[MrJeff]

Current Build:
SHS 18:1 DSG (8tooth)
G&G 18:1 Bevel & Spur
SHS 14T (8 teeth cut off, 3rd tooth lightly shaved down, AOE corrected)
Lonex POM Piston Head (bearings removed, spring guide has them)
Lonex Full cylinder
Lonex Cylinder Head (one with the pads at the front of the gearbox)
Retro Arms 8mm GB
Matrix Meteorite bushings
Maxx 21.25 nozzle
Tappet plate cut to 12ish mm (Guarder)
Real Deal Tappet Spring
ASG 30k (not the fancy CNC)
Perun Hybrid (in DSG mode, no AB in semi or in full auto, no precock)
SP150 (Guarder)
2 Barrel setups:
6.03 155mm brass Krytac barrel & purple prommy w/ maxx hop
6.03 275mm lambda barrel &G&G green bucking w/ maxx hop
11.1v 3000mah 50c Zeee
11.1v 3000mah 15c Titan
7.4v 2200mah 120c Zeee
7.4v 2200mah 30c some RC car brand


Gun works fine on full auto on big boy 11.1v, put a mag in and it screeches like hell once it's up to speed after a few bbs. Doesn't make the noise on super short bursts, also won't make the noise with ROF control to ~35rps. It was shooting ~48rps before whatever I changed made it happened, but would lose ~40-50fps on full so I was trying to figure that out and ended up with a non feeding gun unless ROF control is on.

I went to a tappet plate ~.5mm longer than the one that dropped on full thinking it might be a weird feeding issue and nothing changed.

I read that someone had their sector gear shimmed too high and was rubbing on the tappet plate. I doubt this is my case as there is only a .2mm shim on the bottom of the sector gear. (Tried 1-4mm below the sector and sounds the same)
My bushing under the sector also happened to start spinning (was pressed in with a C-clamp originally w/o glue) around when I believe this problem started occurring, but I haven't checked under it in quite a while. It is now glued in place and doesn't move, sector gear shimming was rechecked and it stayed the same.

Any ideas as to what could've gone crazy?

 

[Leo Greer]

Easy. Your BBs are jamming either partially or fully and it’s causing a complete or partial compression jam, which is giving you PME at higher RPS.

When you lose FPS on full it means you’re having mild PME. When it screeches because you put a mag in, it means the compression resistance is pushing you over the edge.

More knowledgeable techs, am I crazy?

 

[MrJeff]

Longer tappet plate? Same mags, used to feed 48rps with shorter tappet plate, just would be slightly inconsistent so I've got a slightly longer one in now (started screeching on the shorter one) just wondering why it's happening all of a sudden. I figured that they were jamming, just unsure why, if you have any insight. And how am I hitting PME with an sp150 and 18.5g piston assembly at a "measly" 50rps? I checked for the faulty shs rack, piston falls freely back and forth in an empty gearbox, not knocking 3rd tooth on piston. What else can I check to help stop pme? I've got an assortment of different thickness 3d printed spring guide spacers that I've tried as well.

 

[Leo Greer]

A compression block will create PME under those circumstances, depending on how bad it is. I suggest inspecting your hop rubber carefully, and trying to push BBs through the lips while the barrel is installed into the hop unit.

Check and make sure your spring isn’t binding.

Make sure that the Perun is putting your gears at the right spot.

Check for midcap syndrome—it occurs when the nozzle is pushed upwards by the mag spring and this can potentially cause blocks as well.

Make sure your cylinder doesn’t have hardly any friction with the piston head—I had an issue where my gun was hitting PME even when it shouldn’t have, because the piston was off spec and was slowing the return down.

Make sure your gearbox is aligned perfectly with your hop unit/upper receiver. I notice you have the RA shell—since it‘s an aftermarket shell, there’s a chance it’s causing issues. I have a continuing build that uses an E&C shell in a Classic Army body, and though it fed perfectly as an SSG, as a DSG it became apparent that the shell wasn’t perfectly aligned with the HU, causing nozzle stick and issues.

Airsoft is weird man—things have funky issues. Try testing everything you can possibly think of: look at and cycle the gearbox with just the motor grip attached. If you have zero issue, it means the barrel group and feeding assembly may be at fault. Simulate a mag being inserted by pushing around on the hop unit and gearbox with your fingers. If anything starts screaming, it’ll tell you stuff.

 

[MrJeff]

Thanks for the ideas, I have already sanded the grip to align perfectly, and shimmed the GB into the lower for great nozzle alignment, as I have had problems with this before this build yet I will make sure they stayed true.

How do I check to make sure the spring isn't binding?

I believe the Perun is picking up my gears correctly, in sensor mode with an open GB it flashes as soon as the teeth cross the "sensor line" that it has marked.

How do I check for midcap syndrome? I believe I may slightly be running into this just due to the cylinder head wear. If so, any good solutions?

Will double check cylinder-piston head friction, but I believe it is quite smooth with little resistance.

Even with the upper and hop up unit in, on fully charged big 11.1v, there is no screech, at all, which is what leads me to the feeding issue.

Thanks again for the quick response and tests to try.

 

[Leo Greer]

Spring binding is when either the spring sticks to the spring guide for some weird reason, or your spring is too long for the gearbox. This was the most random of my suggestions, though—I generally don’t expect this issue.

Midcap syndrome can be checked for by: using a hicap, trying different mags, and just trying to shoot the last few round of a mag.

I have to say, I think it almost has to be BBs causing a compression block. It’s possible someone smarter than myself will swoop in with the answer, but I don’t see why you’d only hear the sound when actually firing unless it was related to jamming. Both of your barrels are normal—I trust the Lambda to be the right inner bore, and the Krytac is actually a 6.05mm, so that leaves your hop unit, hop rubber, and potentially your tensioner and nozzle. The best way to try and diagnose a jam ime is to just get inside the thing and take a look, searching for wear, damage, and other indicators.

One note is that your Maxx nozzle is actually shorter than standard M4, which could possibly be related. Obviously every gun is different, but stock M4 is 21.4mm, and I tend to find works well.

What model of gun are you using?

 

[rm105]

Video would be helpful here. Screeching noises usually indicate meshing problems but what you're calling a screech may be different than what I do...

 

[MrJeff]

I've had good luck with the 21.25mm nozzle pairing with the Maxx hop and sealing well in other builds, but I haven't really testes in a retro arms gb. They also recommend a 21mm nozzle so I may try that as well. These are the 2 nozzles Maxx recommends for their hop in a standard m4 build. I'll also try a krytac and g&g hop and see if it's a nozzle length to hop issue.

Will post a video if it screeches again next time it's together.

What hop do you use the standard 21.4 nozzle with?

 

[MrJeff]

Reshimmed using FLT bushings, now it double shots on semi, I'm assuming this is due to lower resistance overall. Feeds on full auto now, no screeching, but drops from 350 on semi to ~300 on full.

I'm assuming mild PME. What is everything I should look at that could help resolve the issue?

Piston assembly weighs 18.5g, almost no resistance from piston head/o ring to cylinder. Teeth mesh correctly, piston glides in gear box rails with no resistance when cylinder is removed.
Sp150 spring

 

[rm105]

You've still got a lot going on here, slow down and be careful.

I use 21.00mm with Maxx units when paired with Lonex shells, 21.25mm if the gearbox is a little thicker up front. I've seen that 0.25mm either way make a significant difference, both in resolving feed issues and fixing air leaks. The Maxx units are very good when they work, but very picky when they don't.

Double feeding in semi is to be expected with a build with your spec, turn on AB to get the semi cycling under control.

Bushings do not have less resistance than bearings, more likely whats happened is you either had a bad bearing or your shim job is looser now and that's allowing the gearset to cycle easier. The noise was likely due to meshing of tight gears.

50fps loss in auto is a lot. If you're down to 18.5g for the complete piston assembly and cannot go any lower, might have to increase the spring tension to stave off PME - can do this by adding a spacer to the spring guide, might try 5 or 10mm and see if that does the trick. FPS will increase a bit, but if that's what it takes to cycle smooth you'll just have to tune it back down another way. Other options would be using a less powerful battery, short stroking the DSG and upping the spring, using a slower motor, using rof control, etc etc.

 

[MrJeff]

The old meteorite bushings had left a weird orange rust looking powder on the shaft of the bevel and the inner ring of the bushing was shiny, the coating had rubbed off. So I picked up FLTs this time around, reshimmed and now I believe there is less resistance than with the meteorites, definitely less side to side shaft play. I believe I shimmed the FLTs just slightly tighter than the meteorites, but without the shaft wiggle side to side.

I turned AB on lvl 1 and it stopped the double shots.

My nozzle is straight and appears to be perfect, however it skips here and there on full, unless I pull back on the magazine, then it will feed quite well. Could this be because the nozzle is ever so slightly long (possibly .25mm.....) And I am causing the hop to be pushed away from the gearbox when pulling on the mag? Or is this a different issue with magwell shimming...

 

[MrJeff]

Pushing the hop forward slightly with a screwdriver through the dust cover also relieves the issue

 

[Leo Greer]

That would make sense—good deduction.

 

[rm105]

Would be my guess, yep

 

[MrJeff]

21mm nozzle feeds great, same fps basically. Thanks again for the insight everyone

 

[MrJeff]

I lose ~10fps on semi with the 21mm nozzle vs the 21.25mm nozzle, but it feeds far better. To shift the nozzle forward ever so slightly when seated, should I sand down the front of my tappet plate so it moves ~.1mm more forward to regain the seal?

Edit: it fails the tissue test on semi, with some shots seeming to be sealing okish while others can launch the tissue. The tissue absolutely flies on full auto.

 

[Leo Greer]

Feeding is annoying man—keep on fiddling! The Maxx adjustable nozzle may be of some use to you here. When guns aren’t feeding right, I always pop one in to find the perfect nozz length, and then either use the right nozzle, or shave a plastic one to fit.