[mr. potatohead]

Hey guys, I recently have been working on a DSG build, but I've run into either one or multiple problems depending on if they're related. My fps is inconsistent on semi, fluctuating between 380-360 fps. On full auto, fps is even worse. One day it was around 360, so not bad. But the next day it was in the 250-300 fps range, so I don't know what happened there. Full auto shot pattern literally looks like a shotgun spread, though at least it is feeding.
Here's my parts list:

-18:1 stock gear set with shs DSG gear and stock tappet plate (<11mm)
⁃ shs high strength polycarbonate piston (SS + SC)
⁃ Matrix silent piston head (aluminum)
⁃ ASG 30k boost
⁃ Gate titan mosfet (dsg setting)
⁃ Maxx Model CNC aluminum hopup (pro model)
⁃ G&G hopup bucking (cold-resistant)
⁃ Matrix CNC aluminum air seal nozzle (ground off <.25 mm)
⁃ Matrix silent cylinder head (aluminum)
⁃ m150 spring
⁃ 265 mm inner barrel
⁃ ported cylinder

DIY mods done:
⁃ AOE
⁃ Shimming
⁃ Tappet return spring -4 coils
⁃ Everything greased/lubed accordingly

***tested with 11.1 v 2200mah 25-30c lipo and .20 gram bbs***

Most of these parts, including the hopup, bucking, nozzle, and tappet plate were used in my 12:1 SSG build, which worked just fine. I had this same problem with the SSG build but solved it by grinding <.25 mm off of the nozzle which made my fps more consistent; when it did change it was +-10 fps. Still using these parts, but encountered the same problem again albeit worse. I had already cut 2 coils off my tappet return spring, and then did the tissue test. Paper was blown off, so I thought that my nozzle wasn't returning fast enough. I then cut off 2 more coils. Paper moved slightly after that (but still quite noticeable on auto). So in total there are at least 4 coils cut off my tappet return spring and I'm still having the tissue kinda pushed off. At this point I'm even considering that the nozzle might be a hair too short (thanks to me grinding a bit off), causing hopup compression issues.

Note: This is my first DSG build, so by far I don't know everything. Any help is appreciated.

 

[Guges Mk3]

I do think your nozzle is too short.

And burping pressure early is going to cause your FPS fluctuations.

 

[mr. potatohead]

I do think your nozzle is too short.

And burping pressure early is going to cause your FPS fluctuations.

Yeah I was like no way there's 4 coils off and still have this problem. Imma order a new nozzle. Got some stock plastic ones lying around, but their seal is pretty bad.

 

[Guges Mk3]

Are you referring to the seal between nozzle and air tube from cylinder?

 

[rm105]

could be nozzle, could also be the tappet trimming isn't spot on?

 

[Ben3721]

A dsg is a high pressure setup. You need to spend a lot of time on every aspect and evaluate every part for proper function.

Even small things like the hopup moving around will kill fps. A ripped packing can also do that. Or a improperly trimmed tappet.
If a low grade spring was used it could have cracked.
You can also sand the front of the tappet to allow it to seal deeper into the packing.
The cylinder head needs to seal tightly, in a dsg the pressure can spike much higher and leak out only when under pressure.

 

[mr. potatohead]

Are you referring to the seal between nozzle and air tube from cylinder?

Yes

 

[Guges Mk3]

Fluid Dynamics say that pressure loss via that direction is "irrelevant". Most people actually don't understand how "flow" moves.

All the top makers of AEG's know this and that is why their nozzles don't "seal". Krytac, Tokyo Marui, VFC...etc

Try is with any nozzle on hand...and see what you get.

O-Rings were used early on to address tolerance and alignment issue and not sealing issues...

There are so many things that are misconstrued in this sport....cough...bearing on spring guide and piston head...cough.

 

[mr. potatohead]

Fluid Dynamics say that pressure loss via that direction is "irrelevant". Most people actually don't understand how "flow" moves.

All the top makers of AEG's know this and that is why their nozzles don't "seal". Krytac, Tokyo Marui, VFC...etc

Try is with any nozzle on hand...and see what you get.

O-Rings were used early on to address tolerance and alignment issue and not sealing issues...

There are so many things that are misconstrued in this sport....cough...bearing on spring guide and piston head...cough.

All right so I cracked open my gb to swap the nozzle and found my tapppet return spring had snapped (most likely too much strain from the -4 coils). Changed the nozzle to a stock one and also changed the spring to a stock one (-2 coils). Chronoed my gun and fps was in the 360-340 range. I redid the tissue test and had the paper blown up. I'm guessing the spare tappet return spring I was forced to use is weaker, which is why the fps dropped and the gun's still leaking air. I don't know if I should just cut off another coil, or order another spring.

As for incorrect tappet trimming, I can post a pic of my gb later. Maybe you guys can catch something else I haven't considered. I'm hesitant to grind down the front of the tappet plate because I'm afraid that the nozzle won't get pulled back far enough during the cycle, which could cause feeding issues.

 

[rm105]

Shouldn't need to touch the front of the tappet unless you've got some weird tolerance stacking going on. When I trim the tappet I open the gearbox, take the mainspring + tappet spring out, spin the gears by hand to manually pull the piston back, and adjust the tappet length such that it 'releases' when the piston is in contact with the last 1.5 - 2 teeth on the sector. Very small adjustments after that if feeding issues. If using active breaking on a neo motor might need to shape the bottom of the tappet slightly to avoid the opposite 'pickup' pin on the sector from crushing it.

 

[mr. potatohead]

Shouldn't need to touch the front of the tappet unless you've got some weird tolerance stacking going on. When I trim the tappet I open the gearbox, take the mainspring + tappet spring out, spin the gears by hand to manually pull the piston back, and adjust the tappet length such that it 'releases' when the piston is in contact with the last 1.5 - 2 teeth on the sector. Very small adjustments after that if feeding issues. If using active breaking on a neo motor might need to shape the bottom of the tappet slightly to avoid the opposite 'pickup' pin on the sector from crushing it.

Thanks for that. I'll try it out when I get some time.

 

[mr. potatohead]

Is trimming the tappet plate to <11mm the same thing as you mentioned, or is it a little different?

 

[rm105]

The above trims the tappet to the correct length for your particular gearbox, whatever that length may be. Trimming the tappet an arbitrary amount such as 11mm might work, might not - it depends on tolerances, part selection, etc etc.

Basically, 11mm is an 'educated guess' whereas doing it this way is 'how to find the right length'

 

[mr. potatohead]

Makes sense.

 

[Ben3721]

The actual tappet fin has sections with different purposes. The whole 11mm thing is more of a rough estimate
Here's a shs v2 tappet for example.

The red zone is where the sector cam pulls back the tappet all the way back, it has the most impact. Some tappets have this spot further back.

The orange zone keeps the tappet 100% back ideally.

The purple and green zone help release the tappet slowly. The purple zone also rests against the sector shaft some in some setups. Having the tappet release slowly helps prevent the bb from kicking past the packing lips and hop application. The downfall to this is in full auto the cycle rate speed is faster than semi, resulting in the bb kicking deeper into the hop causing slight hop loss.

However a dsg doesn't have the time to release the tappet slowly, it slips off the fin and rams the bb into the chamber, the packing lips should have a bit of resistance to help prevent it from kicking it straight past the hop up. There is a plus however, because it slips off it will equally kick the pellet the same depth into the hop in auto and semi.

Ideally for a dsg you would want to cut the green and purple sections off.

Some tappets lack all of this and are just a curved fin that actually only has 1 spot where the tappet is pulled 100% back. Lack of proper design is to blame.

Then there is the opposite, a few tappets I've seen have the fin designed to keep the tappet 100% back as long as possible, basically as a sector chip would do.

 

[rm105]

Good info. When trimming for DSG I try to get the best of both worlds by slightly rounding off the front corner after finding the right length - this slows / controls the release to slighly reduce the 'ramming' effect but not enough to cause timing issues. Have to really be careful with this, to much and you'll have to start over.

 

[Ben3721]

A triple sector gear requires you to cut everything below the first red zone off. Those things are experimental however.

 

[mr. potatohead]

So while adjusting my tappet fin, I noticed that my piston is pretty snug when the gb is closed and screwed up (as in it doesn't slide down the track). Is it supposed to be like that cuz last time I checked in my SSG build it was able to slide around pretty easily when the main spring was out.

 

[rm105]

Piston should slide freely when the gearbox is closed - the more freely the better.

With the gearbox open, does the little rail on the gearbox fit into the slot along the side of the piston? Sometimes different oem specs cause issues here.

Could also just be the piston is a not sitting in the rails on both halfs of the gearbox.

 

[mr. potatohead]

I think I'm gonna grind down the rails in the gb a bit. Weird cuz it worked in my SSG build with the same exact piston...