accelerator pump fuel feed?

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Kala1984

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Quick question regarding an 01 GTS with the Mikuni BN-40i-38 with the accelerator pump. From what I can see the fuel must go from the fuel pump through the body of the carb to the needle valve and from there through a hole in the metering chamber to the little hose that feeds the accel pump. If the needle valve will only open under the prescribed PSI its set at which only occurs at higher RPM's, how does the accel pump have fuel for starting before the engine is even running and the needle valve is closed?

Either I don't understand the fuel route to the accel pump, or that once the system is primed, the accel pump will "pull" fuel through the needle valve when you pump the accelerator? Thanks for any clarity on this. I ask because my accel pump stopped squirting and after careful inspection and rebuild it still is not squirting even though the fuel pump is working fine.
 
The accelerator pump actually pulls from the same chamber under the diaphragm as the kidney shaped aluminum block. So if you pump the throttle without the ski running you will actually deplete the fuel in this chamber pretty quickly It won't refill until the fuel pump is pulsing and the diaphragm opens the needle.
 
The accelerator pump actually pulls from the same chamber under the diaphragm as the kidney shaped aluminum block. So if you pump the throttle without the ski running you will actually deplete the fuel in this chamber pretty quickly It won't refill until the fuel pump is pulsing and the diaphragm opens the needle.

Yes, that is what I meant. The whole metering block chamber including that kidney shaped piece are only fed by the needle valve, right? So is the only way to prime the accel pump by running the engine?
 
Basically yes, cranking it over with the choke on can also increase the vacuum on the diaphragm and get fuel in there also but it will probably start.
 
So is the purpose of the choke to increase vacuum in order to open the needle valve and let fuel flow, or is there a different source of fuel during start and idle that does not involve the needle having to open, and the choke works the same as on a lawnmower just restricting airflow to increase vacuum and fuel/air ratio? I thought the needle only opens at mid rev range when pop-off pressure is achieved? Thanks for clarifying this for me.
 
Choke allows more fuel in and less air but also increases the vacuum in the carb.

Pop-off is just a test and isn't haw the carb actually functions when running.

When the engine is turning over there is a vacuum created in the body of the carb......this is less than the outside air pressure...the outside air pressure actually pushes the diaphragm down and depresses the lever arm... overcoming the spring pressure and finally lifting the needle off the seat and fuel flows in. You get a little help from fuel pressure but the actual opening is from the diaphragm pushing down the lever arm.
 
If the needle valve will only open under the prescribed PSI its set at which only occurs at higher RPM's, how does the accel pump have fuel for starting before the engine is even running and the needle valve is closed?

Either I don't understand the fuel route to the accel pump, or that once the system is primed, the accel pump will "pull" fuel through the needle valve when you pump the accelerator? Thanks for any clarity on this. I ask because my accel pump stopped squirting and after careful inspection and rebuild it still is not squirting even though the fuel pump is working fine.

Engine vacuum pops the metering needle off it's seat, at high RPM it's fully off it's seat and low speed it will be bouncing on and off a little. The accel pump gets fuel from this metered fuel source. The reason for this is so the accel pump isn't exposed to positive tank pressure, which might force fuel through the accel pump check valves into the carb throats.
 
Ah, I finally understand. Makes sense. So even turning the engine over with the starter should create enough vacuum to allow fuel flow through the needle to start.

Separate question: anyone know approximately how low the reserve pickup is in the fuel tank (or how many inches of fuel from bottom of tank is reserved for reserve) and if you "run out of gas" with fuel valve in the "on" position, would you have to re-prime the carb and fuel lines after switching to reserve? I am asking if it will it run dry or does fuel simply stop flowing causing the engine to quit but the lines are still full? Thanks again.
 
If everything is primed and full of fuel except the chamber then cranking it should get fuel into the chamber.

For reserve there is a long and short tube in the tank, the short one is for ON and when it is out the fuel system will be drained and suck air. Once you switch to Reserve it will take some cranking to get it primed again but it will eventually on it's own. The 2001 GTS has a 15 gallon fuel tank including 3 gallons of reserve.
 
Ah, I finally understand. Makes sense. So even turning the engine over with the starter should create enough vacuum to allow fuel flow through the needle to start.

As long as pop spring pressure isn't too high, there will be enough vacuum while cranking to lift the metering needle. It may even be necessary to use the choke to enhance the vacuum for additional fuel.

Once you hear the engine fire, you're getting fuel so be gentle with the choke to avoid fuel flooding.

If pop spring pressure is set too low, the metering needle might be pushed off it's seat too easily by positive pressure in the fuel tank, this would flood the engine with fuel.
 
I think there might be a little confusion her or maybe it's just me or maybe we are saying the same thing.

To clarify the fuel doesn't actually push the needle off it's seat. The fuel pressure created by the fuel pump can only reach a set pressure because any excess pressure is bled out of the return hose, this is why the return fitting has a restrictor orifice properly sized in it. Some engine builders will even install carb jets inplace of the factory orifice so they can tune their inlet fuel pressure. Typical pressure is 3-4 psi at idle and 5-7 at WOT.

Fuel tank pressure can only reach 3-5 psi and the minimum pressure the largest seat and lightest spring can hold 12 psi so the fuel pressure in the tank will not flood the engine. On the flip side the highest spring pressure and seat can be combined to equal 55 psi which the fuel pump wouldn't be able to overcome.

What actually moves the needle off the seat is the diaphragm pushing down on the arm, compressing the spring and allowing the arm to remove the spring pressure off the needle opening the seat.

It acts exactly the same way a float carb works in that the float can easily close the needle against the fuel pump or your car would flood the entire time it is running. The diaphragm acts like the float and moves in when the carb throat air pressure is less than the outside air pressure. THe beauty of the diaphragm carb is that it works in almost any position and is less susceptible to rough operating conditions.
 
Yes, I get it. Its the atmospheric pressure on the diaphragm reacting to vacuum in the carb that opens the needle and not fuel pressure.
 
The fuel pressure can lift the needle off the seat if pop spring tension is in left field. About the lowest pop pressure I've seen is 10psi, I think that's about minimum to keep a tank of fuel from emptying out onto the floor?

Here they talk about the vapor pressure of gasoline:

"The 1990 Clean Air Act (CAA) Amendments added a new section, 211(h), that prohibited the EPA from establishing a volatility standard more stringent than 9.0 pounds per square inch (psi) in an ozone attainment area, except that the EPA may impose a lower (i.e., more stringent) standard in any former ozone nonattainment area that has been redesignated to attainment. "

Gasoline Reid Vapor Pressure | US EPA
 
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Just in case the OP needs an idea of why he is still not getting squirts after rebuilding a-pump, it is common for the brass nozzles in each carb to get clogged. Take the a-pump fuel line off nipple of each carb and blow compressed air through. If it’s really bad you can shoot a little carb cleaner through.
 
Just in case the OP needs an idea of why he is still not getting squirts after rebuilding a-pump, it is common for the brass nozzles in each carb to get clogged. Take the a-pump fuel line off nipple of each carb and blow compressed air through. If it’s really bad you can shoot a little carb cleaner through.

^ This! And don't attempt removing the nozzles, too likely to break off.
 
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