Getting frusterated....need a troubleshoot
 

For starters, big time new at this....I just got my first AEG, a CA M15A4 SPC
I upgraded.....first time doing it and followed mechbox.com instructions.
From stock I added:

m120 spring
tightbore barrel
new tappet plate (with this gun I heard stock one imminantly breaks)
Gun came stock with metal bushings.

Bought a polycarb piston but did not install it yet, going to wait for a new piston head as well. Read that this gun with the spring I have did not need new piston off the bat so im waiting.

Symptoms:
substantial power loss, at 2 inches does not penetrate a soda can, just dents it.
After 4th reasembly the bbs are not feeding properly, one comes out approx every 3 pulls of the trigger.

Things ive checked:
After 4 times in my mechbox, each time I could find nothing wrong with the assembly. Timing is correct, cylinder hole is towards the rear, spring is inserted correctly, greased and siliconed. Never had a problem with the gun acctually firing and have found no wear on anything internaly.
I also checked my hopup sleave and barrel and siliconed the barrel. Only thing I noted is that when I drop a bb down the barrel it will stick in the hopup sleave, but the slightest push with send it through (not sure if this is normal?)

Anyways, I am at a loss and need some help. Only thing I can think of is an airleak but I checked my seals and it looks the same....brand new!
I did fire 600 rounds with no problem before I took it apart, so it was a working gun before hand. Thanks

Pretty observant new guy if you ask me!

Not sure what the prob is yet, will ponder for a while. To answer your question, yes it's normal for the BB to be held in place by the hop up rubber in the barrel.

Is it safe to assume you have the tappet and nozzle hooked up correctly?

I would just like to inquire about the type of battery you are using.
I know its only a small possibility but perhaps you do not have enough power to crank that spring. If I can think of anything else I'll let you know.

As far as I could tell there is really only way to assemle the nozzle and tappet plate. I looked in detail at a still pic of the mechbox.com video to compare and I couldnt see a difference. Now the hole in the cylinder...does the orientation matter, the rotation? I know it has to be towards the rear but if I was to point it straight up I can cover the hole.

My battery is a 8.4v 3000 mah. I talked to a guy who owns this exact gun and spring setup and he has a 8.4v 3600mah and its going ok for him.

I guess ill take my gun apart again and give er another look, but I dont think im going to find anything new. Any pointers would be great cause im missing my first game today because of this.not a happy capper.

That " tightbore barrel" you put in is it longer than stock? Because if it is longer you could be having bb suck back which would explain the loss of FPS.

No, its the same, I even put the old barrel back in and tested it, same deal. so I think its safe to say its not the barrel, which leaves the hop-up ( dont think its that ) or somthing internally. Im kind of tempted to put the old spring back in and see if that clears fixes anything. But far as I know a 8.4 should be able to drive a M120.

Yep if it is not obvious, I would put the old spring back to isolate the part that is not fuctioning properly. Also your shimming is good right?

take the gun apart so you can access loading nozzle.
plug in you batt and fire.
does the nozzle move back then forward with every shot?
if it does, place a finger over the nozzle, and press just firmly enough to make an airtight seal.
fire and see it there is good compression in the cylender.

If youve got the time, I say put it all back together with stock and see if it works like that, unfortunetly you may have had a certain part fail somewhere along the ways. But well no harm no foul, might as well put it together stock and see she goes.

You are correct about the battery though, that batt should pump that spring fine.

I tried this, it moves back then forth, I put my finger over it but dont know how to tell if it has good compression. what am I looking for? im not feeling much air on my finger covering the nozzle. but when I take my finger off I can feel the air about 2 feet away on the palm of my hand. 3feet away I cant feel it. I can also feel air pushing out of the cylinder hole, bit not from any seams nor the rear hole that exposes the spring.

Compression test:

Open up the gearbox, removing the spring guide and spring.

Cover the air nozzle with your right index finger.

Using your left hand, draw the piston back and with a good amount of force, slam the piston forward. If you have a good seal, you should actually be able to get the piston to come to a complete stop, held back only by pneumatic pressure. The more forceful your push, the more likely you should come to a complete stop. If you feel air releasing on to your right hand anywhere, you'll need to localize it.

So far........ took it apart....again... and dropped in the stock spring, no change.
Did the compression test and with the plastic nozzle piece taken off, putting my finger over the metal spout on the cylinder head (not the plastic nozzle) made the piston stop immediatly, even without having to slam it (Piston and cylinder head ruled out). With the plastic nozzle on, I could not bring my piston to a stop no matter how hard I tried. There was some resistance but I could not make it stop and could hear air escaping from the base of it. But since there is nothing to seal it, this must be normal. So I put the M120 spring back in, fired it again, and got 1 shot out of 5 to go through the side of the can at 1-2 inches. ugh.. stress.

Also, the last 2-3 bbs in my mag never load, they just fall out when I release the mag. Any ideas? im thinking of replacing the nozzle and maybe get a new piston head and cylinder head. Just to get everything covered, but dont really want to spend much more $$$

This is normal...no worries.

2-3 bb's fall out of your mag is normal happens to me to its cause it loads 4 into your hopup chamber, the only time this doesnt happen is when my mags has a piece that protudes the top when empty.

Check your hopup rubber as well its on your barrel in the hopup for any cracks or breaks, that can reduce FPS if its not a tight enough seal or if its bent and getting in the way of the bb

Ugh, ok so my rifle is now firing 250-280ish according to my soda can crono, regardless of me using the stock or upgraded m120 spring. It fires at the same FPS it seems, and yes the 2 springs are obviously different in stiffness. ive taken the mechbox apart 6 times now looking for faults....nothing. Im getting good compression between the Piston head and cylinder head, not including the plastic nozzle. It fires straight at 20 ft ( its all the space I have for testing). Problem with all of this is I have no other mechbox to compare this all to. This point im almost ready to replace Piston and head, Cylinder head, nozzle, hopup rubber. Even though I dont see anything wrong with these parts, I dont know what else to do here, and Id hate to install it all to find out its not the issue. Far as I know this 120 spring should be shredding cans at 25-30ft, and well.....its not even close. Thanks for all the help so far, ive looked into every suggestion, but so far nothing. Im sure someone who has done this alot would take a 1/2 hour and go......"oh here, this is the problem", but I dont have the luxury of knowing anyone locally here who knows this stuff. Thanks again, and ill still take suggestions.

What's your location? Someone Might be able to find you a gun doc in your area.

Why don't you try putting the old tappet plate back in and see if it fires better? You probably have a part failing along the line somewhere.

Im in Penticton BC.
And I held both tappet plates side by side, they are IDENTICAL, I dont know why I was suggested to get a new one cause the new one isnt made any better than the stock one. At least I have a spare now.
Can the nozzle in anyway cause lack of power? I think it may be a compression issue. When I fire it, sometimes every round just causes a dent, then suddenly a round will go right through and dent the other side. Before I took it apart though, the stock spring was putting 2 out of 4 bb's through both sides at 2 inches. Soda can im talking about.

ex-royal = RCR?

Well I guess it's not that then. Any kind of deformation in the barrel? Yeah I'm pretty sure the problems lack of compression but whats causing it?

That is not normal. I just tested this and I was able to stop the air from escaping with the nozzle on.

You may have a problem with your nozzle.

Good Luck

Try this with your AEG set on semi:

1. clear your gun (no bbs in hop up or barrel, remove mag)
2. invert gun so the magwell faces up
3. plug the inner barrel tightly with your finger
4. fire mechbox on semi only (a restricted barrel prevents the piston from returning to full front, firing in full auto can jam the piston teeth against the sector gear and kill the piston)
5. listen to the tone of the report

This test leak tests the entire pneumatic series:

barrel-hopup-nozzle-cylinder head-cylinder-piston head

If you have a sharp piston impact against the cylinder head, you have a significant leak somewhere in the pneumatic series. If you have a very flubby piston impact with the piston observably gradually returning full forward then you have a decently sealed system. Feel for very strong puffs of air out the mag well bb inlet.

Also, where are you located?
Filling in your profile more would help if you need local help, etc.

Wish I knew this before I took it all apart again. heh. Anyways, im positive my piston head and cylinder head are tight. I checked them again with the cylinder removed. But my nozzle is leaking from where the nozzle meets the cylinder head. I put some silicone around the base of the nozzle and sure enough...compress and I could see the air getting out all around the base as my finger plugged the nozzle opening. But would this seemingly small leak translate into loosing 100fps? abouts. Im unsure about the barrel and hop up and am anxious to try the above test. But it can wait till tomorrow.

How would I plug the barrel and the hopup (bb feed "hole") and fire the action at the same time. I mean, if you plug the barrel wont the air still escape out the bb feed?

It's normal to have a low leak between a plastic nozzle and the cylinder outlet.

Usually the most difficult to troubleshoot leaks end up between the nozzle and hop up. They're typically caused by something that results in crummy fitment between the hopup body and the mechbox. Sometimes you just can't get a reliable fit between the hopup and the mechbox so the nozzle doesn't pop forward far enough to seal against the rubber sleeve in the HU.

If you are using a 1piece HU body, try hand loading pellets into the HU with the mag removed and chronying. With 1pc HUs, the magazine can pull the HU forward and open a gap in the nozzle-sleeve mate. TM really had it right with their 2pc HU design. It frees the upper HU to float against the mechbox and solidly mounts the bb inlet so a wiggly mag can't affect the barrel-HU-mechbox alignment.

I'm starting to think something is wrong with your hop up rubber since both springs spit the BB out at the same rough fps. Check the semi-diaphram at the rear of the hop up (the rubber ring that the BB passes through, and that which seals the nozzle in during firing) and see if it's still intact. If you or no one else has farted around with the rubber, it might still be intact but shifted forward. Also check the hop up unit and make sure your plastic C clip is still intact and holding the hop up unit to the barrel properly. I've had a low fps reading on an upgraded AK before, was to be shooting around 360ish, only sat around 280ish, I dug around and found nothing wrong until I made the nozzle sit full forward in the breech, shone a flashlight into the magwell and looked down the barrel to see light getting in from the bottom. I checked the hop up unit to find the C clip that holds the barrel and unit together had two nice cracks in it. Because the hop up unit didn't hold the barrel solidly, the barrel would shift forward a little bit with each shot, causing a bad airleak.

Well, I feel much better, and cant believe how much time was wasted on opening my mech box. I payed close attention to my hop up rubber and the way the barrel sat in it. After fidgeting to try to find the perfect "sitting" on the barrel in the rubber, and rubber in the hop up I tried my rifle out again. My can test was inconsistant but positive. One bb only dented the side, then another went through both sides, and then consistantly was knocking the can over going through one side. Then I shot the bottom, first shot almost penetrated, second shot hit a different spot and went right through the bottom. This was at about 3 feet away. Redwolf site says if it penetrates the bottom of a can at 2 inches its firing at about 420Fps. I hope this isnt the case, I only wanted 380 - 400max. Anyways, the power is now there though inconsistant. But I at least know where my issue is and I will be ordering some new hop up rubber.
Thanks for all the advice, i was pulling my hair out untill today. Great community. Cheers!!

Redwolf site is wrong, and is most likely only valid for asian style cans, which are much thicker. Through the bottom of a can is like 380-385+ FPS

Im ginna guess that you barrel is too long for you cylinder. The barrel has more volume than the cylinder so when the piston is all the way relaxed the bb still isnt out the barrel. Maybe im wrong but that is my guess