Weak 1377

[slivershooter]

Hey Fellas,  My latest 1377 project has got me confused.  I suspected an air loss at first and buttoned up the transfer port and bolt seal. Chronying this thing, I get less than 400 fps with 12 pumps. ?  My attention is now on the valve.  Could it be that it's not pumping up and I have air going somewhere other than into the valve? It has a stock valve. I haven't monkeyed with it. I don't want to spend on getting a flat top setup cause if I could get the velocity that other stock 1377s do, I would be happy.  Any ideas my friends?

[BDS]

Project? So what did you do to the gun or add on to it? What weight pellets shoot at 400 fps? What did it shoot (fps) prior to the project (pellet weight, pumps, etc)?

And, 12 pumps with a stock valve? Not good IMO, it's just a plastic piston and parachute cup seal.

Hard to diagnose the problem without a lot more info. The early stages of Crosmanitis seems to come with advanced "lack-of-detail-aphobia" symptoms? 

[1377x]

as bds said more info is needed
did the gun gun start out as a complete stock 1377? not made from left over parts

[slivershooter]

You're right guys. Lack of information. Sorry for my bad case of crosmanitis or one of its  many affiliated conditions.  This gun is a rebuild from parts that I had laying around. 760 barrel that I milled on my lathe to match a 1377 barrel. The valve is a change out from a new 1377 I put a flat top in.  The barrel port and chamber I have closely looked at and tested with oil for leaks and at first there were some blowout on the sides. I tightened up those areas and I have no signs of leakage. I did replace the o-ring on the valve before installing it. I used a hardware store variety and maybe this is the culprit. Are Crosman O-rings a special thing? I don't feel it's taking on air like it should. It should have a nice bark to it and it sounds subsonic. 

[1377x]

some o-rings are softer crosman doesnt use anything special just a 70 duro o-ring. viton,buna-n, polyurethane all work good. dont think you want silicone o-rings
now you ported and chambered a 760 barrel, i assume it is smooth bore?, i am kinda thinking this is the problem. can you swap out the barrel for a stock 1377 barrel?
maybe to much air is getting past the pellet as its going down the tube
or
maybe your barrel port isnt the same size as a stock 1377 barrel smaller perhaps. it might be off not quite in line with the other ports
i think i would try a stock 1377 barrel then report back
also
get some o-rings you know are at least 70 duro made from either viton,buna-n or polyurethane(spendy)

[BDS]

X2 to Ed's comments, the barrel swap is a big red flag, both port alignment/size as well as the smooth bore, or is it a VERY old 760 with rifling?

[slivershooter]

No, the barrel is rifled.  The port holes are all the same size so there are no restrictions.  It may be that the chamber isn't tight enough. I was thinking if the pellet was sitting a bit loose, some of the air could slip by before it started down the barrel.  I may recut the breech end or push the pellet further into the barrel with a longer bolt probe.  Does that make sense?

[BDS]

A longer probe can help but, you still need room to load your pellet so that's not the answer (maybe just a component of the answer)

Try this, take barrel off and push a new pellet thru the barrel by hand with a cleaning rod from breech to muzzle, take note as you go of fit of the pellet or restriction of the breech and barrel as you push it through.

The ideal condition is minimal restriction at the breech, a slight bit more through about 80% of the barrel length and even more at the muzzle end. This is not always the result from Crosman barrels but, it's the ideal situation. (match barrels are "choked" this way) Also look at the pellet to see how much (if any) rifling grooves you can see in the pellet. If you need to do this over, use a new pellet each time.

If the pellet is sloppy throughout the travel, the barrel is a problem. Also, if this was a "760" barrel, it may have seen 1000's of steel BBs thru it regardless of the single shot pellet capability of the rifle and that surely ruined the rifling which is very shallow in an airgun. Was it your 760?

[WyoMan]

+1 to that...and a variation is to push a pellet from the muzzle to the breech...can do both sometimes helps find restrictions better

[BDS]

Hijack, hey Wyo... hows the weather been over in Wyoming?  Hotter than the hinges o' hell here in SW Idaho this summer, ok maybe not that bad but, 90+ degrees most days by 5pm. Started early this year and keeps on going. 

[WyoMan]

Way hotter than normal here too, we got to 90 a few times and normal is high 70's
Smokey too from the fires, seems like everyone has allergies now....do you have the smoke too?

[BDS]

Yup, cough-hack, mostly blowing in from west norwest from Yosemite CA fires, about 500 miles from here as crow flies (dies), guess we are sending you our Teton and Sawtooth fires smoke? 

[DaveB50]

Have you done the tissue test? Drape a tissue over the breech and fire the gun, if it moves, you have a leak.

[1377x]

that or the powder test
i have been using that lately because you can cover the bolt and transfer port at the same time
powder blows off the bolt if there is a bolt leak and powder blows off the tube/breech area @ the transfer port

[BDS]

Good call on the powder Ed