CA MP5 issues after upgrades

[Bellerophon]

I just finished rewiring my CA MP5SD with 16AWG wire + deans, as well as installing a 3/16" sorbo pad and the requisite grinding and sanding of the piston teeth to correct the AOE. I'm extremely impressed by the improvement in responsiveness and rate of fire, especially considering everything else including the battery (8.4V NiMH mini type) is stock. However, in doing these upgrades myself, I've encountered some problems:

I. FPS loss, was shooting 320 ± 5 before and now shooting 260 ± 10. I expected some FPS loss when installing the sorbo pad due to the reduced cylinder volume, but not this much. I'm already testing various parts for compression as per Thundercactus' thread. I suspect the cylinder's side vent is now too far forward.

II. Tappet/nozzle in 'open bolt' position, sitting fully rearward at the beginning and end of the shot, moving forward with each cycle of the gearbox. The tappet plate isn't apparently stuck against anything, moving freely back and forth at a finger press when the shot is paused in forward position. The tappet and nozzle still seem firmly attached to each other. Also, when I was working inside the gearbox, I noticed the sector gear has only a little round post to draw back the tappet plate with, in all other gearboxes I've opened there is a little cam on the sector that engages the tappet, is this normal?

III. Short trigger pulls on semi will produce a burst of full auto, which ends once the trigger is pulled fully to the rear. I'm pretty sure I reinstalled the contacts correctly when I was finished soldering, but it seems like there is some pre-engagement going on here. Resolved, thanks pestobanana!

IV. Probably related to both II and III, misfeeds and double feeds.

If I can combine the power and reliability I had before with the responsiveness and quietness I have now, I think I'll be having a very fun summer. Thanks for reading this far.

[lurkingknight]

1
what is your barrel length? anything over 363 you'll probably need to get the next step up in cylinder. Which would probably be the hole at the end.

2
might be normal, the nozzle doesn't have to park in a specific location. But could be an indicator of the answer for issue 3. Some sectors have a post, some have a cam, some have a big circle. As long as it's pulling the tappet back with the correct load and shoot timing, it's fine.

3
you may be seeing overspin so you're getting 1 full cycle plus a little bit due to the increased electrical efficiency. Could also just be starting to reveal a worn cutoff lever. Other things it could be is the trigger gate is holding the shuttle too long (too tight) so the shuttle spring can't pull it out and requires a couple hits from the cutoff lever.

4
When you installed your sorbo did you glue it on to the rubber pad on the cylinder head or did you strip the pad off? When you corrected your AoE, how much of the 3rd tooth did you take off if you took the 2nd tooth completely off. (1st tooth being the pickup)

If everything else is stock it's unlikely to be seeing overspin or rate of fire faster than the magazine can feed. It's usually RoFs getting closer to 30 that will expose feed timing issues.

But, it looks like you have what, a m90 spring? It's possible overspin can be occurring with such a weak spring, but it's alo a rather weak battery.


Does the gun gravity feed upsidedown?

[pestobanana]

Your full auto in semi is because your trigger prongs are too close together. You are closing the circuit before the trigger sled is far enough for the cutoff lever to knock it up an release it. Pry them apart a bit.

[Bellerophon]

Originally Posted by lurkingknight

1
what is your barrel length? anything over 363 you'll probably need to get the next step up in cylinder. Which would probably be the hole at the end.

Barrel length is stock 275mm, my concern is because the piston starts its stroke from so much farther back that the vent might now be in the wrong spot

Originally Posted by lurkingknight

4
When you installed your sorbo did you glue it on to the rubber pad on the cylinder head or did you strip the pad off? When you corrected your AoE, how much of the 3rd tooth did you take off if you took the 2nd tooth completely off. (1st tooth being the pickup)

I glued the sorbo to the rubber pad, 2nd tooth I completely removed, I filed down only the top and leading edge of the 3rd tooth.

Originally Posted by lurkingknight

If everything else is stock it's unlikely to be seeing overspin or rate of fire faster than the magazine can feed.
~
Does the gun gravity feed upsidedown?

It isn't that it won't feed, it's that sometimes it double feeds.

Also, thanks pestobanana, that's a plausible explanantion for the semi issue as I had to remove the trigger prongs for soldering and probably bent them back too closely.

[Hectic]

for the fps drop there is a leak somewhere.
the stock cylinder has a fair bit of extra volume.
arnies use to have a nice list but i can find it atm but this post on another site shows the barrel sizes in relation to the port location
http://www.airsoftforum.com/board/Co...Ch-t95744.html.
with that length barrel you have actually increased the efficiancey by making the cylindet a lil smaller

[lurkingknight]

how flush did the pickup tooth of the sector meet with the pickup on the piston?

they should be as flat together as possible for correct aoe. If this is the case, then you may need to go 1 cylinder up since you've doubled the buffer on the cylinder head. Depending on your parts, it may be what you needed to get the proper aoe, I did the same on my p90. But my g36 required the rubber pad be removed.

Also when you check your AoE your spring and spring guide should be in so the piston is full forward under load.

double feeding suggests the hopup isn't sitting well against the gearbox if it's not dry firing and shooting double the next shot. It could also explain some air leakage.

for comparison my p90 has a 274 barrel and the stock ported cylinder at the 2/3 mark and shoots 390 with a 110 spring. So maybe your barrel is not sitting well in the hop unit or the unit is not sitting correctly or when you reassembled it didn't come together properly. Your cylinder should be ok, but to be sure can you take a picture of your cylinder just for shits n giggles to confirm it's the standard 2/3 port and not a weird one super long port like the ones in kwas.

And as dumb as it sounds, are you sure you didn't put the cylinder in backwards?

[ThunderCactus]

Could actually be your mechbox is misaligned, it does happen sometimes.
Take a flashlight and look down the barrel, difficult to do but possible. Make sure the air nozzle is in the full forward position, it should be perfectly centered in the hop rubber.

Otherwise all the above advice is solid

[Bellerophon]

Success! It looks like everything is working properly now after I
1. pried the trigger prongs into a better shape
2. lightly sanded a bit more off of the 2nd and 3rd piston teeth, where there was some slight wear
3. greased the sides of the tappet plate, after I noticed it was bone dry
4. added teflon tape to two points, the cylinder head and the end of the hop up unit where it meets the gearbox
5. adjusted the way the hop up rubber was sitting on the barrel, might try the dental floss trick later on

Thanks for the advice everybody! If I don't screw the damn thing up again, I'll be having a lot of fun with this.