150cc BMS Heritage fuel delivery problem?  By  william42 (Read 14,621 times)

tvnacman- you said in an earlier reply that "cranking vac and running vac are different". What did you mean by that. How are they different?

If, as alleyoop claims, that running vac is indeed lower than cranking vac, then I would have to agree that the floatbowl isn't being replenished fast enough to supply the need of the increased rpm's at full throttle because the amount of fuel coming from the fuel pump at idle was low to begin with, as shown in the video.

How I took the quote was that just because the fuel pump output was low at idle doesn't mean that it's low at running rpm's and that I shouldn't be concerned with the cranking rpm's as shown in the video. But now, your above reply seems to contradict your first reply. Unless I misunderstood.

The pulsing feature of the fuel pump is created by a spring and vacuum (supplied by the downward movement of the piston). A vacuum is created inside the fuel pump sucking fuel from the tank into the pump. When the vacuum is removed from the pump (the piston no longer in it's downward movement) then the spring returns the diaphram inside the pump to it's normal state forcing the fuel through the fuel line to the carburetor. The faster the piston moves the more pulses it's going to give to the fuel pump therefore increasing the amount of fuel to the carburetor.

The volume of air that the piston displaces remains constant. However the volume of air being moved inside the system (vacuum lines) should change with the changing speed of the piston. The air pressure (vacuum) created by the piston should not change as the rpm's increase or decrease, however the volume of air will increase or decrease with the changing rpm's because more air is being moved inside the system. This, then, should produce more vacuum pulses at the fuel pump supplying more fuel to the carburetor.

I would like to think that the fuel pump would by designed to supply more fuel than the carburetor can use and I think that is supported by the third line on the pump returning to the fuel tank. As the carburetor float closes it would create a back pressure in the fuel pump and that pressure would somehow return the excess fuel to the fuel tank.

With all that said, because the fuel pump at the beginning of the video wasn't pumping any fuel at all, and very little at other points in the video, does logic suggest that the fuel pump return system is faulty? That it is stuck open and supplying most of the pumped fuel back to the tank instead of the carburetor? Then again, maybe the rag I had stuffed into the intake manafold was letting enough vacuum escape through the rag to lessen it in the hoses so as to not have enough vacuum at the pump?

The routing of the vacuum lines is different now then it was when I had the problem. I changed them because I was having the problem. The way they are routed in the video is the original set-up and I believe are the way alleyopp want's them routed.

Comments???

I'm going to get a couple of "Y" connectors and hook up some test hoses to check vacuum pressure and the output of the fuel pump while the engine is running. That should give an indication of problems in those areas. I have to start somewhere.

Bill , I'm going to make a few statements . The number one rule in trouble shooting is to never take any thing for granted . There is something called a "PDI" most of the things you take for granted are checked and or fixed . Here is sn example , I would check and adjust my valves before I put fuel in the gas tank . So I understand when you say its new why would there be a problem with the ground . I have been around scooters for a short time and ground problems come up often .

As far as vacuum , the starter only turns the engine over so fast . When at idle or upper rpm the vacuum is more consistent .

John

John, I reset the valves to .004 right after I wrote "Crap!".
Skuttadawg, the fuel tank is beneath the floorboard.

I got a few feet of gas line today and am going to somehow see if I can see the gas output from the fuel pump while I'm driving it. A can or something between my feet might work. The lengths I will go to I tell ya!

I see your point and it makes perfect sense. If the pump were below the tank it would get it's gas supply free of charge via gravity. No vacuum at all would be needed to get the fuel to the pump. On the other hand, however, with the pump being lowered, gas would have to be pumped further up-hill to get it to the carburetor using more enegry getting it there. Does the math really work out in favor of your set-up?

This is my first scooter really, so I have to use logic and my advanced old age of aquired knowledge to try to make sense of things. So another question I would have to ask myself is "why would the manufacturer install the pump in a place that might cause problems within the system?"

I do value your opinion and insite though as you have a hellofalot more knowledge in this matter than do I. I hope you keep replying to my queries and put up with the lack of my understanding.

Not a problem William, my tank is up under the hump further than yours is. The hose from the pump to my Fuel filter is about 3 feet long then it goes up pretty high to my carb as you can see in my second picture. The pump will pump more than enough. Your problem is not enough gas is GETTING TO THE PUMP so it can pump it because the GAS has to go UP to the pump.
Really man I caught that right away when you said the tank was under the floorboard all this time i taught the tank was behind the seat above the carb.

IN the Video the IDLE IS SET TO LOW , turn the IDLE SPEED screw CLockwise about 1/16 does not take much to bring up the IDLE. The Carb has an ENRICHER and the motor warms up about 3-5 minutes the enricher will extend and shut off the extra fuel and it should idle about 1500-1800 after it is warm. Raising the Idle is just like twisting the throttle a little that is all the IDLE SPEED screw does same thing.
Alleyoop

After u fix the fuel delivery , if it still is having a hard time idling u may need to adjust the air fuel screw on the carb .