P90 inconsistent semi cycling

[Lollingsgrad]

I recently had a bit of fun working on a really awful CYMA P90. Turned out pretty well, went from shooting 170 to 310fps on 0.2g and the accuracy isn't bad. However it does occasionally dribble a BB out of the end, about 1 in 50 shots and only on semi-auto.

Something odd I noticed about it is that compared to my G&G CM16 (the only other AEG I've opened) the start and finish position of the nozzle is very inconsistent. Could this be the cause of the occasional semi misfire? How would I address it?

If it's relevant the internal changes are hop chamber (TM), barrel and bucking, then nozzle (SHS aluminium single o-ring), shimming, spring (shortened SHS M100) and motor (16tpa SHS).

 

[Asomodai]

I am very interested in this as I have bought my own Cyma P90 for a project. 

When it does double feed, does the motor feel like it does more then one cycle? If so it may be that the motor is too powerful for the spring. If not, I would be looking at the barrel for imperfections and the hop unit. 

 

[Lollingsgrad]

It doesn't double feed, just sometimes a BB will roll out of the barrel instead of being propelled. The semi action is crisp and never double fires, but the nozzle will be all over the place. Sometimes deep in the hop, sometimes fully retracted etc. Can post a video if this is unclear.

 

[Asomodai]

It doesn't double feed, just sometimes a BB will roll out of the barrel instead of being propelled. The semi action is crisp and never double fires, but the nozzle will be all over the place. Sometimes deep in the hop, sometimes fully retracted etc. Can post a video if this is unclear.

I understand. This makes me think the Tappet plate spring is broken or poor quality. Though tbh I think many gearboxes do the same with no issues. 

Occams razor makes me think hop unit and barrel still. But check the tappet plate spring as well. 

 

[Lollingsgrad]

I could believe tappet plate issues, the spring is okay but the plate itself is a bendy piece of garbage. The reason I suspect the random nozzle positions is I'm thinking a BB could be pushed through the lips prior to firing. That said grippier bucking lips might solve that. Maybe I'll try this G&G green bucking I've got laying around and use it as an excuse to try flat hopping.

If I'm right that this inconsistent nozzle position is the problem would a sector chip help?

 

[Asomodai]

I could believe tappet plate issues, the spring is okay but the plate itself is a bendy piece of garbage. The reason I suspect the random nozzle positions is I'm thinking a BB could be pushed through the lips prior to firing. That said grippier bucking lips might solve that. Maybe I'll try this G&G green bucking I've got laying around and use it as an excuse to try flat hopping.

If I'm right that this inconsistent Nick's position is the problem would a sector chip help?

Hop bucking first, softest you got. Tappet plate next, King Arms or Guarder. If it doesn't make a difference then maybe try a sector chip. 

 

[Lollingsgrad]

Tappet plate next, King Arms or Guarder.

Why those brands out of curiosity?

 

[Asomodai]

Why those brands out of curiosity?

Better then the Cyma one! I have the King Arms one, and seems much sturdier. I imagine Guarder is the same or better quality. 

 

[Asomodai]

I recently had a bit of fun working on a really awful CYMA P90. Turned out pretty well, went from shooting 170 to 310fps on 0.2g and the accuracy isn't bad. However it does occasionally dribble a BB out of the end, about 1 in 50 shots and only on semi-auto.

Something odd I noticed about it is that compared to my G&G CM16 (the only other AEG I've opened) the start and finish position of the nozzle is very inconsistent. Could this be the cause of the occasional semi misfire? How would I address it?

If it's relevant the internal changes are hop chamber (TM), barrel and bucking, then nozzle (SHS aluminium single o-ring), shimming, spring (shortened SHS M100) and motor (16tpa SHS).

So I finished my project, I have the same inconsistent air nozzle position, but that is just down to harmless overspin. 

However I did have misfires and large drops in FPS every other shot on semi. I found that the SHS Nozzle has a thicker diameter then the stock one and is slightly longer (SHS is 20.8mm and Guarder is 20.7mm), it would catch on my metal hop unit causing it to scratch. This probably wouldn't appear so easily on yours as you are using the plastic TM one, (More likely you are rubbing away on the inside of the hop unit). 

If you have a set of digital callipers, see how long exactly the original Air Nozzle is. We might be able to find an O-ring nozzle of the correct length. 

View attachment 41273

View attachment 41274

Go back to the original Air nozzle. However you WILL take an FPS hit as the original is not an O-ring one and is shorter, it cut mine by about 35 FPS.

 

[Lollingsgrad]

I should have posted about this but it slipped my mind, the problem is the magazines. I use the MAG brand 170 rounders. The first 20-30 rounds have these occasional misfires. I've seen the same problem with my 190rnd Socom Gear Lancer M4 mags and read about similar issues elsewhere. I think this is just something that happens with higher capacity midcaps with their strong springs. I see this problem less and less so it's probably the spring getting weaker.

For others who might be interested I don't seem to have the nozzle rubbing issue with the plastic TM chamber.

 

[Asomodai]

I should have posted about this but it slipped my mind, the problem is the magazines. I use the MAG brand 170 rounders. The first 20-30 rounds have these occasional misfires. I've seen the same problem with my 190rnd Socom Gear Lancer M4 mags and read about similar issues elsewhere. I think this is just something that happens with higher capacity midcaps with their strong springs. I see this problem less and less so it's probably the spring getting weaker.

For others who might be interested I don't seem to have the nozzle rubbing issue with the plastic TM chamber.

That was my second explanation! Good to see you worked it out same as me! 

I just got a Lonex nozzle through. Will give that a shot tomorrow. 

 

[Asomodai]

I should have posted about this but it slipped my mind, the problem is the magazines. I use the MAG brand 170 rounders. The first 20-30 rounds have these occasional misfires. I've seen the same problem with my 190rnd Socom Gear Lancer M4 mags and read about similar issues elsewhere. I think this is just something that happens with higher capacity midcaps with their strong springs. I see this problem less and less so it's probably the spring getting weaker.

For others who might be interested I don't seem to have the nozzle rubbing issue with the plastic TM chamber.

Things are better with the Lonex Air Nozzle. Has gained an additional 5FPS and none of the obvious rubbing that the other aftermarket ones had. 

I still have midcap syndrome but should quiet down the more I use it. I noticed that .2's seem to be more of a problem when feeding then .25's and above in those midcaps. 

 

[Lollingsgrad]

Good to hear it, my mids seem okay with 0.28s and 0.3s though muzzle energy was higher with the former so that's what I use.