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Hypoth-eor-etical Airflow Question

stl360+450

stl360+450

Veteran Member
Joined
Aug 6, 2021
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Location
St. Louis, Missouri, USA
Suppose one chose to install automotive air filter paper on stock CB450 air boxes and said material was more restrictive of air flow than the original filter medium. How might this manifest itself in the function of the stock CV carburetors? (You may assume Keihin 723A with aluminum vacuum pistons in this particular case.)

The obvious answer is that the bike would run rich, but I'm more curious to hear theories about how the reduced flow could affect the function of the vacuum pistons.

I ask because I have unwittingly conducted experiments of this nature and am trying to make sense of the preliminary findings.
 
In my experience, greater restriction ahead of the carb on the intake tract causes a rich condition. Since CV carbs are involved and I'm not the carb guy, I could easily be wrong.
 
ancientdad said:
In my experience, greater restriction ahead of the carb on the intake tract causes a rich condition. Since CV carbs are involved and I'm not the carb guy, I could easily be wrong.
Click to expand...

Actually I should have said the obvious answer would be rich. So I'll fix that now. You're absolutely right that it would act almost like the choke (in the extreme).
 
A restricted airflow prior to the carb will increase the vacuum. The engine is trying to pull in X amount of air that it needs. The stock filter element allows X, or higher, amount of air past so vacuum in and at the carb is able to function normally. When the supply air is reduced the vacuum increases due to insufficient supply.
This creates 2 issues. 1) the piston will rise earlier than intended and 2) the increased vacuum will draw more A/F mixture out of the jetting.
 
LongDistanceRider said:
A restricted airflow prior to the carb will increase the vacuum. The engine is trying to pull in X amount of air that it needs. The stock filter element allows X, or higher, amount of air past so vacuum in and at the carb is able to function normally. When the supply air is reduced the vacuum increases due to insufficient supply.
This creates 2 issues. 1) the piston will rise earlier than intended and 2) the increased vacuum will draw more A/F mixture out of the jetting.
Click to expand...

The piston rising earlier goes against my intuition.

Doesn't the piston rise due to the increased pressure as the air flows through the [restricted] venturi towards the lower pressure on the engine side of the carb? If there is less air flow, it seems to me that the piston would rise later, since the motor will have to pump faster than normal to pull air at the rate required to lift the piston.
 
When and how much the piston of the CV carb rises is indeed due to the pressure differential between the pressure in the cylinder (and engine side of the carb piston) and the atmospheric pressure on the air cleaner side of the carb piston....
This is why the "straw test" works... Blowing into the port raises the pressure at the "air cleaner/underside of the diaphragm (or piston) with respect to the at atmospheric sitting above the diaphragm/piston (on/from the engine side of the slide)....The piston rises, but only until the two pressure areas equalize.....
Now to the question.....
Yes, with restricted air coming in the piston does rise sooner and momentarily higher (greater differential) until it can equalize, but the lesser number of air molecules passing over the needle jet lifts less fuel.... However, the higher atmospheric pressure above the fuel in the bowl during that interval pushes more fuel out the now wider/larger jet opening and is primarily responsible for the richer condition....
Also, a fuel/air mixture that is not as well atomized may not fully burn and the unburnt fuel droplets will eventually accumulate enough to foul the plug.....
 
Thank you, Steve and Jim, for your explanations. These CV carbs were well designed and, apparently, I have some more studying to do before the test. I'm going to invest some more time looking at the diagram below in the mean time.

hNt5GSk.png
 
The piston rises due to the difference in pressure between the intake port and the drop in pressure created under the piston from the air flowing through the venturi (thanks Bernoulli). Because the air flow increases slightly sooner than the pressure drop to the intake, it may rise a little sooner than with the normal restriction, resulting in a somewhat richer mixture, until the air filter pressures reach steady state. While increased restriction in the air filter may exaggerate this effect a little, the biggest change in mixture is due to the less dense air, as the increased restriction simulates running at a higher altitude. Because the float chamber is vented outside the air filter, the difference in pressure can also add to the extra fuel in the mix.

Have you thought about putting more square inches of filter media to reduce the overall restriction? Or, is the filter already too packed?
 
WintrSol said:
Have you thought about putting more square inches of filter media to reduce the overall restriction? Or, is the filter already too packed?
Click to expand...

Thanks, Rick. Your explanation helps as well. I do think I could increase the area of the filter medium since the pleats are not as densely packed as on the OEM filter. Here's what my rebuilt air filter looks like.

WYQY1wa.jpg


The funny thing is that I felt one of the pistons sticking (left) and I could feel it rise suddenly as I gradually managed to increase RPM, which would provide a jolt of power. If I held the throttle in place at that point I could ride "on top" of the obstacle, but it seemed that whenever the piston dropped I would have to overcome the obstacle again.

When I removed the air boxes completely, I could see the pistons rising on both sides with increasing RPM (no load) and the bike felt much better when I rode it up and down the street. I then put on Uni foam filters (pods) and it still felt good. This made me begin to suspect the air boxes.

I plan to go through the carbs again to make sure all of the air orifices are clear, especially the one shown in the innermost portion of carb top. I suppose it's possible that I need to look for pistons/tops that are in better shape than what I have.

I appreciate everyone's responses - they have been very helpful.
 
You could plumb a line into the air cleaner tract, connect it to a vacuum gauge and see how much,if any, restriction there is.
 
MrPepper said:
Did you not want to use a strip of uni foam instead of paper?
Click to expand...

It wasn't so much about not wanting to use foam. I was just trying to understand the behavior I was seeing with the slides. In retrospect, I think my slides were not moving freely enough and it was probably not related to the air filter medium. I've cleaned them a couple now times and they're moving better. I'm also running Uni foam pods for the time being.

mike in idaho said:
You could plumb a line into the air cleaner tract, connect it to a vacuum gauge and see how much,if any, restriction there is.
Click to expand...

That could make for an interesting experiment when I have time to try it.
 
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