Low FPS from Hop Up

[RainyEyes]

I'm looking for some insight as to why an AEG would lose FPS due to the hop-up unit. Both guns are classic army with different gearboxes:

I have 2 gearboxes, one TM and one CA.
Both shoot 320+ FPS on hop up unit A.
Both gearboxes shoot 220-260 FPS on hop up unit B.

I've checked the hop up rubber; nothing is torn and bb's do not double feed.
I've checked the distance between the hop-up unit and the gearbox; it fits snug with no play.

Symptom: no matter what spring i change it to (m90-m110) it will always consistently shoot 260 FPS or lower. I've also tried swapping cylinders, cylinder heads, etc. Air compression is not an issue within the gearbox.

So far i've isolated the problem to the hop up unit itself after swapping the hop up bucking and barrels as well.

Interdasting feature: If i turn the gun upside down, insert a mag, remove the mag, and put my thumb over the bb feed hole and shoot it, the bb's will shoot 300+ fps and gradually drop after the 1st shot. I can feel the first shot suck my thumb and air into the hop up unit. The FPS stays consistently below 250 with just the magazine feeding into the gun.

What does this mean, and what can i do to fix it aside from replace it? Is it possible that the nozzle is too short?

This is quite a head scratcher and i'm trying to get some theories on what went wrong and why replacing the hop-up unit seems to fix the issue.

[devbro]

what you describing sounds like the air nozzle and the rubber are not making proper seal. maybe the rubber is a millimeter too far on hop up unit 2 for air nozzle to make a seal.

[RainyEyes]

We tried separating the nozzle from the hop up unit and the fps dropped. Went to 140 fps when we move the nozzle further from the hop up unit 1-5mm away.

Will definitely check with a longer nozzle but the gearbox was gutted so I have no idea what the stock length is for the nozzle.

[Rossco66]

I had the same thing happen to a Stoner I was working on for a friend. Couldn't figure out what in the hell was causing the FPS drop. As it turned out the Bucking wasn't sitting in the correct position in the Hop up unit. The barrel was too deep into the unit. Once I backed it out 1mm or so the FPS went to where it should have been.

[Maximus]

Sounds like a bad Hopup. I would just suggest replacing it with a Lonex unit. Spend the $20 and get rid of the headache. How much time have you spent on this already?

[JiamingQ]

What hopup and air nozzle are you using?

[RainyEyes]

Using the stock CA MP5 hop up and an SHS/ACE1ARMS AK nozzle. It's about 1 mm longer than the stock nozzle it came with. I left the thing with panda since i isolated the problem and now I'm still waiting to see if it's going to fix it later.

And we collectively and embarassingly spent more than we would like to admit. It was a headscratcher for the both of us because none of what we were doing made sense.

Everyone said the same thing: compression issue in the GB. But we figured that wasn't true when we tried a different hop up unit with the same/different barrels.

[lurkingknight]

mismatching brands of hopup unit to gearbox shell and nozzle has a potential to cause this. This is why you try to not change any of the 3. The nozzle is usually easiest because you can generally find one the exact same length as stock or just a bit longer to shave down.

You make this harder by changing the hopup unit as the feed tube may not sit proportionally the same location from one unit to another or the barrel is held more forward than backwards or the hop unit itself sits farther forward or closer to the face of the gearbox, or the gearbox shell is thicker or thinner at the front or the tappet channel is not as forward, etc etc.

Changing shit around so now you have to play with trying to put longer nozzles in to reach the lips of the bucking can cause feeding issues causing you to sand the nozzle tip which can then lead back to air seal issues. Some CA units are even too narrow for the nozzle to protrude all the way into touch the hopup rubber.

And assuming the dimensions are all the same the hopup unit could be cast too tall or too short or at a slant that causes a nozzle to hopup misalignment causing the seal to break.

[RainyEyes]

SO, update. I've recently visit my local airsoft shop and asked for advice. Our consensus was generally either nozzle length or the hop up rubber. Considering I didn't have time to futz around with the nozzle length we opted for a 1mm longer hop up rubber and voila~ now the gun shoots 350~ on .20 and 310fps on .25g bb's.

Seems like swapping the g&g green bucking for a lonex one did the trick for one probable reason or another.

Now I need to actually level my shots and tune the hop.

Edit: i am so smart i put the barrel in upside down so that the hopup window is on the bottom ROFL

[BobbyDangerous]

Originally Posted by RainyEyes

SO, update. I've recently visit my local airsoft shop and asked for advice. Our consensus was generally either nozzle length or the hop up rubber. Considering I didn't have time to futz around with the nozzle length we opted for a 1mm longer hop up rubber and voila~ now the gun shoots 350~ on .20 and 310fps on .25g bb's.

Seems like swapping the g&g green bucking for a lonex one did the trick for one probable reason or another.

Now I need to actually level my shots and tune the hop.

Edit: i am so smart i put the barrel in upside down so that the hopup window is on the bottom ROFL

G&G Green is finicky with certain Hop up units. It’s a wicked bucking but in come Hop Up units it runs like haggis. I just cut one short and threw it I. My EBR. Waiting to test it after I build the gearbox up.


But a long story short I had the same thing in my old VFC Mk18

Swapped it for a modify flathop and that did the trick.