Odd, but apparently not completely unheard of... 177 airflow

Crosman Challenger 2009 (PCP). It maxed out at 530fps from factory- so I decided to pull it apart and expand the transfer port sleeve a bit. Seems I left the transfer port ('detuning' sleeve from a Crosman 13xx/22xx) at the office, so I had to drill another out. My drillbit collection seems to have diminished (seems someone borrowed a few and didn't return them yet)- so I ended up drilling the port out to 7/64ths.

So far so good. I test the rifle- shooting 680-ish with 14.3gr pellets. I take about 10 shots to ensure it's consistent. I note that by screwing the hammerspring adjuster in, I actually lose power, so the adjuster comes all the way out and things stabilize at 680 pretty much even.

Then after shooting for awhile, when the pressure drops from it's initial 2000psi to 1500psi-ish, the bolt starts being blown back on firing (was not occurring previously). This goes on until I reach 1000psi and decide to re-fill the reservoir and shoot some more before determining what to do about the bolt issue. Rifle at 2000psi now shooting 580fps, and then when pressure drops to 1500psi, bolt starts moving again and power goes up to 725fps. Hammerspring adjuster has little effect, but when screwed fully 'in' (i.e. hammer spring as tight as possible) velocity actually drops.

Without having more time to fiddle with it just yet- I am guessing I have a couple issues- hammer bounce and possibly not enough hammer travel to open the valve properly at higher pressures.

Any ideas?

Yup and this being a target gun it actually fits well.

Target guns without regulators are often set up with too much hammer spring and then choke the bejeez out of the transfer port. I run my AR this way and it gives a really flat string. Ideal for target shooting.

But when you removed the constriction you let the beast out of the bag.

I'm sure Bob will be along shortly with 6 different ways to tune it

Lots of little issues.... I have heard (but cannot confirm) that the bolt handle lifting up on a Disco is caused by the hammer cocking pin hitting it.... That would happen more as the pressure drops and the valve opens further.... which is consistent with your symptoms.... On a Disco, shortening the back of the bolt helps.... Dunno if this is happening on your 2009, or if it even CAN happen....

You said you were getting 680 fps with 14.3 gr. pellet.... I though the 2009 was .177 cal?.... Colour me confused....

In my Disco with a 7/64" transfer port I was getting ~780 fps with 14.3 gr pellets at 1600 psi, and 750 at 1200 psi, dropping to 690 at 850 psi.... With a stock 9/64" TP, the gun shot 827, 797, and 737 at the same pressures.... More hammer spring gave more power.... You seem to be WAYYYYYYY down from those power levels.... However, the 2009 has a "transfer bar" assembly that transfers the hammer hit to the valve stem.... Any possibility that limits the travel of the hammer to prevent tuning for high velocities?.... Maybe when you screw the preload in, the hammer is hitting the transfer bar holder and bouncing?.... Something strange going on there for sure....

I'm guessing that the gun is setup to use the backpressure of a tiny transfer port (didn't you say it was about 0.030" stock?).... You removed that, "unleashed the beast", as Sean says, and now the way the gun is constructed/tuned is fighting you.... On a "normal" gun increasing hammer spring preload won't usually decrease velocity.... Something strange going on, for sure....

Bob

Pardon me, 7.9gr pellets- got myself hung up on my "standard" test weights.

I suspect that 'hammer bar' or adjustable "striker" as Crosman labels it might need some fiddling. I wouldn't have expected this sort of inconsistency mind-you, seeing as how the Disco valve is similar and runs at the same pressure, and the newer Crosman PCP's (Marauder and the pistol) also have this adjustable-striker-on-hammer doo-hickey.

Yes, but they don't have that "Transfer Bar Ass'y" CH2009-100.... It also has a completely different valve than the Disco.... at least on the outside.... I wonder if the inside is a lot smaller?....

Bob