A simple question about pressure testing fuel pump

[jmcmul2]

So, I am trying to bring a K100RS back to life. My question is about pressure testing the fuel pump. I am rigged up to run the pump without cranking the engine and get a good 38 psi at the rail. Life is good. But my question is, when I crank the engine (without out it starting), I can see the injectors opening on the pressure gauge and pressure drops maybe 10 psi on each pulse! If this correct, I would have expected a barely perceptible drop in pressure........

[Laitch]

What's your simple question?

[jmcmul2]

Question is, should I be seeing a large drop in pressure when the injectors open?

[Laitch]

Question is, should I be seeing a large drop in pressure when the injectors open?

No.

How long have you had the bike? Have you ever ridden the bike? Do you know how many miles the bike has been ridden? Do you know when the last time was that it was ridden. List every diagnostic and replacement procedure you've done to the bike up to this time. Please post some photos of the bike.

Welcome!

[The Mighty Gryphon]

Here's my simple question:

Have you changed the fuel filter and checked the screen on the fuel pump? 

Here's another one:

How fast does the pressure come up when you start the bike?

The fuel pump is capable of pumping at least twice as much fuel as the engine can use running at highway cruising speed.  It shouldn't take more than a second to build pressure, and the injectors don't flow enough to drop the outlet pressure of the pump.  If the pump is working properly, the only thing that would drop the pressure in the rail would be an obstruction in plumbing in the tank.  A blockage in the pump screen or filter are the first places to look. 

Another possibility would be loose hose connections in the tank causing leakage of fuel before it can get to the rail.

[jmcmul2]

Thanks so much for your reply. I actually have the fuel tank removed from the bike and the fuel pump is in a new clean paint can of gasoline sitting on the concrete! The way I rigged the connection, there is actually no filter or screen so that is out of the equation. Return flow from the bypass looks great when I just run the pump and pressure is correct. But I get these large, maybe 10 psi, pulses down in pressure when I crank which didn't seem correct..


I think you have answered my question, I should not see these large pulses. I am using a cheap, aftermarket fuel pump and will try replacing it. I will let you know if the pulses persists.


Thanks once again. John

[The Mighty Gryphon]

Have you tried measuring how much fuel is pumped in one minute?  I believe the specification on the Bosch OEM pump is 33 liters per hour.  That would work out so approximately 8 1/2 gallons per hour for those of us who are metrically deprived. 

Your pump should be able to move a quart of gas in a bit less than two minutes.  17-18 ounces in one minute.

[Martin]

What brand pump are you running a picture would be nice. A picture of your test set up would also be handy. Are you running a regulator in the bypass line?
Regards Martin.