Lct Air Seal Issues

[Liam Porter]

LCT VSS air seal problems... When the gun is apart, air seal seems good, but I only get 350 FPS from an SP120 spring (hop completely off).

 

Pic 1, gearbox in half, no barrel - near perfect seal - I can barely push the piston in with my finger over the end of the nozzle. When I stop pushing, the piston returns to original position.

 

Pic 2, gearbox in half, hop/barrel attached to gearbox - near perfect seal - I cannot push the piston far with my finger on the end of the barrel. When I stop pushing the piston, it returns to original position.

 

Pic 3, gearbox, hop unit and barrel assembled in body of gun - not sure best way to test, but I cannot blow down the barrel, so assuming good seal.

 

SP120 spring, and the best I can get is about 350 FPS with a 0.20g, worst is 250 (depending on combination of parts below).

 

Anyone have any ideas?

 

Tried with all combinations of the following:

- Stock hop unit;

- Stock nozzle;

- Stock barrel;

- Stock hop rubber;

- Combat Union hop unit;

- Combat Union nozzle (came with unit);

- Prometheus 6.02mm barrel;

- Prometheus purple hop rubber;

- Maple Leaf Macaron Yellow hop rubber.

 

I've found the stock nozzle doesn't seal very well with the stock cylinder head, but the Combat Union seals perfectly. And the nozzle and hop unit only seal in pairs (i.e., stock seals with stock, Combat Union seals with Combat Union) - mixing and matching gives air leaks.

 

(Apologies for some pictures being upside down.) 

 

Thanks in advance. 

[Adolf Hamster]

sounds like you've tested all the normal ports of call with air seal- cool idea trying it with the barrel mounted on the gearbox imma pinch that one.

 

whats the fps consistency like? if you've got good static seal (which it sounds like you do) and there's not much variation in the fps on the chrono then i'd be inclined in this case to be thinking that the spring maybe isn't as strong as it claims to be.

 

edit: also worth chrono'ing with the hop on a tad (for whatever weight you normally run or therebouts), if it's completely off can give bad readings.

[Liam Porter]

18 minutes ago, Adolf Hamster said:

sounds like you've tested all the normal ports of call with air seal- cool idea trying it with the barrel mounted on the gearbox imma pinch that one.

 

whats the fps consistency like? if you've got good static seal (which it sounds like you do) and there's not much variation in the fps on the chrono then i'd be inclined in this case to be thinking that the spring maybe isn't as strong as it claims to be.

 

edit: also worth chrono'ing with the hop on a tad (for whatever weight you normally run or therebouts), if it's completely off can give bad readings.

 

FPS is quite consistent, its 350 +/-5.  I've found air leaks tend to give a lot more variation... 

 

I've had 430 FPS out of this exact spring before, but can't seem to get it in this gun.

 

Will give it a try with hop adjustments, and will double check the spring in a different gun. 

[Iceni]

First thing I'd probably look at would be the sector shimming and the tappet plate. If the sector is a little high and the tappet is catching on the shims/bearing you end up with the tappet not going fully forwards or been slow, this in turn can cause FPS issues.

It's also not enough to just test it in a neutral gearbox position. You need the piston to be fully tensioned as it will pull the sector to the side and take up slop in the bearings. This might just be giving the tappet a little nip.

[Adolf Hamster]

17 minutes ago, Liam Porter said:

 

FPS is quite consistent, its 350 +/-5.  I've found air leaks tend to give a lot more variation... 

 

I've had 430 FPS out of this exact spring before, but can't seem to get it in this gun.

 

Will give it a try with hop adjustments, and will double check the spring in a different gun. 

 

5fps doesn't sound too bad, not perfect but good enough.

 

could be the preload difference compared to whatever you had it in previously, say a shorter box with thrust bearings on spring guide and piston compared to only one here.

 

i'm guessing by the targets you're going for this is going to be a dmr platform, and the titan suggests precocking, in which case having thrust bearings on both ends isn't the worst option (i tend to avoid running piston bearings on auto guns to save piston weight/avoid pme but that's not relevent in a semi-only pew)

[Liam Porter]

Quick update:

 

I ordered some SHS nozzles (long - 20.7mm and short - 19.7mm) and an SHS tappet plate - still getting the same issue with the gun.

I spoke to AirsoftWorld, where I ordered the hop unit from, who suggested removing the upper screw from gearbox (holds hop to gearbox, but can pull it at a funny angle, causing an air leak) and said to put the barrel assembly in the gun loosely, then insert the mag, and then tighten all screws.  Done it this way - still getting the same issue.

 

If I fully assemble the gun apart from the stock, outer barrel and gearbox spring guide and spring, I get a perfect air seal (even with magazine in).  If I put my finger over barrel and push the piston with a screwdriver, there is no air leak whatsoever, and the piston returns to original position when I stop applying pressure.

 

FPS is consistent (+/- 2, with these new nozzles) but still 50-80 under what I would expect and what I can achieve with the exact same springs.

 

So, in short - tried more things, still stumped.  Will update when (if) I get this thing working properly!

[Iceni]

How many teeth on the sector? 15 or 16.

I'm wondering if the delayer chip could do with a little shave. If you can just drop in a picture of just the sector with the last rack tooth at 12 o clock and the delayer in place.

I might be wrong (often am), but that chip looks like it might just be big enough to be still holding onto the tappet a little as the piston fires. There's also sometimes the inverse of that issue where the delayer opens the tappet a little too soon and causes a weaker shot.

If you have a spare smaller delay chip you could try that, or Just to test try shooting in single shot without any delay chip installed just to see if it affects the FPS. If the FPS goes right back up then at least you have your culprit.

[colinjallen]

It looks like the delayer chip is on upside down.

[EDcase]

1 hour ago, colinjallen said:

It looks like the delayer chip is on upside down.

Images of factory gearboxes show it that way around.

The shape would suggest that is the correct orientation.

 

What's Lukes @negative airsoft opinion?