Post by norfolknchance on Jan 28, 2013 19:40:05 GMT 1
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.
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.