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· T25 powered R1
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Discussion Starter · #1 ·
Good write up even though carbs are a thing of the past. Full article at: http://www.drpiston.com/Cvcarbs.html

CV (Costant Velocity) CARBS
Both Hitachis and Mikunis are CV carbs.
In respect to these carbs, I'll be talking about the intake side, and the exit side. Air comes into the intake side and exits out of the engine side as fuel mixture.
The CV Carb has a more complex air control system than the two carbs described above:
---The butterfly valve is back, and sits toward the engine side of the carb. It is opened and closed by means of the throttle and throttle cable and controls the amount of air that can flow through the carb.
---But the slide is retained. It sits in the middle of the carb on the intake side, before the butterfly. But instead of being pulled up and down by the throttle cable as in the slide carb, it now has no direct connection to the throttle cable at all. It is now attached to a rubber diaphragm and is raised and lowered by vacuum (depression) introduced on the top side of this diaphragm through holes drilled up through the slide. The slides in Hitachis are round, and in Mikunis they are flat.
Now we'll try to figure out how CV carbs work.
When the butterfly valve is closed, very little air is moving in the carb bore. (The engine is getting some air and fuel through the pilot circuit, which we'll describe later.) With little to no air flowing, the air in the carb bore and the air in the closed chamber above the diaphragm are at close to the atmospheric pressure of the outside air.
Open the butterfly, and several things happen.
1. Air now speeds through and venturi effect (depression) at the point of the slide (variable venturi) is created.
2. The depression at the venturi is transmitted up through the holes in the slide to the closed chamber above the diaphragm. This lowers the density of the air in that chamber.
3. The open air below the diaphragm now wants to rush into that chamber to equalize the pressure, but it can't because there is no passage.
4. So it does the next best thing and tries to push its way in through the underside of the diaphragm.
5. The diaphragm can't let the air in, but it is flexible so gives way it is pushed up by the outside air pressure.
6. As it goes up, it pulls the slide with it, and the slide pulls the tapered fuel needle up in the fuel hole.
7. More air flows, more fuel is pushed into the air stream, and the engine accelerates or runs at higher revs.
But how does this improve things over the simpler slide carb?
When the throttle is cranked on the slide carb, the slide is pulled up immediately by the throttle cable, expanding the variable venturi suddenly, and causing the lean stumble described above.
When the CV butterfly is opened, the slide does not immediately jump up to a much more open position. It raises gradually as the increasing engine revs provide the needed depression (at the venturi), which is then transmitted to the chamber above the diaphragm. As the slide rises, the increasing depression also encourages more fuel to enter the carb bore and combine with the greater air supply now available. And the higher the slide goes, the more fuel the tapered needle permits to flow. In other words the genius of the CV carb is that the fuel from fuel hole can now "keep up" with the increasing air available--maintaining the mixture at proper ratios during the accelereation process.
In summary, the CV carb provides quick enough acceleration (no lean stumbles to slow things down) which is also smooth. And overall we get a "kinder, gentler" carb which gives us less twitchy responses as we make small throttle adjustments.
Now we'll get into:
(Article cut very very short to allow to be posted on the R1-forum.com)
This circuit takes its "airbleed air" from the main air jet, and its fuel from the float bowl. The fuel travels up through the main jet, and is pre-mixed with air from the main air jet in the needle jet (called by Yamaha in the Hitachi years the "main nozzle"). This needle jet is a long jet with holes in the side to permit the air to enter and be mixed with the fuel--before this mixture plumes out into the main bore to be further atomized as it heads to the cylinder. The tapered metering needle rides up and down in the needle jet and meters out more fuel the higher it goes. At wide open throttle (WOT) the slide and needle are fully raised and the needle is effectively "out of the way" in the needle jet, allowing maximum fuel to flow into the carb bore, regulated only by the size of the main jet.
When does the needle taper cut in? If you put a digital caliper on a Hitachi needle, you will find that it does not taper for the first 3-4 millimeters residing down in the needle jet. So presumably until the slide raises more than 3-4mm, we are still in "pilot circuit county", since with no taper the needle jet is pretty well filled with the needle and little additional fuel will be pushed out that hole. Once the taper cuts in additional fuel starts to flow out and this progressive metering continues until the much steeper taper of the needle drops it out of the game as WOT is approached, and the main jet becomes the only restirction..
Note that jet changes typically involve the main jet, and to a lesser extent the pilot jet. In these carbs we never seem to get into the air or needle jets.
An interesting thing to do is to make some marks on your throttle showing its position at 1/4, 1/2, 3/4 of rotation and WOT. You'll be surprised to see how little the butterfly is actually open at steady cruising speeds. You'll see some serious throttle rotation on brisk acceleration, but just try to maintain a steady speed at high throttle openings on the freeway. This will get you too much speed or too many speeding tickets, whichever comes first.
So those are the three circuits. Half the trick is diagnosing carbs is to figure out fuel, air and outlet paths for the various circuits. Note there is a degree of independence between the pilot circuit, the needle/jet, and the main jet. For example, we are told that the engine will start and idle on the pilot circuit with no needle or main jet in the carb at all (I've never tried it).. Also, the engine will in theory run up to around 3/4 throttle (mixture being controlled by the metering needle) with no main jet in place. (Not to be tried with Hitachis, since the main jet screws into the needle jet and holds it in place.)
As we have now seen, the CV carb needs the presence of outside air (at atmospheric pressure) inside the carb for several reasons:
--Outside air is needed under the diaphragm to push it up.
--Outside air is needed above the fuel in the float bowl to push down on the fuel and force it up past the various jets and into the starting, pilot and run circuits.
--And outside air is needed to service one or more of the air jets that reside inside the carb body..
How does this outside air get in? It gets in through the breather pipe which sits toward the top of the carb just under the diaphragm. Air jet(s) also reside in this space and have access to it. And drillings in the carb body allow the outside air to go down and enter the space above the fuel in the float bowl.
A breather tube typically attaches to the breather pipe and extends to some point (e.g., behind a side cover, or inside the aircleaner pod) where the air is relatively calm. Why? We don't want wind to be changing the airpressure at the end of this pipe, because this will disturb the operation of the air jets, the diaphragm, and the fuel delivery. These functions need consistant outside air pressure to work properly against the various levels of depression created in the bore, above the diaphragm, etc., as the carb goes about its work.
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