25 Caliber Marauder help.

[Skip-in-WV]

Here is the setup: Lane reg for 25 cal set to 1800 lbs. 25.4 grain pellet gets 750fps. Valve port opened to 0.172 in. Fps 750. Larger transfer (refridgerator tubing) port to 0.163. Fps 750. Opened barrel port to 0.182. Fps 750. Home made transfer port 0.172. Fps 720. Tissue paper leak test. Negative. Reg set to 2000lbs. 760fps. Lighter valve spring tension. 760fps. Im trying to get about 850fps (40 ftlbs). What went wrong? I'm out of ideas.

 

[Xbowsniper]

Skip, 

Even though you are increasing the reg pressure, your velocity is going down, even with a lighter valve spring. This is due to the high pressure of the reg, it is still too much pressure for even a lighter spring to have any affect.

Try these things

A)....ultra light spring in the valve. You should be able to push it down with 1 finger, and easily

B) .reset your regulator to around 1500 PSI

C) use the largest transfer port between the valve and barrel, try to use a proper transfer port, not tubing. Tubing could leak air on the shot, and you'd never know it.

D) back your hammer throw all the way back for your starting point. 

E) bottom out the valve transfer port, and come out 4 full turns as a starting point. 

F) use stock spring for a .25, or stronger.

If you need some of these parts, Tim Hill of Hill Airguns is the man.

If you set all of your adjustments close to above, you should be able to get the velocity you need.

Start with all adjustments out. Play with hammer spring first, see what your velocity is. Then, adjust hammer throw by one turn. Test velocity. If it goes higher, keep adjusting hammer throw. If it does not, back off 1 turn and stop. Now adjust the transfer port. If velocity is too low, back off 1/2 turn increments. Once you cannot get any more velocity, stop and put it in to where it was on the edge of not doing anything. This tells me the transfer port is at the optimum spot.

It'll be a hit and miss game with the hammer spring and throw, but there should be no problem in getting it to shoot at your stated velocity.

What is more important in this equation, is volume over pressure, but within reason. By lowering the regulator psi, the valve can stay open longer, thus more volume of air to do the job.

You can also give Tim Hill a shout, he is the real power guru when it comes to tuning the Marauder for power.

To give you an idea, I put Tim's valve, transfer port, spring, hammer in my Marauder. With all adjustments all the way out (except for his valve and hammer,they are not adjustable) and the SLOWEST was able to get a pellet to leave my Marauder was 1150 fps with an 8.4. My reg was set to around 1500, and I ended up lowering my reg pressure to around 1000 to even come close to my 12 ft. Lbs.



Hope this helps, don't be afraid to give Tim Hill a shout 



Tom Holland 

 

[ackuric]

The most common cause of that severe kind of fps loss would be a breech o-ring, very common. Make sure its present, It should be around .05" depth inside your barrel on the breech end.



When you ported the valve + tp, did you port the barrel? If not the barrel is still at .14" which would cause an issue...



Also if you didn't open up the THROAT at all, it is likely at the stock (.203-.21) I forget which, which is limited to .156" porting, going beyond that does nothing but create more wasted volume...



You have two remedies to the throat issue, run a slightly thinned down poppet stem from poppet face (flush) to back of throat, I'd advise around .09"~...even just to .1" thinned in the throat would work...that or you can open the throat up to .2188"...which would be good for up to .175"~ porting...



Good luck and the above will fix your issue 100%. If you need any more help feel free to ask. Careful if you do drill out the throat, the seat is around .224-.24" total on a gen 2 being raised, so that leaves little room for error even going to just .2188"...Many people drill upwards of .24-.26" and then re-do the valve seat but that is overkill and not necessary especially for your porting. I'd simply drill the throat to .2188" if I were you and verify barrel is ported equally to tp/valve and give her a rip



-Matt

 

[addertooth]

Skip-in-WV,

All of the advice above is good. They comprise the two basic camps of "roll your own", or "buy parts with known results".

Pay close attention to the O-Ring mentioned above, I will include a picture of it's placement. I took the buy known parts from Hill approach, and have my regulator set to 1800 PSI. I had to use right at minimum hammer strike with the TSS dual spring system to get my power down to 40 FPE with JSB 25.39 grain pellet in my .25 caliber Marauder. It actually wanted to deliver a much higher punch. I used, TSS with softest springs available with the kit/gun, Hill flyweight aluminum hammer with lightweight striker, Hill valve, Hill plastic transfer port. Barrel ported to 0.187, I polished and deburred the tube where the hammer slides and used graphite lubrication, and reshaped/recontoured the bolt in order to make it obstruct the transfer port less. And the Huma Regulator set to 1800 PSI. It gets me 42 shots at 40 FPE with less than a 1.8 percent velocity change between the fastest and the slowest shot in the 42 shot string.

If you want to take the existing factory valve and re-work it, be sure to do more work on it than just making the transfer port larger. Also make sure the screw adjustment on the side of it is not set to create restriction (it should be adjusted as far out as feasible).

Picture of potentially problematic O-Ring, forgive the dust on the Marauder in the picture. 

 

[Skip-in-WV]

Is that oring a common numbered one?

 

[Elbowgrease]

I replaced my breach o-ring with one from a big o-ring kit I have. I would say that its a very common item to get and could be found at home depot or most hardware stores.