Older 1377 expected FPS?

[Methuselah]

Backpacker made in 1983, fresh pump cup, using 7.9 grain crosman premier hollow points. I just resealed it and cleaned/recrowned the barrel.

Pumps          Fps
3                    278.8
5                    342
8                    399.9
10                  410.2
15                  449.9
20                  470

Not enough retained air to push out a second pellet at 20 pumps.

It's not the old dump valve type, should I be getting higher numbers?

[JPSAXNC]

With a stock length barrel it should be making higher velocities. It could be that the compression is not high enough (piston not touching the valve) a leak past the o-ring, a  slow hammer strike. A few things to check. hth

[Methuselah]

With a stock length barrel it should be making higher velocities. It could be that the compression is not high enough (piston not touching the valve) a leak past the o-ring, a  slow hammer strike. A few things to check. hth

Oh boy - I reckon it's time to don the dunce cap...

It does get pretty stout between 15 and 20 pumps, seems to dump all the air at 15 pumps, and the hammer cocking is also stout. What I did not do was polish the hammer or lube it (not even oil).

Which o-ring were you referring to, breach? That I could tissue-test for, but how do you "adjust" from pump cup to valve?

How much higher should the numbers be?

Thanks James!

[maraudinglizard]

Steve, I have a rear cocker the same age and did a full make over on it. The numbers should be a lot higher at 10 pumps. Here is the link to what did with the gun.

https://www.gatewaytoairguns.org/GTA/index.php?topic=83670.msg802677#msg802677

[Methuselah]

Steve, I have a rear cocker the same age and did a full make over on it. The numbers should be a lot higher at 10 pumps. Here is the link to what did with the gun.

https://www.gatewaytoairguns.org/GTA/index.php?topic=83670.msg802677#msg802677

Nice thread Kris, yours has a different pump arm than mine, metal and adjustable where mine's plastic. Great shooting for 50 yards! So far this one's doing OK at 25 yards (perhaps an inch with the stock irons).

I thought maybe I put the transfer port ring in the wrong spot, but it matches on-line schematic at JGAirguns website. I'm puzzled why it's about 200 fps slower than yours...

I'll probably tissue-test the breach tomorrow.

Edit: BTW, I also did some trigger work, shimed the sear and added a spring guide (.22 casing).

[maraudinglizard]

David Grimes has all the parts and will answer any questions you might have about the 1377. Just give him time to return your messages, you can call or email him. His member name is Davio. Great guy to work with. Just let him know how old your gun is to make sure the parts will fit. These guns have change just a little bit in some of the parts.

https://www.ebay.com/str/alchemyairwerks?_trksid=p2047675.m3561.l2563

https://www.alchemyairwerks.com/

[JPSAXNC]

Hi Steve, You can do a simple test for leaks by pumping the gun and let it sit for an hour, fire it and see if you get the same velocity. I was thinking of the o-ring sealing the valve body.

[dukemeister]

Check to see if you have a Canadian detuned version. The chrony numbers match well for 1377c. The main detuning method is a bleed hole in the valve, so look to see if there is one.

[Methuselah]

James/Duke,

Both good thoughts, today was spent sighting in two scoped air rifles, replacing pump cup and check valve on a 1300, and cleaning up the parts on a torn down 1377, so will try the suggestions during the upcoming week.

Took more than half the day to get that cotton picking pump cup off!

Thanks!!

BTW, I think I had seen the screw hole and transfer port were not 180 degrees apart...

[Methuselah]

Ten pumps, 5 minute wait, 416.0 fps
Ten pumps, 32 minutes later, 426.6 fps

Didn't want to wait longer , sun was going down and didn't want to factor temperature in.

My pics don't show the side of the valve with the poppet, so no idea if it's got a bleed hole there, but that seems like something I'd have noticed when I lapped the seat with Flitz.

I've read it's common for the holes not being a perfect 180 degrees apart, I wonder if the transfer port got partially blocked. If that turned out to be the issue, is best course of action a replacement valve?