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Full Auto Droop

762 Views 21 Replies 4 Participants Last post by  Jimps
Current Build:
SHS 18:1 DSG (8tooth)
G&G 18:1 Bevel & Spur
SHS swiss cheese piston (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 11ish 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, no ROF control)
Maxx ME Pro Hop-up
6.03 155mm brass Krytac barrel
G&G green bucking
SP150 (Guarder)

Semi is consistent at 355-360, full auto drops to ~330 and then the last shot ends around 360fps yet again. Any thoughts where i'm losing the fps in full or why the last shot returns to normal fps?
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Run a 7.4v lipo to see if the FPS drop goes away in full auto. That will tell you a lot.
I don't think that is the issue. If your piston weighs 17 grams all together, I cant see getting much lighter or it mattering. I have a build at 47RPS with a 20 gram piston and no loss of FPS in auto SP150 at 370FPS.
When I put a SHS tappet plate in a DSG after 38RPS the FPS would drop in auto. The SHS tappet was shaped wrong causing tappet PME. I swapped to a guarder tappet and the issue went away. You are already using a guarder tappet right? Just double check the tappet spring. I always remove as many coils as I possibly can before compromising the spring/tappet. Put a little lube in the nozzle to be sure it has a smooth action. I was convinced that I had a vacuum issue. I found a clear hose that fit on the end of my nozzle and added some red water in the hose. I then would pull the piston out of the cylinder as fast as I could and measured how much the water moved. I widened the holes in my cylinder head and used a thinner o ring. I tinkered with it until the water did not react to me pulling back the piston. So no vacuum at all. I got around 6-8 FPS gain in auto. My main issue was the tappet plate PME.
The ASG Ultimate cylinder head is OP. The OD of the stainless nozzle rail insert is greater than most cylinder heads. It allows for a tight fit on the nozzle. Less worry about midcap syndrome. Don't use the Maxx cylinder head in the RA gearbox. It will knock the guide pins off. You don't even really need an o ring nozzle. Air wont leak there unless it is REALLY loose. I would remove 2-3 coils from the tappet spring. You have plenty of coils to remove some. Yes is is normal for the nozzle to not seat all the way in the tappet. Is it good, no. Not much you can do about it unless you have an injection molding factory and knowledge of CAD, a CNC machine would work as well. LOL. Some tappet plates are different and the nozzle will seat further down the tappet. I have designed a 3 piece tappet in CAD that fixes that issue. I need to cut it on my friends CNC mill for testing. I used a hand drill that came with a micro drill bit set to widen the holes in my piston head. Probably 0.3-0.5mm bigger.
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This is the ASG Cylinder Head. Sold at Brill. I also like the Tienly Cylinder Heads.
Rectangle Gas Electric blue Magenta Metal
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Not sure what piston you used. I had a bunch of the SHS 15 tooth pistons have issues. The rack is a little off alignment. Not sure if it was a bad run or if all of them were bad. I use Rocket 14 tooth pistons and have not had issues. Look closely at the gap between the teeth and rack when the gear spins. I had like 15 piston racks I had to trash, this was one of them.
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I like the ASG with the stainless steel insert. It is slightly oversized and nozzles fit snug. That prevents the bbs pushing the nozzle up because the is very little play between the cylinder head and nozzle. You probably don't need to change the one you got really. If you do I like the one I pictured. They are $13 on Brill Armory. I would still shorten the tappet spring coil. At least 2-3 coils. OP= On Point. I still don't now if I'm a believer in the vacuum thing. I do it on all my builds because it does not hurt anything to do it. I did bump up a few FPS after but I bumped up FPS at 38RPS and 48RPS. Tappet timing was my main issue. Yea best to check the piston. One way is to spin the gears by hand, without the spring in obviously, with the gearbox together, and see if you feel a slight snag. I use a metal pick to spin the bevel gear, see if you feel it snag right after the pickup tooth. Probably not related to FPS issue but could be an issue nonetheless. You can check it like in the video as well.
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Well, a better (more convoluted) way to do the "tissue test" would be to get a 6 mm (outer diameter) clear hose. Stick it in the hop unit and put some colored water in the tube. Fire full auto, and if there is any air escaping there, it will move the water in the tube. Even the tiniest bit of air will move the water line in the hose. I'll share my tappet spring, and be sure you have something similar. I don't know how many "pounds" my tappet spring is, but it is a lot. I have bought a couple "upgrade" spring sets before, I still removed coils from the tappet spring. Not sure the actual ratings but they were "better" than some factory tappet springs. Most normal builds they would be fine, but getting close to 50RPS I rather be sure. Maybe that is one bad *** spring, I don't know. It might be fine.
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