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AirGunEric · Site Admin

Well- I think I've had someone finally do it- come up with the perfect "dilemma".

What I've got is a 1377 with a flat-top piston kit (flat top piston and cut-down valve nose to build pressure faster with less pumps).

Seemed like the FTP was defective as it would only pump (very hard to pump) a few times and then a start ejecting air from around the piston seal. After much dinking about, I discovered it's problem wasn't that the FTP was not working correctly- it appeared to be working too well. It took 4 cocks and release shots for all the air to come out of the valve- it was basically a couple pumps away from complete valve lock!

So- get rid of stock hammer spring, install a 2200 spring, fit things together. Now, 8-pumps no air retention in valve (didn't have chrony set-up but sounds like it must be zinging those little .177s through it's 10" barrel at about 700-ish fps). 10-12 pumps and air is retained and noise level from it sounds to me as though the second shot must be around 450fps. If 8 pumps done, then shot, then 4 more pumps, this will be what appears to be a 'full-power' shot with not quite as much retained air for the next shot.

WTF is causing this? 12 pumps to retaining air? It does have a 1322 transfer port in it and it is only a .177 barrel- but 12 pumps and it retains air for a second shot???

Other than hogging out the valve port or lightening the internal valve spring- anyone got any simple ideas how to get all the air dumped on the first shot?

Doc Sharptail · Member

The Cdn market 1377's are partially de-tuned with a restricted transfer port. It's opening is pretty tiny. Switch to a 22-89/22-40 t.p., or drill that tiny hole out. They may have done the same with the 13-22 cdn version?

My 13-77 was retaining air at 18 pumps with a reduced valve spring and delrin flat top piston. Switching to a Discovery hammer spring dumps all the way up to 42 pumps before air is retained. There isn't much sense in pumping that much- as my gun (24" bbl) tops out around 36 pumps velocity wise. Pumping any more than that is just sending air out the bbl after the pellet has departed...

You may have to polish/clean/replace the valve stem. I had one on a 22-89 that liked to bind, and prevent full valve exhaust...

Regards,

Doc Sharptail

rsterne · Moderator

Most of my pumpers are .22 cal with longer barrels.... but in limited experimenting with a 1377 with a stock valve and hammer spring, plus an FTP and valve.... I was getting retained air at 15 pumps at 614 fps (with a 180 fps 2nd shot).... Transfer port size was 0.140"....

By replacing the hammer spring with one from a 2240 (weaker), the gun would retain air at about 500 fps at 7 pumps.... still with a stock valve spring....

If your gun is retaining air at only 12 pumps with a 2200 hammer spring (stronger), my guess is that it has a very strong valve spring.... Knowing the velocity at the point where it retains air might clue us in.... If you really are getting 700 fps in 12 pumps, I'm speechless.... Shocked

I have seen two versions of 13XX valves, one with a short check valve and a long spring, and the other with a long check valve and a short, very stiff spring.... I have never tested the latter in .177, my results above were with the short check / long spring valve.... I have found that .177 cal guns tend to respond well to a short, high-pressure pulse.... not for maximum performance, but in terms of efficiency.... ie good velocity from minimum pumps....

I wonder if the valve in your gun has the long check (or even an internal restrictor) which reduces the internal volume.... Smaller internal volume builds pressure more quickly, so with equal springs the valve would retain air with fewer pumps.... However, it now has less air to do the work.... Generally speaking, larger valves can deliver more energy, providing the valve can dump that larger volume of air....

Chrony that puppy and with that information we can probably figure it out.... Wink

Bob

AirGunEric · Site Admin

Update:

Never did chrony the gun. Never had time to work on it until last week.

It appears my audible "impression" of gas release/velocity was way way off- I had taken the rather loud, rapid "pop" to be an increase in gas flow- when in fact it was a severe reduction in flow. Turns out the adjustable sear in the gun, which I had assumed initially came from me (turns out it did not) had poor geometry and was a useless piece of crap- it was actually dragging on the hammer and slowing it down. No amount of adjustment could correct it, and it was the source of the retained air issue.

Unfortunately, I didn't figure this out until I had eliminated everything else in the valve/hammer path (i.e. valvespring adjustments were made, hammer spring modifications and so on).

After replacing the counter-productive sear with a proper unit the retained air issue went away and the gun performed quite well. I will post pictures of a proper sear vs. the poorly designed POS that was found in the gun for comparison a bit later.

rsterne · Moderator

Interesting about the sear problem.... I had a similar issue with my Disco right after I bought it.... I had removed the safety so that I could get the gun out of the stock and when I chronied it, I had LOST a couple hunderd fps and had terrible inconsistency.... Shocked

The gun was still stock at that time (hammer spring, including preload).... from what I remember, anyway.... The safety is located by the same spring as the sear, and when you remove it the sear spring has so little tension it doesn't even cock reliably.... For some strange reason, it appears what was happening is that somehow the reduced spring pressure on the sear caused it to drag on the hammer on firing, slowing the hammer down and affecting the velocity.... You would think the opposite would happen, if anything, but as soon as I replaced the safety the problem went away.... Rolling Eyes

Anyway, it's possible for sear interaction to alter the action of the hammer in a negative way.... Crying or Very sad

Bob

AirGunEric · Site Admin

Well, here is the source of the problem:

Sear, by vendor who shall remain unnamed:

A properly done sear (Steve Woodward unit):

And together:

That narrow channel and the higher trigger contact point on the non-functional sear seems to jam things up, and no amount of adjustment would overcome it.

I think the point is, be careful where custom parts are bought- they are not all equal.