'78 Benjamin 347 rebuild
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'78 Benjamin 347 rebuild
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Topic: '78 Benjamin 347 rebuild (Read 3145 times))
Kidder
Shooter
Posts: 9
yes
Real Name: Travis
'78 Benjamin 347 rebuild
«
on:
September 14, 2018, 01:59:13 PM »
Good day all,
I recently bought a '78 Benjamin 347 that shoots weak. I can pump it up with ease. I started out just replacing the pump cup/seal and didn't seem to help. So I ordered an entire seal kit with the valve tool on eBay last week that should arrive next week. Maybe the check valve is bad and that is why it's not building up enough pressure? I can hear a slight air hiss as soon as I push out the pump handle but it seems to hold pressure fine, with the little that is built up.
I really thought the pump cup was the problem and was expecting that would have fixed it. Is there anything special I should have done after replacing? I dropped a little ND-30 oil on it on the advice of a guy on YouTube that was describing an almost identical issue as mine. The pump handle also falls loose after shooting. The piston on this particular 347 is a fixed length, no adjusting.
Any help would be appreciated.
Thanks much,
Travis
«
Last Edit: September 14, 2018, 02:08:15 PM by Kidder
»
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Enon, OH
ped
Sharp Shooter
Posts: 548
yes
Re: '78 Benjamin 347 rebuild
«
Reply #1 on:
September 14, 2018, 06:02:20 PM »
the pump arm falling is probably a damaged /missing pump arm spring
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uk
TerryM
Expert
Posts: 1559
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Re: '78 Benjamin 347 rebuild
«
Reply #2 on:
September 15, 2018, 09:41:10 AM »
You might have a bad check valve. Does it build up more pressure than one pump? I would replace all the seals if it were mine. Good luck with it.
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TX
Crosman model#
101, 108, 110, 112, 113, 114 (x2), 118,
140, 180 (x3), 187, 400, Mark II, 1377, 1322, 150, 122CG, Sears 126.19311, Ted Williams Match Rifle (2nd variant 160) Self made 180-400-114 hybrid carbine
Benjamin model#
312(x2), 317, 342, Discovery
Sheridan:
Blue Streak (x2: '64, '67)
F model CO2 Blue Streak (converted to bulk fill)
DanD
Nod and smile.
GTA Senior Contributor
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A solid maybe
Real Name: Dan
Re: '78 Benjamin 347 rebuild
«
Reply #3 on:
September 15, 2018, 11:28:42 AM »
Quote from: ped on September 14, 2018, 06:02:20 PM
the pump arm falling is probably a damaged /missing pump arm spring
I concur. I've had this problem with a used gun that had a worn out arm and missing spring. I replaced the pump arm, piston, and piston pin with current 397 parts and deepened the groove of the 347 handle to fit the new arm. Cost me around $30 and works great, but I wish I had spent more and bought NOS 347 parts to keep the arm looking original.
Sounds like the OP has other issues, too.
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USA, upstate NY
ped
Sharp Shooter
Posts: 548
yes
Re: '78 Benjamin 347 rebuild
«
Reply #4 on:
September 16, 2018, 07:44:09 AM »
I have a 347 with the same issue but i'll be cutting the piston and threading it like I do my non adjustable bluestreak ones
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uk
Hoosier Daddy
The sound of silence
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Real Name: Scott
Re: '78 Benjamin 347 rebuild
«
Reply #5 on:
September 16, 2018, 09:36:01 AM »
The Arm tension spring is more like bent flat steel that an actual coil spring. Should be able to tell if it is one piece or broken / missing. Item #123...
Here in the 'states I would just call Rick at Precision Pellet and see if an adjustable one from an older 342 would fit and if he has one for sale. Probably does, and NOS too boot.
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Kendallville, IN
ON THE GTA MAP!
JimQwerty123
always said ...
"Shoot safe and have fun!"
Kidder
Shooter
Posts: 9
yes
Real Name: Travis
Re: '78 Benjamin 347 rebuild
«
Reply #6 on:
September 17, 2018, 08:24:28 AM »
Thanks all. I checked the arm and there is a tension spring but it is laying the bottom of the pump arm, not really serving any purpose. I need to pry it out of there and see what the deal is. Not a big deal that it hangs loose, I'd rather have this rifle shooting at full power. It doesn't seem to build up additional pressure after a couple pumps; it's just as easy to pump it 10x as it is 2x.
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Enon, OH
ped
Sharp Shooter
Posts: 548
yes
Re: '78 Benjamin 347 rebuild
«
Reply #7 on:
September 17, 2018, 05:54:07 PM »
wants a rebuild in that case
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uk
Hoosier Daddy
The sound of silence
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Real Name: Scott
Re: '78 Benjamin 347 rebuild
«
Reply #8 on:
September 17, 2018, 07:12:23 PM »
Ditto.
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Kendallville, IN
ON THE GTA MAP!
JimQwerty123
always said ...
"Shoot safe and have fun!"
eeler1
Expert
Posts: 1364
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Real Name: Jon
Re: '78 Benjamin 347 rebuild
«
Reply #9 on:
September 18, 2018, 02:00:22 AM »
You’ve ID’d two issues, here’s some things to try;
1- check head space on the pump arm, and adjust till there isn’t any
2- try the pump lever test for the check valve, 8 pumps, open the arm, let it sit overnight and see if the pump arm moves towards or past the air intake hole.
3- the link spring is held in place under the rivet. If spring is missing, then you can maybe force a new one into place without removing and replacing the rivet. Otherwise it gets kinda involved.
4- cock the gun before you pump
Start with #4, then #2, then #1, see what happens
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West Sacramento, CA
Kidder
Shooter
Posts: 9
yes
Real Name: Travis
Re: '78 Benjamin 347 rebuild
«
Reply #10 on:
September 21, 2018, 12:45:44 PM »
Thanks all for the suggestions. I disassembled it this morning after a real struggle with the valve nut breaking free. Heated it up with heat gun and that seemed to have helped. Looks like the rubber insert in the check valve is coming out of the valve. Maybe this is why it wasn't building up sufficient pressure? The outlet valve shaft also has a bit of rust/residue on it, causing some friction in the the valve seat. Not sure if that could be causing my problems or what. Everything else looks OK, though there is a lot of corrosion on the springs and washers. Someone had tinkered with it in the past because there was a rubber washer on the valve side of the valve seat instead of a lead washer. Shouldn't there be a lead washer on both sides of that? There were also two valve spring washers (brass) inside of it. I'm guessing they were doubled up? Does anyone know what the thickness of this washer should be? Maybe they lost the original and only had thinner ones? These are measuring .040" each.
Anyway, gonna put it back together today and hopefully start plinking with it this weekend. Oh yeah, the tension spring is cracked in two, just underneath the rivet, so I'll have to replace that sometime in the future. Not a big deal right now.
-Travis
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Enon, OH
Hoosier Daddy
The sound of silence
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"Eye for an eye" leaves everyone blind
Real Name: Scott
Re: '78 Benjamin 347 rebuild
«
Reply #11 on:
September 21, 2018, 12:51:05 PM »
Seems to me one of my rebuilds had two washers as well, or maybe I read about it here following somones rebuild.
The determination was they were an accident and only one was needed... Like they were stuck together on original assembly.
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Kendallville, IN
ON THE GTA MAP!
JimQwerty123
always said ...
"Shoot safe and have fun!"
Kidder
Shooter
Posts: 9
yes
Real Name: Travis
Re: '78 Benjamin 347 rebuild
«
Reply #12 on:
September 23, 2018, 11:12:59 PM »
Well, it seems that my 347 is fixed! Shoots so much harder now, even after just 3 pumps. Unfortunately I don’t have a chronograph to see what FPS it’s shooting. The only issue I have it that it doesn’t exhaust all of the air from the chamber. A second dry firing exhausts the remaining air. I can also feel a puff of air on my cheekbone after firing. Should I worry about either of these?
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Enon, OH
eeler1
Expert
Posts: 1364
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Real Name: Jon
Re: '78 Benjamin 347 rebuild
«
Reply #13 on:
September 23, 2018, 11:42:05 PM »
puff of air might be an adjustment issue for the cam plate. Loosen the two screws and reset it so you can just close the bolt or just before the bolt is fully closed.
So, not dumping air at 3 pumps? Or how many? Factory set up was to dump at 8 pumps. More pumps, maybe didn’t dump.
Oh, and cool you got it shooting again. And so the journey begins!
«
Last Edit: September 23, 2018, 11:44:31 PM by eeler1
»
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West Sacramento, CA
Kidder
Shooter
Posts: 9
yes
Real Name: Travis
Re: '78 Benjamin 347 rebuild
«
Reply #14 on:
September 24, 2018, 12:08:11 AM »
Yeah, I’ll adjust that plate back to its original position. It’s not dumping after 3 or 4 pumps, haven’t pumped it up 8 times yet.
Can’t wait to start obliterating some pop cans now. Ha!
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Enon, OH
eeler1
Expert
Posts: 1364
yes
Real Name: Jon
Re: '78 Benjamin 347 rebuild
«
Reply #15 on:
September 26, 2018, 02:51:12 AM »
Not dumping air isn't the end of the world. You can learn it and live with it, with some effort. Or just shoot out the retained air with a dry fire after each shot.
Retaining air is usually due to one of several factors. Could be that double washer, but not likely at 3 pumps. More likely the exhaust valve stem isn't running true or isn't sitting all of the way in the exhaust valve spring, or both.
If you don't want to tear it apart again, you might shim the hammer spring until it just cocks, so it will hit the valve stem harder. Might help some.
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West Sacramento, CA
ped
Sharp Shooter
Posts: 548
yes
Re: '78 Benjamin 347 rebuild
«
Reply #16 on:
September 28, 2018, 04:17:51 PM »
not dumping all the air is because of insufficient hammer strike
this could be due to
too stiff a valve spring -highly unlikely
binding stem -doubtfull
binding hammer-unlikely
weak hammer spring -more than likely
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uk
Hoosier Daddy
The sound of silence
GTA Moderator
GTA Senior Contributor
Posts: 24935
"Eye for an eye" leaves everyone blind
Real Name: Scott
Re: '78 Benjamin 347 rebuild
«
Reply #17 on:
September 28, 2018, 06:05:13 PM »
Give the hammer spring a stretch... 1/8" to 1/4" in length should be plenty.
It is a balancing act between that and the valve spring tension...
I had to "cock" the bolt before pumping at first but it settled in some what.
Mine now dumps all of 6 but not 8 pumps.
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Kendallville, IN
ON THE GTA MAP!
JimQwerty123
always said ...
"Shoot safe and have fun!"
Kidder
Shooter
Posts: 9
yes
Real Name: Travis
Re: '78 Benjamin 347 rebuild
«
Reply #18 on:
October 04, 2018, 01:31:43 PM »
Thanks guys! I'll give that hammer spring a shot (no pun intended...) and see if it dumps better. I'm not too worried about it at this point, been dry firing after every shot.
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Enon, OH
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'78 Benjamin 347 rebuild