Problem with plenum

[villaman]

I have been running my .25 impact on 160-170 bar and 970 FPS , b on the dial and 3 lines on front adjuster achieving 35 shots .

So I thought I would install a plenum kit from the UK, which comprises of , transfer port with two holes , valve housing which has been modified, pellet prob which has been modified , c3 and plenum. After installing the kit I am getting the same amount of shots , less FPS about 925 with everything maxed out on 135-140 bar . 
what have I done wrong , it’s mk2 impact 

 

[edosan]

I do not see a hammer weight in your description, you need one.

 

[Fire_at_will]

Have you tried higher reg pressure?

Also don't see anything about added hammer wieght? 

970 is doing pretty good with 25cal, I probably wouldn't have changed anything.

 

[villaman]

Sorry forgot to mention hammer wight and spring guide fitted

 

[villaman]

The only reason for fitting plenum , to gain more shots

by the way using 33.9 gr mk2 JSB 

 

[edosan]

You should probably ask the vendor of the PK about it. He should be know about it.

 

[biohazardman]

Not an expert or anything, but if I were faced with that problem, I would turn the hammer spring down a bit, from where it was with the 160-170 bar, and open the front adjuster a couple, three turns towards the forth line just to see if it would help any. If that didn't werq I would call the vendor like Edosan said.

 

[Airgun-hobbyist]

Without seeing the original parts and the new parts, and the new settings compared to the original, it is hard to say where the speed loss came from. 970 FPS is a high number, so going to anything higher does take a substantial change in parts and tuning. The FX MK2 plenum is only 5 cc's bigger in volume than the original FX plenum and does not add anything noticeable to the shot count.

That said, after I have installed these new items the spring pressure and relative position changes compared to the original parts. Because of this change in position you need to adjust the power wheel set screw, so it would be in a similar position before the mods. This temporary setting will show the true gains in hammer weight and air flow. Then you can tune for the new power from there.

 

[villaman]

Yes had a tune before fitting this kit , just thought by adding this kit , it would give me more shots at the same speed 

 

[biohazardman]

Now that you have added the kit you will need to completely re-tune your gun. Same thing if you change slugs or pellets.

 

[villaman]

Tried everything to get this kit to work , but I have tried different settings but no luck 

 

[PigeonMan]

The only reason for fitting plenum , to gain more shots

by the way using 33.9 gr mk2 JSB

Larger plenum will not give you more shot. Who told you that? The air in the plenum is regulated. Even if you have a 100 Cf for your plenum. Once you shoot, it is falling in pressure so you are not getting additional shot. If you want more shot, why not get the 580cc bottle? It has 100cc more air at 3600psi vs your large plenum which I'm guess is about 40cc at 150 bar.

 

[heavy-impact]

The only reason for fitting plenum , to gain more shots

by the way using 33.9 gr mk2 JSB

Larger plenum will not give you more shot. Who told you that? The air in the plenum is regulated. Even if you have a 100 Cf for your plenum. Once you shoot, it is falling in pressure so you are not getting additional shot. If you want more shot, why not get the 580cc bottle? It has 100cc more air at 3600psi vs your large plenum which I'm guess is about 40cc at 150 bar.

You do get more shots if you reach the same velocity at lower regulator pressure.

 

[nalby]

I have Stuart's kit also , before the change I was shooting 44g jsbs at 840,when I changed all the parts I left my reg at 160 it was shooting at 650 (what the ?) I lowered the reg to 130 adjusted the hammer spring turned the front adjuster out to 4 lines. now I get 40 shots at 930 and is a lazer to 85 yrds ( my yard limit)

 

[villaman]

Which I can’t and that’s the problem 

l will give it another go , after that I will sell the kit . My mate has gained another 10 fpe on his mk 1 from 46 ft/lbs up to 56 and shooting fantastic . 

 

[Hobbyman2007]

I’d be looking at how thick the C3 bumper is. Another thing is you might be hitting the valve wayyyyyy to hard and it’s bouncing closed. Sometimes less is more . You might also be maxed out at that regulator pressure . Have you plotted out the velocity curve ? You might very well need more pressure to get the velocity you’re after . If you look at it this way , you’re getting close to 65 FPE from 120 bar . That is extremely good IMO. 

 

[GQ]

Is your valve seat have been modified to 6.5 mm ?

And the rear block is more efficient when modified for less air obstruction.

Am I wrong ?

 

[heavy-impact]

Is your valve seat have been modified to 6.5 mm ?

And the rear block is more efficient when modified for less air obstruction.

Am I wrong ?

The block on the right is like mine and has been milled and threaded M14 for high flow. This isn't required for normal external plenums. They use the block on the left with the center hole drilled to 6mm.

 

[heavy-impact]

I have been running my .25 impact on 160-170 bar and 970 FPS , b on the dial and 3 lines on front adjuster achieving 35 shots .

So I thought I would install a plenum kit from the UK, which comprises of , transfer port with two holes , valve housing which has been modified, pellet prob which has been modified , c3 and plenum. After installing the kit I am getting the same amount of shots , less FPS about 925 with everything maxed out on 135-140 bar . 
what have I done wrong , it’s mk2 impact

Did you drill the gauge hole in the rear block to 6mm ? If not you need to disassemble and carefully drill it through the teflon seal. Remove any burr on the inside where the o-rings will slide over to prevent nicking them. I use 3M pink polishing paper for this.

 

[Bob_O]

Start the velocity tuning from scratch.

It's not good enough to just lower the reg pressure and run the VA and HS maxed out if I read that correctly.