1377 Low FPS, Need some assistance

[zeroz52]

Hi Everyone,

I have been doing some mods to my 1377 recently in order to bump up the FPE and I'm getting a lot less than I'd expect.  I've torn the gun apart and rebuilt it so many times now trying to get it figured out, I'm getting a bit frustrated and looking for some direction.

Here is the list of mods I've done:
Steel Breech
24" Barrel
FT Piston and Valve
Trigger Spring and Guide
Aluminum Barrel Band
Larger Pump Arm
Crosman Stock

With all of that @ 10 pumps I'm averaging about 430FPS.  I only got the Chrony recently so I don't have a baseline for comparison, but I was expecting to be Closer to the 650FPS range. 
I think it is the infamous crappy TP seal, but I'm looking for some ideas on how to investigate it.  The one thing I did notice the last time I assembled it, some portions of the seal look to sit below the Pump Tube OD, allowing air to leak out. 
I also am pretty disappointed in the quality of the FT Piston and Valve, as there are some definite issues with ports being concentric, not awful, but when I put the seal in I can see material where things are not lining up.  And I'm a bit concerned that it may be restricting my flow a bit, at the very least it will cause turbulence and pressure drop issues.  But I do not know enough about airguns yet to know if that would translate into the low numbers I am seeing.

For those with a FT Piston & Valve, did you notice it became noticeable harder to pump? 

Thanks for any info.

Z

[BenjiHunter]

There is one thing I have seen that can give you less FPS.
That is if the pellet isn't seated deep enough in the breech.
It lies on top of the exhaust hole.

[breakfastchef]

You likely have a leak or misalignment.

Check for a leak around the transfer port. Lay a piece of tissue paper over the junction of the compression tube and breech where the transfer sleeve is located. Pump three times and fire the rested gun. If the tissue paper moves, you likely have a leak. Alternately, you could use talcum powder or corn starch. I find the powders provide better visual evidence of a leak. If you find there is blow by, check that you installed the transfer sleeve correctly (longer part into the barrel) and that the seal on top of the valve is in good shape.

Make certain the exhaust port of the valve is lined up with the appropriate hole in the compression tube. Often you need to unscrew the rear section of the valve a wee bit to get things lined up.

I assume you know how to adjust the flat top piston. If it is not properly adjusted, you will not get full compression. Check that the piston o-ring is not nicked.

The hammer dragging on the top of the sear can slow it down and cause a drop in velocity. If you used shims in the trigger group that are too thick, they can bind when the cover plate is installed. If the sear cannot fully retract when you pull the trigger it can interfere with the hammer.

[K.O.]

Wow there are problems for sure.  My stock PC77 gets 505 fps and if I use a seating tool to push the pellet just past the port I get 530.

That is Average velocity over 15'.

Did you make sure that you did not put the transfer port upside down? 

I think you should register @ the Crosman pistol owners forum there is a good article about barrel vs valve volume in the tech section there

and some knowledgeable folks also...

Did you check the pump tube for burrs before reassembling, to check for a cut seal pump it up like normal get fps then pump the same let it sit for a few hours and check fps...

[BillK]

Hard pumping sounds like an improperly adjusted piston.  The stroke may need to be shortened.  Short end of transfer port goes down into the pump tube.  Holes all should line up.

[zeroz52]

Thanks for all the assistance guys.
I have the standard bolt, I have not upgraded that as of yet.  I really doubt that would drop me below stock performance, but I will give it a try and seat it a little further in.
I definitely have the TP and seal installed the correct way, so I know it isnt that.
I'll give the tissue trick a try to check for a leak, thanks for the idea.
I found a link on how to adjust the FT.  That is something I did not know I had to do.  Could be part or all of my problem there.  Bit of ignorance on my part there.
I'll report back when I have a chance to tear it down again.