I have a 96 cbr900rr with a fuel delivery problem. I get a dribble of gas into the float bowls. Not enough to reach the cylinders, but enough to drain from the bowls. Not a chance on turning over thou. If I remove the fuel pump cut-off relay, and run a jumper wire, everything works. That tells me that I have 1 of 2 problems.
1. The relay is bad.
2. The relay is getting a false signal to cut off.
Either way, the best thing to do is replace the 80$ relay, and hope it solves the problem. However this is just my stunt bike. Its not even registered for road use. So here is my idea, pick it apart, or agree with the theory. I need some other opinions to make sure that my idea is a good one.
Instead of replacing the relay and hoping that solves the problem, how about do away with relay all together. Either gravity feed the carbs, hook the fuel pump to a toggle, or (my favorite) install a vacuum fuel pump and run it off the vacuum for the carb sync of #2carb. I am aware of these problems with the 3 choices. GRAVITY FEED will work flawlessly as long as tank is always above carbs. Not so good when 80% of the bikes life is on the back wheel. TOGGLE SWITCH/JUMPER WIRE would ensure fuel pump operation, but most likely will over pump, and force the float needles open, inevitably flooding the bike. Last option is the vacuum idea. I can't think of any potential issue as long as I get a vacuum pump with the same output pressure as the stock fuel pump. That's why I am here. Does anyone know from experience any reason the last idea won't work? Advice? Ideas? Problems? Solutions? What do u techs think?

 

Few ideas.
Fuel filter?
Can you hear the pump running?
Test for 12v at the pump.
You can jumper 12V to the pump to test it.

 

Yes I have a inline fuel filter,
"If I remove the fuel pump cut-off relay, and run a jumper wire, everything works. That tells me that I have 1 of 2 problems." (posted in first post)

 

If you can find a wiring diagram you can test to see if the relay is geting the signal that turns it on, then you can say it is the relay for sure.

 

No offense but u are not comprehending what I'm asking. Maybe ur not reading my post, I don't know. But thanks for trying

 

What is your question? You have quite a few up there?

Run gravity feed?
May work if you can run bigger lines. Will not likely work with the stock ones. There probly is a reason Honda used a fuel pump.

Run a togle swtch?
If you lay the bike down the origional system probly shuts the fuel off. Carbs will overflow when the bike is on its side. Could result in a burnt bike or worse.

Myself I would jsut fix the problem. It is most likely the relay. Find a used one after finding out the real problem.

Basically what I am saying is you have to test the function of the relay. Supply it with power to turn it on.
If the relay does turn on and supply the fuel pump with power then you have a problem with what supplys the relay the switching signal.

If you supply the relay power to turn it on and the pump does not work you have A a bad relay or B no power to the swich part of the relay.C maybe a broken wire in the circut.

I am trying to help you diagnose the problem without guessing.
And yes I am a licenced mechanic.

 

I have a 96 900 and it has neither a fuel pump or filter inline. It never has since I bought it in 2000 with 2500 miles. Does it have a vacuum actuated petcock that maybe sucking air? Try using a vacuum pump to open the petcock and see what type of flow you get. I do this every spring to fill the float bowls to prevent fouling plugs.

 

1996 cbr900rr is the last year that comes stock with a fuelpump. If u don't have it, it has either been converted, or has a swap with a newer motor. Yes I have a vacuum petcock, and yes it works or there would be no fuel after using a jumper wire at the relay. Thanks

 

My question was does anyone see an issue with installing a vacuum pump.

I came here to test and idea, before I installed it. I am a licensed (but not practicing) mechanic thru suzuki and honda.

You are correct about what u said about testing the relay, however. I have heard that the fuel pump on this year model is actually a "pulse pump" and instead of just working on volts, it requires a certain amount of ohm's. If this is true then the relay could be working correctly, but over time the pump could have malfunctioned due to built up resistance somehow. This is the only thing besides the relay getting a false cut off signal that would cause the pump to pump a tiny bit of fuel. If it was the relay it would either pump or not pump. That is why I'm confused. The fuel pump circuit is so maticulios that its a headache. I just think it would be quicker, simpler, and cheaper to convert to a vacuum pump.

Fido thanks for helping so much, tell me what u think now.

 

Hmmmm. It's got me stumpt. I am not familiar with a fuel system using the resistance thing.

If you dio have a vacume cut off the vacume fuel pump should work + be safe in a crash. Only one way to find out i guess.
Good luck.