p90 issues

[lurkingknight]

I've run out of ideas on this. My p90 that was once shooting awesome has now developed an inconsistent fps problem.

The issue is that on a full midcap, it will shoot inconsistent fps on non consecutive rounds until the mag is 1/2 empty then it will resume normal function.

Feeding from mags is fast, and there seems to be no issues with BBs leaving the magazine.

nozzle is new, cylinder head is new (replaced as a precaution also the sorbo was tearing the rubber patch of the old cylinder head.

When it shoots consistently, it will shoot +/- 3 fps.

I've also tried different brands of BBs with the same result, so I think it would be save to assume it's not a BB issue.

I have reseated the barrel in the hop up bucking 3 times, each time with the same result.

This gun functioned properly for about 2 years, it's now out of the blue decided it wants to cause trouble for me.

When it's shooting fine, it shoots 305 with .32s When the mag is full, every 1-2 out of 3 shots will come out at about 120fps.

The only 2 things I haven't replaced are the tappet spring and the tappet plate. The tappet spring is a bit tricky since it's a compression spring and nobody sells that.

[RainyEyes]

Perform a compression test with the cylinder head and piston if you haven't already.

Since you've installed a sorbo pad on a previous cylinder head and replaced it, ensure that the new cylinder places the piston in the correct position for AoE with the (i believe it's called) sector gear.

[Short Round]

I've ran into this issue with my P90 lately as well ever since I put a new Lonex a1 Titan motor to replace my old G&P M120 motor. Oddly if I use my old Echo1 magazines it feeds fine but with my new TM low caps I run into the same issues as you. The same thing happens with my Scar too.

Maybe Stealth or Pestobanana can chime in on this issue? Pesto I remember talking to you a while back about this issue and you mentioned something about the nozzle and hop up unit but I cannot remember exactly what you said about it.

EDIT: does the issue only occur on Semi? If you use full auto I assume the issue goes away?

[pestobanana]

He's using semi because auto would strip the piston. Problem is simply nozzle misalignment. I guess after two years your nozzle might have worn and developed a wobble or something.

Is there scratching on just one side of the nozzle and a matching mark in the hop unit?

[lurkingknight]

no marks that I can tell.. also just put in a new nozzle.

Compression is perfect.

I'm pretty sure I saw the issue occurring in full auto as well.

[pestobanana]

Issue should have nothing to do with compression. What happens if you try a nozzle with a smaller internal bore? What cylinder head and nozzle are you using?

[lurkingknight]

lonex and lonex

[jordan7831]

Did you sand the tappet plate down? I did that with my old P90 and found to have reliable feeding I had to add a drop of superglue to secure the nozzle. Otherwise it would bounce around too much when the gun was being fired which caused misfeeds.

[lurkingknight]

in fact, this time I did glue the nozzle to the tappet. also nothing is obstructing tappet movement.

[DipTwit]

I'd hazard a guess that the pressure of bb's in a full mag is pushing up on the nozzle as it chambers the bb, causing it to drag against the top of the hopup chamber.

If it doesn't slide fully forward of course you'd get a poor seal. So it might just be that the top surface of the chamber has gotten roughed up and has more friction than before. it might also be worse if the shiny new nozzle you put in has the tiniest bit more width or taper to it than the previous one.

You could try a little lube on the surfaces, but only to test the theory, not as a long term fix.

I suppose it could also happen if you switched brand of bb's and they aren't as polished, but I'd try lubing or changing out the hopup chamber first.

[jordan7831]

Could you go back to the old nozzle you had in there? Does the problem still persist if you do that?

Also, what brand of P90 are you running?

[lurkingknight]

Originally Posted by DipTwit

I'd hazard a guess that the pressure of bb's in a full mag is pushing up on the nozzle as it chambers the bb, causing it to drag against the top of the hopup chamber.

If it doesn't slide fully forward of course you'd get a poor seal. So it might just be that the top surface of the chamber has gotten roughed up and has more friction than before. it might also be worse if the shiny new nozzle you put in has the tiniest bit more width or taper to it than the previous one.

You could try a little lube on the surfaces, but only to test the theory, not as a long term fix.

I suppose it could also happen if you switched brand of bb's and they aren't as polished, but I'd try lubing or changing out the hopup chamber first.

changing brands of bb theory holds no water. The gun shot fine on the brand I use, it developed this issue this year, 2 weekends ago was the first time this gun has seen action this year. It went away last year working properly, all mags fed fine, fully loaded.

The only thing that changed in the transition was that I replaced the piston body with one that was swiss cheesed. I put the original one back in to check if it was an issue with the piston binding from pme but that is not the case.

The only 3 bits that have not been replaced as of this point is the hop up unit, the tappet plate, and the tappet spring.

[targetGspot]

are you sure it's not just double feeding?

[lurkingknight]

not double feeding.

[DipTwit]

Originally Posted by lurkingknight

changing brands of bb theory holds no water. The gun shot fine on the brand I use

Agreed, if you haven't changed brands - just seen it happen before

You could try stretching / replacing the tappet spring to see if it's just weak, that could cause the same issue I described, and may be more likely than wear on the hopup chamber. Murphy's law I know but since those are the only things you haven't changed.... guess you know what to try next lol

Good luck!