation. On "this side" of the butterfly, it's all about the <span style="font-style: italic">amount </span>of airflow. A closed throttle pulls less air through the throat of the carb, and over the venturi, than an open throttle.

There's atmospheric air pressure in the float bowl, and atmospheric air pressure on the bottom side of the piston diaphragm. On the top side of the piston diaphragm, is the same low air pressure as is flowing over the venturi.

At idle, with the butterfly closed, there's raging vacuum in the manifold, but in the carb, only a small amount of air is pulling past the venturi. The piston is sitting low, with the needle deep in the jet. The small space for the air to squeeze by causes it to move faster, making a spot of low air pressure right over the jet. The gas is pushed out the jet past the needle. The piston has the low air pressure of the venturi above it, and it is pushed upwards--just a little--by atmospheric pressure.

Now, stomp on the gas. The butterfly opens up, and the manifold vacuum drops as the engine breathes deep. In the carb, a <span style="font-style: italic">much </span>larger volume of air is forced through the carb throat and through that tiny space over the venturi. The air has to move much faster to make it past, and the air pressure over the jet plummets. A large amount of gas is pushed out the jet into the airstream (despite the needle still being deep in the jet). Meanwhile, that huge drop in air pressure is also over top the piston, and atmospheric pressure tries to force the piston upwards. It can't, immediately, because the dashpot oil slows it down. That gives you the extra shot of gas for acceleration.

The piston is slowly pushed up, because the low air pressure over the venturi is also atop the piston, relative to the high atmospheric pressure under the piston diaphragm. The space under the piston opens up. All that air isn't squeezed through the venturi as quickly, since the venturi is bigger now, and the speed of the air over the jet slows down. With slower speed comes higher air pressure, and the gas isn't pushed up the jet as vigorously. The needle, mounted on the bottom of the piston, is further out of the jet, so the gas can flow easier, even with the increased air pressure. This increased air pressure is now also over the top of the piston, remember, so it stops rising.

The results of this ballet dance is that the piston always finds a spot where the volume of air flowing past is balanced with the speed through the venturi, so --get this-- the actual venturi <span style="font-style: italic">pressure</span>, whether idling or running WOT, is <span style="font-style: italic">always the same!</span>

To use an electrical analogy, airflow is Amps, venturi speed is Volts, and the piston keeps the Watts constant. More amps, fewer volts; less amperage, more voltage.

When you let off the throttle, the butterfly shuts, and the volume of air through the carb drops dramatically. The wide-open venturi now has so little airflow through it, and thus such high pressure, that the piston drops like a rock. The space under the venturi squeezes down until the air speed through it is fast enough to drop the pressure again. The piston balances that pressure up top, and holds its position so that the constant pressure over the venturi is maintained. Ain't it a beautiful system?? :banana: Just a handful of moving parts and the laws of physics.

Here's where blocking off the air cleaner comes into play. Restricting the intake decreases the pressure slightly through the carb throat. (It's the same thing that happens in the manifold when the throttle's shut, but on a <span style="font-style: italic">much </span>smaller scale.) The airflow through the carb would then start out as <span style="font-style: italic">slightly less </span>than atmospheric pressure. Remember, the piston has atmospheric pressure pushing it up. So, when the air, already slightly low pressure, runs through the venturi, the venturi pressure would be slightly lowered too. The piston would then be pushed up slightly higher throughout all the engine's range, as the piston maintains that same low pressure--<span style="font-style: italic">lower pressure than it would have been if the air cleaner were free flowing</span>. The higher piston opens up the venturi, slowing the airflow through it. The gas isn't pushed out the jet as easily, and --voila-- the engine runs leaner at all speeds.

It is certainly possible to fit different jets and weaker piston springs to compensate for the increased pressure: basically you're resetting the carb balance. Nothing wrong with that. But restricting the intake does the same thing. Ever wonder why SU air cleaners have those teensy tiny snorkles...?

Whew! :tired: I can try to find/make a diagram, too, if that'll help out any. :thumbsup: Sorry for being so long-winded.

 

[bigjones]

Bill,

Many thanks for taking the time to write all that down! You have got me convinced.

I will have to do some experiments with duct tape- my ZS has a 3" deep K&N but I cannot get it running weaker. Judging by the plugs I think it may be beneficial to get it a bit weaker. I'll report back.

Thanks again.

Cheers, Adrian