Marauder 25 tuning questions

Purchase or make a larger i.d. transfer port 

Check out Wicked Air Rifle. Simple changes such as adding a new Hammer, getting a larger diameter transfer port, and the new TSS will not only add power but shots. These mods have enable me to get 28 shots averaging 844 fps. Addertooth has gotten 40 shots averaging 840 fps. These are very simple mods to add. He has a demo video on the TSS. After adding the TSS you can change the speed with a pocket screw driver.

Springs do wear out and you can always start with a new spring. With a stock .25, I was able to get 880 fps. If you don't want to add these mods., be sure to check your transfer port adjustment. This is located on the barrel just in front of the action. You want it wide open, which is 4 turns out from closed. There is a cover grub screw that has to be taken off first. If you over tighten the grub screw when you put it back on, you can actually close the adjustment you just made. Not sure why they have this feature, as most people want more power than less.
Having a smaller transfer port diameter mean less air moving through therefore less power. Having your current transfer port open all the way could also solve your problem. If you are new to messing with pcps, there are alot of videos on the Marauder. It is very difficult to screw the Marauder up and if you do there is a lot of help out there. There is another forum devoted to the Marauder if you can't find answers here. 

Adding a HUMA regulator will only help with you spread making sure your shot is consistent. That's what a regulator does, it regulates the amount of air flow. It may not add shots ( but can). Many have not had to add this feature if the shot spread is consistant, (but I did). At 880 fps you will only get around 16 shots. Doing the mods I suggested and adding a regulator, I get 28 consistant shots averaging 844 fps. I have followed other's example and have not tried to get more speed (therefore power), I wanted more shots. At 840 fps, the .25 Marauder shoots a 25 gr. pellet at about 40 FPE. This is plenty of power for small game out to 75-85 yards. I can stretch it to 100 but the wind can be a factor. Trying to get 880 fps (therefore power) would result in reduced shots. If you are looking for power meaning, 45 FPE and above, I am not sure I would do all this mods right away. AJshoots was able to tune his .25 Marauder with an extreme spread under 5 fps at 50 FPE but only got 8 shots. He did enlarge the transfer port. This was his coyote rifle, he didn't need more shots. You will have to decide what you want it to do and tune it accordingly. I am happy to get 28 shots at 40 FPE. 
The good new is that this rifle is able to accomodate a lot of needs. Once you understand your purpose, that will help you determine the adjustments you need. Travis at WAR can be a great resource. The upgrades he offers are very affordable and you can do them yourself.
Best of luck with your new project
Doc

What size for transfer port?

I am uncertain to the size he used. I know he has a lathe and made his own. I would check the Maruder forum for suggestions as to the best size openings for specific power levels. You can also contact Travis. If you just want to improve shots just adding the TSS and Hammer may be all you need.