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Old Lampy · Member

I put this thing together about a decade ago. I dusted it off a few months ago and put it over the Chrony today.
Shooting RWS Superdomes at 8.3 grains, 2 pumps 396 fps, 4 pumps 553 fps, 6 pumps 610 fps.
It stays at around 610 fps all the way up to 8 pumps at 10 pumps the fps drops to 600 fps.
My plan is to replace the hammer spring with a heavier one to see what happens or add weight to the hammer.

Old Lampy · Member

The first mod is done. I pulled the hammer assembly and added a brass spacer ring to the hammer, besides adding weight to the hammer it adds preload to the hammer spring.

With the added preload the hammer holds the exhaust valve open and has to be cocked before pumping up.
The new Chrony readings are 2 pumps 390 fps, 4 pumps 564 fps, 6 pumps 631 fps, 8 pumps 670 fps, and 10 pumps 689 fps. It's no longer stalled at 610 fps at 6 pumps. A gain of 80ish fps at 10 pumps is a start.

Next will be changing the Frankenstien valve I have in this thing.

I wish the mod went as smooth as this post makes it sound but it did not.

Old Lampy · Member

The next change was to the exhaust valve. I wanted to increase the volume of the valve to see what effect this has on velocity. I did this by switching the long check valve for a short one.

At the start of this project my thinking was that less volume in the valve would take fewer pumps to bring the pressure up. From what I see from the numbers this is true. The drawback seems to be the hammer is not able to overcome the high pressure and open the valve quickly enough. The other issue could be the restriction of the exhaust valve slowing the air down...

The list bellow is the number of pumps with the "As built" "Heavy Hammer" and "More Volume" FPS

Looking at the numbers again I'm going to try making a valve with less restriction in the exhaust valve...

AirGunEric · Site Admin

That is some great improvements in output.

The amount of pressure restricting things at a certain point has been common in the PCP world- but with newer valves containing only a single plunger the only way to deal with it seems to be a lighter valvestem (most PCPs a few year back had alot of metal in the stems, so people were switching to more plastic). In any event, you seem to be on the right track..

Old Lampy · Member

I tried increasing the size of the exhaust port for increased flow. I ran into the problem you mentioned. The larger exhaust valve means more area for the air pressure to push against making it harder to open. At lower pumps/pressure it was looking great, close to 600 fps at 3 pumps. At 4 pumps fps dropped to 400 fps and the valve was holding air, it took 3 shots to empty the valve.
I took a break and asked google for some help. I found an article in Hard Air Magazine written in 2018 explaining how a balanced valve works.
I'm working on my version now. Squeezing a check valve in with the extra bits needed for a balanced valve is a bit challenging.

https://hardairmagazine.com/ham-columns/balanced-valves-for-pcp-airguns-theyre-here-today/

AirGunEric · Site Admin

Did you make your own stem and exhaust valve, or where did those pieces come from?

Old Lampy · Member

I have a stash of old parts from when I was into airguns a decade or so ago but the new valves (aluminum) are what I dreamed up and turned on my lathe.
I'm really bummed that Crosman as I knew it doesn't exist any more. Gamo seems to be taking over, and that explains why the Crosman Vapor Max looks more like a Daisy pumper than a Crosman pumper.

AirGunEric · Site Admin

Old Lampy · Member

I called Crosman yesterday to order a .177 barrel for a 1760 (just to have for future projects) and was told they had none in stock. They referred me to Alchemy Airwerks and it looks like they bought up some of Crosmans inventory of parts.

Old Lampy · Member

Just some pics of what I've been doing with the valves. One of the main mods is opening up the exhaust port from 1/8" to 3/16"

Just for fun I made my own valve out of aluminum. I used o-rings for sealing instead of PTFE or whatever Crosman uses for their valves. My valve showed promise but it wouldn't empty all the air after 4 pumps.

I have a third valve I'm working on, trying to make a "balanced" valve. I haven't had much luck yet.

AirGunEric · Site Admin

Is it possible your valvestem is just too heavy to be able to be hit fully open and stay open for any length of time?

Old Lampy · Member

Old Lampy · Member

I'm calling it done.
I put it back together with the original brass valve. Making my own valve was fun but it's time to move on.

The final stats

2 pumps 362 fps
4 pumps 578
6 pumps 652
8 pumps 713
10 pumps 725

Next on the bench is a .22 cal build I started around the same time I originally built this.

Old Lampy · Member

Went to the range last week and shot a few targets.
5 shot group with 4 out of five inside the 1" bullseye at 25 yards.

AirGunEric · Site Admin

That's great shooting for a pump-up 1377-based gun- nice work!