Low FPS despite actions to bring it back...

[brooxy]

Right...quick question for you lot.

My MP5 used to be awesome. However one day, the FPS dropped off the side of the planet, plummeting to 120fps from about 280fps.

It's since had the following bits put in it to make it work:

- Guarder SP90 spring
- SHS v2 cylinder head
- SHS green polycarb piston (couldn't get the old Marui one apart)
- SHS piston head
- G&G MP5 air nozzle for Marui MP5

The FPS has come back up - from 100 up to about 150. I've done all the basic compression tests and they seem to check out. I also shot myself in the hand point blank to make sure it wasn't the chrono playing up. No marks at all, so I'm assuming the chrono is correct.

Also if you put the hand over the nozzle and dryfire, it feels like it's delivering a similar amount of pressure to my 320fps AUG, so I'm guessing the problem is in delivering the power to the BB / seal with the hop unit.

Any ideas?

[tandy]

Re: Low FPS despite actions to bring it back...

have you replaced the hop rubber? maybe that could be letting it down possibly

[Caveira]

Re: Low FPS despite actions to bring it back...

My 2 guesses would be hop rubber - either a very small split, or it being mis-aligned - or some crap in the barrel. A split hop rubber would reduce the fps, but should also naff up the compression, so a twisted rubber is the first thing I'd look for. A dirty (or in theory damaged) barrel would leave you with good compression but poor fps.

[North West Outpost]

Re: Low FPS despite actions to bring it back...

You have swapped a lot of parts that need checking.

Have you striped the barrel/ hop assembly cleaned, checked and put it back together. Then made sure the hop up is pushed back into the gear box.

It only needs a tiny bit of movement or misalignment to drastically effect fps.

[brooxy]

Re: Low FPS despite actions to bring it back...

Thanks, I'll disassemble the hop unit tonight and take a look. The rubber was replaced about a year ago and ran fine for most of that time, but as you say, it may have got torn or misaligned.

As for the other bits, I found that the cylinder compression was a bit cack and the general ethos is to use matching parts, hence the huge amount of SHS kit that's been installed, which brought the FPS out of the sewer and into the gutter.

I did notice that the replacement nozzle is a bit tight in the hop unit - originally figured this would aid compression as the marui one was a fairly loose fit (with visible signs of wear) - may switch it back over if the hop doesn't show any problems.

[JOE90]

Re: Low FPS despite actions to bring it back...

May be a silly question but did you silicone grease your orings and cylinder etc? Is the cylinder ported? You could of put the cylinder the wrong way round? Just a few thought there.

[brooxy]

Re: Low FPS despite actions to bring it back...

Yup, everything that should be greased, has been. Cylinder is 3/4 ported and is the correct way around.

The more I think about it, the more I think it's the hop problems suggested above, or the nozzle being too tight - so that it pushes the BB into the hop unit, but doesn't push in as far as it should to get a full seal

Hopefully will find out tonight. It's just aswell that I find rebuilding gearboxes relaxing, otherwise I'd be going mental

[JOE90]

Re: Low FPS despite actions to bring it back...

Put the old nozzle back in, see what happens. With these airseal issues its alot of trial and elimination wor mate.......gearboxes relaxing! Do me a favour!

[brooxy]

Re: Low FPS despite actions to bring it back...

Old nozzle is back in, hop looks OK.

Did a quick test - she's firing 265fps +/-4fps on .2 over 10 shots- anyone know if that sounds about right on an SP90 when paired with a 3/4 cylinder? ROF is a fair bit higher than it was on the old (although not original spring), so I think something is better inside it...

As relaxing as I find gearboxes, self tapping screws on a plastic body are another matter. Once the innards are working up to spec again, I swear to whatever deity that this thing is getting a new body...

[ElZer0]

Re: Low FPS despite actions to bring it back...

pretty basic, but is your barrel clean?

[JOE90]

Re: Low FPS despite actions to bring it back...

Old nozzle is back in, hop looks OK.

Did a quick test - she's firing 265fps +/-4fps on .2 over 10 shots- anyone know if that sounds about right on an SP90 when paired with a 3/4 cylinder? ROF is a fair bit higher than it was on the old (although not original spring), so I think something is better inside it...

As relaxing as I find gearboxes, self tapping screws on a plastic body are another matter. Once the innards are working up to spec again, I swear to whatever deity that this thing is getting a new body...

To be honest given the variables involved 265 on an sp90 isnt far off being normal. Its about 30fps from what it should be but like i said with all the variables in an airsoft gun you wont get it bang on what the packet says. Try to seal your rubber with ptfe tape, i bet that pumps it up a bit again.

[brooxy]

Re: Low FPS despite actions to bring it back...

To be honest given the variables involved 265 on an sp90 isnt far off being normal. Its about 30fps from what it should be but like i said with all the variables in an airsoft gun you wont get it bang on what the packet says. Try to seal your rubber with ptfe tape, i bet that pumps it up a bit again.

That's actually a pretty good shout that I hadn't taken into account. It's running a Madbull Black Python II barrel - whilst the accuracy and range is pretty damn good (especially with the low power output), the barrel is a tad thinner than the original marui one. Wouldn't suprise me if there was a little bit of blow-by along the outside of the barrel...