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Hi,
i am joris, living in the netherlands and just completed my zxr400 project. But i have still one problem with the fuelpump.
i've read some topics about fuelpump problems but i still can't solve my problem.

the bike runs well en starts up fine. no problems at all. but after about 2 miles driving the carberators are empty and won't fill up anymore.
the feulpump got 12V when pressing the start button and the pump delivers pressure then, so that's oke. but i believe it doesn't build up pressure while driving.

i tried to drive without a fuelpump (directly from the tank to the carberators). then everthing works fine and i can drive for many miles and the carberators don't get empty!
the only problem then is that on higher rev's you don't have enough pressure from the fueltank to fill up the carberators quikly enough. normally the fuelpump should do that.

i have already replaced my fuelpump rellais so that should be oke. can anybody help me out with this problem? or tell me how the pump works and why it doesn't work while driving.
thank you in advance

Cheers from the netherlands;)
Joris

Hi. I have investigated the operation of the fuel pump and relay...and still don't understand how it works! But these are my findings: 12V is applied to the fuel pump by the relay as long as the starter button is depressed. After that there is a lower level of voltage applied (I can't remember exactly - maybe 4V?), so that the fuel pump is not operating continuously, but only when fuel pressure drops. I held a screwdriver on the fuel pump body and "listened" to the plastic handle while the engine was running. There were infrequent clicking noises, indicating a short period of operation to supply more fuel to the carburettors. I suggest that you do something similar with the bike stationary. If there are no noises and the bike eventually stops through lack of fuel, then either the pump is no longer capable of operating at low voltage, or the relay is not supplying the low voltage, but can supply 12V for starting.
Since you have confirmed that the bike runs with the fuel supply connected directly to the carbs, it must be the relay or the pump that is the problem. Hope this helps.

my understanding was this, when you depress the starter button 12V is sent to the fuel pump allowing it to prime the fuel bowls.

after this the operation of the fuel pump relay is controlled (at the same time as the tachometer) by the CDi, a signal is sent to the fuel pump relay that is then sent to the fuel pump, as revs rise more signals are sent to the fuel pump relay and the fuel pump operates more. this fuel pump is relatively weak and it cant work against a presure gradient, so as the fuel fills the bowls it gets to a point where no more fuel can flow, the fuel pump relay still activates the fuel pump but no fuel flows.

if your tachometer is working fine, and you have replaced the relay with a known working relay, it can only be 2 things, a blockage in the fuel lines (in lines that aren't used when you connect the fuel tank directly) or a defective fuel pump...

as for the fuel pump internals, if its like other ones (and i suspect it is) it uses a small diaphragm to pull fuel towards it through a one way valve then the diaphragm relaxes and pushes fuel out the outlet etc... if either the diaphragm or the one way valve are damaged this might explain why its able to pump a bit of fuel (with 12V continuous going to it its able to pump a small amount successfully) but then cant work when it goes to a "pulsed" CDi controlled setting.

thank you all guys.
i will trie to solve the problem now with these answers.
cheers.

are the pipes on the right way? sounds stupid but i did it on mine and it went for a bit then died, is your fuel cap allowing air through? i.e. not forming a vacuum, ride along, open the fuel cap, if you hear a "woosh!" as you open it then it could be that, so you will have to take apart and clean