Something odd is happening and I'm not sure it is just a pump cup issue. The pump cup just came off in the shaft so I reinstalled it on the shaft and lubricated it and the same thing happened again. I have put it in a second time and it kind of 'pops' when you push down and I think it will come off again, definitely not smooth. I'm wondering if it is a non-return valve issue. The stove is pretty well brand new and I inspected the cup...no visible issues...
I checked the one way valve and it was fine. I did swap it to test it with the valve assembly in the XGK service kit. It seems that there is something wrong with the pump cup even though there is no apparent damage or swelling or any indication that it could be an issue in any way. I ended up taking the pump cup from the XGK kit and putting it in and it works fine now...but wow this cup is only a few weeks old.
Probably some kind of manufacturing issue. You might be able to bring it up with MSR and get a replacement.
I did some reading and searching. Most of my stoves I run on kerosene but this one runs on naptha. Kerosene is very oily so any kerosene getting past the non-return valve is really not an issue as it helps lubricate the pump anyway...not so with naptha. I'm going to use quite a bit of oil this time to lubricate it and I have put the original cup back in. It is behaving but still not nicely smooth. I will buy some household lubricant and use that and plenty of it...what is the cup lubricant made from anyway?
I tried flooding the cup with WD-40 confident it would be very smooth but...no. It is the cup itself which is just too flexible and I think you are right fjfj, and come to think of it, it did do some weird binding in the pump from brand new and I remember taking out the pump and finding plenty of lubricant on it and then not thinking much about it. I got it from Amazon Global. The last stove the XGK came with a clogged filter, this one with what appears to be a buggered pump cup. It simply has too much flex and with a bit of use it now probably does require replacement. Fortunately I did order the expedition service kit for the XGK and of course the pump cup is the same. I quite like the Whisperlite, it is a unique and interesting stove but it is not a great stove for priming. I wouldn't use kerosene on this stove, not without adding some more wicking for priming, as at least one user has done. Nice stove though and it runs very very hot, hotter than any stove except for the XGK on kerosene...and the Soto Muka but then that is in a league of its own in a number of areas.
Don't use wd40. It's not a lubricant. If you're concerned about fuel getting past the nrv, I would use something synthetic, like full synthetic motor oil or silicone oil or Krytox, if you're feeling fancy. I believe MSR oil is mineral oil.
Consider changing to leather pump cups. I have encountered too many problems with the rubber ones to bother fooling with them anymore. Msr uses leather pump cups in their Arctic pump-they know leather is reliable when it counts. They do not list parts for the Arctic pump on their website, so aftermarket suppliers are the way to go, or make them yourself. the Fettlebox sells leather cups for Tilley lamps, and for a Usgi stove- both of these work and are an easy fit. They are 16mm diameter with a 6mm hole. There are other suppliers too, with good reputations here for quality leather cups.
I got 3in1 oil and that gave it the lubrication needed. I can see how these rubber pump cups could fail quite quickly with insufficient lubrication..they would just tear…definitely not as resilient as a leather cup.
