What is the purpose of the vaccume on the fuel pressure regulator? does it drop or raise fuel pressure? Let see we have an electric pump which should keep pressure constant and the change should be fuel used in the rail by the injectors might cause a slight pressure drop. in the picture of the cutaway of the regulator it appears that as the vaccume increases it would pull in the needle valve anddrop fuel pressure. any one out there run their scooter without this vaccume line?





"entities should not be multiplied greater than necessary" William of Occam

The FPR keeps the pressure at 2,5 bar relative to the outside pressure.

When the pressure in the inlet drops, the vacum help the membran to open to maintain the pressure relative to the outside.

As Inge says, it is intended to keep the fuel pressure at the nozzle tip constant. That tip is situated in the throttle body therefore the link. The newer bikes don't have it any more as the ECU was better.

I think I'll put a pressure gauge on my system and yake a ride and see what affect not using vaccume has.

If you disconnect the vacum line, you would read a constant 2,5 bar pressure (or slightly lower if the spring in the FPR have become a bit weaker during the years in service).

my fpr was delivering insufficient pressure, so, i've replaced it with an aftermarket one designed for cars. I'ts adjustable and has a guage on the side. being bigger than the original, i've relocated it to behind the left sidepanel.I've tried it with and without the vacuum pipe connected, and found slightly better fuel economy with the pipe connected. I,ve also experimented with the pressure to the injector rail and found about 40 psi is best, giving a smoother idle, good throttle response, and reasonable fuel economy ( slightly less than standard mpg) the plug tips are slightly browner than before, but the engine seems happier, smoother and more responsive. Before this mod, the plugs were whitish grey, and the engine less responsive, with lumpy idle.

Basically what you are doing is putting more fuel in your engine. The set-up is for 2.5 Bar (36 PSI). Putting the fuel pressure up to 3.0 Bar (42 PSI) will result in more fuel being injected with possibly a slightly better spray pattern/atomisation.

sven k100 wrote:my fpr was delivering insufficient pressure, so, i've replaced it with an aftermarket one designed for cars. I'ts adjustable and has a guage on the side. being bigger than the original, i've relocated it to behind the left sidepanel.I've tried it with and without the vacuum pipe connected, and found slightly better fuel economy with the pipe connected. I,ve also experimented with the pressure to the injector rail and found about 40 psi is best, giving a smoother idle, good throttle response, and reasonable fuel economy ( slightly less than standard mpg) the plug tips are slightly browner than before, but the engine seems happier, smoother and more responsive. Before this mod, the plugs were whitish grey, and the engine less responsive, with lumpy idle.

sounds like you have vacume leaks from the throttle body rubbers sven ....by adding more presure you are compensating the loss ,,,

best to have a look at that i recon