Mk1 FAC; FPS dropping, low shot count

[Squinty]

Lots of messing about today, had probe issues which made the gun louder and resulted in power loss.
Was running 940fps c/w 18gr...975fps c/w 16gr,  dropped to about 26ft. Fitted a new probe which sorted that.......BUT

Initally thought it was the short metal valve which was causing the problem, just appeared to be more mechanical noise resonance.
I changed to a new delrin one with 5mm port then 5.5mm. Neither picked up the power.
No advantage from the bigger valve and couldn't get a power curve at 210 bar.

Fitted the new probe on advice from Kev and went back to the new valve with 5mm port.
The valve stem o-ring went setting up the probe... irrespectively replaced all valve o-rings again and initially it appeared to be back on form.

But once up at my perm to chrono and zero new mounts...the fps started to drop 975 >970 > 968> 962> down to 920 and shot count was around 28 from a 480cc bottle.
Usually get around 55 shots 195bar  >145ish.

Backed off hammer preload, still made 975 and dropping.

No pops or farts, no external leaks, no air being vented through barrel with the hammer pip resting on valve (under preload).
Shoots quiet at all velocities and is accurate although POI obviusly starts bang on and drops at the end of the bottle

Only 'other' change was a new exhaust spring... bit longer/stiffer, should be closing.

Just seems to be running away with itself...off the bottle, no valve control?

Had this problem when I tried a larger barrel port, big power boost but no control. stem spring cured it but not this time.

[Kernal]

Have you tried fitting a reg on it, Just a thought!

[Squinty]

Have you tried fitting a reg on it, Just a thought!

Considered but its an earlier R7 block with the shallow lower chamber...typically runs 10-15fps extreme spread so consistency wasn't a prior issue.

Really don't want to go the whole hog and lose a great non-reg gun by opening it all up to mk2 spec.
No advantage in my mind, for a few extra shots

[Kernal]

Sure KevG does a mini reg that will fit that gun of yours Squinty,

[Squinty]

Sure KevG does a mini reg that will fit that gun of yours Squinty,

Block will work on a basic shortcore as its been back ported...I may also have one or two in the spares drawer 

But I'd need  a valve chamber extension and another valve, plus the top port and the barrel port opening up.
It's like money..more money...to sort out something stupid simple ....not that I wouldn't fancy a trip up to Donny ...I have a list of jobs

[Kernal]

You should list a complete conversion of jobs done to your rapid as i`m guessing there is alot,...I`d be interested in seeing what you`ve had done to that poor little rapid to put it in the contest,...... 

[Squinty]

You should list a complete conversion of jobs done to your rapid as i`m guessing there is alot,...I`d be interested in seeing what you`ve had done to that poor little rapid to put it in the contest,...... 

Not as much as some sub-12s tbh - less than my air-miser mk2.

Just if it's not shooting exactly as I want I will pull it apart and try a different setup; again and again.

But as I said this is a stupid-simple issue.... where I can't see the wood for the trees

[Kernal]

Maybe seeing the tree`s first,...Before the woods would help

[KevG]

have you thought of a valve chamber extension
KevG

[Squinty]

was kinda hoping for the answer to appear whilst I typed..... maybe  a DOH! Maybe  a Eureka moment 

but all I can think of is either the stem spring isn't as stiff as I thought or the longer valve body is putting the top-hat in the thru-port airflow and not closing.

Either way I believe its valve control?

Miyagi San...I have lost my balance

[Squinty]

have you thought of a valve chamber extension
KevG

Considered lots of things Kev, at the moment I just want it running a consistent velocity again.....rather than getting consistently slower.

[KevG]

I guess you may have tryed this but have tested to see if you are thrashing the gun.ie.say you just fill gun and say your doing 975 fps.have you tryed backing adjuster of say half a turn at a time and see if power is not dropping if it is not then turn out until power starts to drop then just back in just enough to get power back
KevG

[Squinty]

Had considered that,
Came straight in at 975fps / 190 bar when I checked the probe seal, best part of a mag with 5fps spread. Then the stem o-ring split, (PLIP) probably nipped it putting it in the valve body
Valve was taken out, resealed and put back in. Checked for 'pop' and away to the perm.

First bottle was shot zeroing over the chronograph started at 975 with 16s; was not a surprise and besides the mounts being 15moa left of scope central ....5p accurate at 50yds.
On the second bottle; ran a few 16s which started off hot 990fps,  bottle was at 200bar (checking for power-curve plateau).
Backed it off to 975ish but the velocity tailed off
And again on the third fill....from 975 to 950 and  950 to 930 still the velocity dropped after first 14 shot mag.

Got a few more shots at the bottom end of the bottle but on the fourth fill it was up to 975 again.

Just seems to be running to full bottle pressure. Exhaust valve not shutting down?

Had 38 shots off the first bottle
28 off the second 990 > 910
33 on third 975 > 910
gave up after 15 shots 975 > 945  but still dropping again.

Worst case... maybe I'm thrashing a 30ft/lb set up for an extra 3ft/lbs?
But I'm a good bit  short of the consistent 35/36 it was making before.

will have to wait until next weekend to try a different spring combo.

[Brooksy]

I reckon the valve spring is running the gun rather than the air pressure that should be closing the valve.

[KevG]

what length stem spring you using.normaly 19 to 21 mm should be fine as long as correct coil section diameter
KevG