Weird discovery while rebuilding fuel pump

[DJ_Allatoona]

75hp Mercury from 1986. I did fuel pump service today: new gaskets, diaphragm, check valves. I've done it like five times over the years. I noticed in this particular kit I bought today that there was a little round plastic "cap" in the kit. Hmm, what is this? I look at the parts list and it's called a 'spring cap'. News to me that such a thing exists. Diagram showed it fitting into the end of the spring and contacting the rubber diaphragm. Well, what the hell? All these years in my fuel pump, just the end of the spring has pressed against the diaphragm, no plastic "cap" at all. So I put it all together with this mystery cap in place, and go start it up and run it. And I **think** my old motor ran a bit better. I have some fuel-related problems that pop up occasionally (stalling, poor hole shot) but I live with them.

Question: Should I have had this "cap" in place on the end of the spring in my fuel pump all this time? Is it better than just the spring touching the diaphragm, does it have a function, or does it just protect the rubber from tearing? Could this little piece of plastic have improved my running today, or did my intermittent problem just not occur?

Here's a picture of the cap & spring: http://imgur.com/a/ukeU0

 

[wn6ngp]

I just rebuilt my 90hp fuel pump and I noticed that there are lot of versions of this fuel pump so it appears that merc builds motors with whatever is available at the time. M

 

[gm280]

Maybe that plastic cap helps from the spring causing wear to the diaphragm? :noidea:

 

[wn6ngp]

I just rebuilt my 90hp fuel pump and I noticed that there are lot of versions of this fuel pump so it appears that merc builds motors with whatever is available at the time. M

the forum clipped my previous reply,
my 90hp has a little plastic hat like that. Maybe the previous owner had work done and it got left out. Make careful note of the order of the various gaskets, if you get them out of place it may run but not properly. I know

 

[wn6ngp]

Maybe that plastic cap helps from the spring causing wear to the diaphragm? :noidea:

this answer makes sense, I can see how it could work properly without it but not last as long.

 

[mgjtkt]

Yes, plastic cap goes on the end of the spring to protect the diaphragm