How to reduce the FPS of a positive pressure system GBB without changing gas?

[ccyg8774]

Recently got a KSC TMP (hardkick system) as a parts gun, but repaired it using KSC MP9 (Japan version, also hardkick system, not the NS2/S7 system KWA MP9) parts. Seems working fine so far, except the FPS is about 375 FPS on propane, slightly above the CQB limit of the local fields (330-350 FPS).
The TMP/MP9 Japan version uses a positive pressure system so I don't think there will be a NPAS for it. Also don't want to switch to other gas (like the duster gas), as they are more expensive and harder to find than propane. Can I do some thing to the valve (part 198), to reduce the FPS? What about the spring (part 199)? My knowledge for GBB is very limited so I don't know what I should (or shouldn't) do.
What would happen if I block 1/3 of the hole on the valve nozzle to reduce the airflow and reduce the FPS? Some accuracy lose is acceptable. A permanent modification is fine as long as the FPS can be reduced.
TMP%20KSC0020.jpg

I also heard that the TMP can be modified to work as a negative pressure system by placing a small spring in the front of the valve (but I can't find the source or the guide rightnow), would that work? Would that reduce the FPS (although that method is not intended to reduce FPS) depends on the strength of the spring? Thanks.

[Thenooblord]

take out part 198, and with jbweld or something partially obstruct the gas ports
I use this method on all my guns, no way I'm buying 15 NPAS's, can just add or remove material till u get the FPS u want, its not as easy as an NPAS, but its alot cheaper...

[onizukahibino]

that's very smart! i never thought of that.... (looking at all the npas....)

[Kos-Mos]

BTW, that's a NPAS system, not positive pressure.

And you can fit the NPAS' made for KWA MP9's in there.
If that rocket valve (part 198) is metal, you could also drill a small hole through the center into the rear plate, and instal a small setscrew. Basically, you want the valve to sit forward when at rest. That will reduce the amount of gaz that can flow to the BB and push it.

[Thenooblord]

its hard to tell from the pic but I assume its the older version of the KSC with the metal rod protruding thru the front of teh nozzle and if so its not the same as a negative pressure system, this system where a spring is pushing the rocket valve closed as soon as the BB leaves the chamber, as opposed to a negative pressure system where the rocket valve stays open untill the blowby of gas down the barrel sucks it closed
these old KSCs, as well as old western arms use this system, im not actually sure if its called positive pressure system, but it sure as hell aint negative pressure

not that it makes any difference when it comes to blocking gas flow, just more useless information ;p

[Sequential]

#198. (BULB)

Screw in a small tiny screw on to the top of that unit. so that it will block some of the passage of gasses to the nozzle.
I brought my GBB's down from 400 to 320fps by doing that.

Or you can take a small piece of plastic from a unjamming rod and stick it inside of the bulb and slowly drill through it making a tiny space where the gas can pass through.
This method will really lower the fps by a lot.

[ccyg8774]

Originally Posted by Kos-Mos

BTW, that's a NPAS system, not positive pressure.

And you can fit the NPAS' made for KWA MP9's in there.
If that rocket valve (part 198) is metal, you could also drill a small hole through the center into the rear plate, and instal a small setscrew. Basically, you want the valve to sit forward when at rest. That will reduce the amount of gaz that can flow to the BB and push it.

Actually, this is the hardkick system (old KSC system) TMP and most parts are different from the KWA NS2/KSC S7 system MP9.
Some parts are interchangeable to KSC MP9 Japan version (not the Taiwan S7 version MP9). That's how I get parts to fix it.

[ccyg8774]

Originally Posted by Thenooblord

take out part 198, and with jbweld or something partially obstruct the gas ports
I use this method on all my guns, no way I'm buying 15 NPAS's, can just add or remove material till u get the FPS u want, its not as easy as an NPAS, but its alot cheaper...

Thanks, I will try that.

[Kos-Mos]

Looking back at the exploded view more carefully, a stronger spring behind the bulb/valve would do the trick.
Theoretically you could also add a flat head screw from the back of the valve to make a spacer and adjust it. You would need to either cut a slot to be able to adjust it from the nozzle tip or take it apart every time.

[ccyg8774]

Originally Posted by Kos-Mos

Looking back at the exploded view more carefully, a stronger spring behind the bulb/valve would do the trick.
Theoretically you could also add a flat head screw from the back of the valve to make a spacer and adjust it. You would need to either cut a slot to be able to adjust it from the nozzle tip or take it apart every time.

Yeah, it would be great to be able to adjust like adjust a NPAS... But drilling and taping on such small and fragile part is kind of out my ability. Guess I will try the spring method first. Thanks!

[ccyg8774]

Update: Replaced the said spring to a stronger one, FPS changed from 380 to 320. Then tried another mini spring, get fps of 350, which is ideal for me. Thanks for all the help!