Mechanical Fuel Pump - Slow return to start position

[andywalter007]

1971 240z: I removed the valve cover and manually operated the fuel pump arm. When I release the arm, it sends fuel to the carburetors. I noticed that it takes a noticeable amount of time for the arm to return to its starting position on the cam lobe. During this return, the pressure has a max of 6 psi. I can confirm that the return line produces a visible spray of fuel into a glass jar. Once the spray from the return line diminishes and  I see the pressure drop below 3 psi, the pump arm is ready for another cycle.

 

With the engine NOT running, should I assume this is normal behavior?

 

My concern is the time delay. If this delay remains during engine operation, the fuel pump arm would float and never have a full stroke. That's assuming it is supposed to always have a full stroke 100% of the time.

 

 

[Yarb]

What’s the back story on what brought you to testing the pump?

[andywalter007]

New fuel pump was assembled incorrectly. I had to rotate the top 180 degrees to get the flow the correct direction. 

[Zpenman]

Perhaps you need a new return spring? Make sure there is not a build up of gunk around the lever/ return spring area. When I was checking my pump (removed from the car) it would return quickly with an audible click. I also have an original '71 Nikki pump. My luck with after market pumps has been pretty bad, the majority of them are junk.

[Captain Obvious]

Where did you see the stream of fuel? Over by one of the carbs? If so, I'd pull the rubber hose off the return hardline near the fuel pump and see what comes out as you work the pump arm.

Maybe the orifice is plugged with crud?

[andywalter007]

I purchased a new fuel pump from a different manufacturer. It has a much firmer and shorter arm throw for a complete pump cycle. I manually measured the difference between the two, and the new pump has a 1/2-inch shorter throw.  It's behaving more like I expected. It quickly returns to the start position without much delay, and I get approx. 5 lbs of pressure from it. I'll agree with Zpenman at this point and chalk it up to poor manufacturing of the original replacement part. 

Thanks, everyone.