CP88 intermittent discharge on hammer release

[charliemow13]

Good evening all you pistol heads. Hope you've all enjoyed a sunny bank holiday.

My friend passed me his CP88 over the weekend as hes been having issue with intermittent discharge when the hammer is released. Its hit or miss whether is actually pops the valve or not. Having taken it down check through the only thing I can see that might potentially cause this issue is the level of wear on the hammer, theres quite a dimple worn in it, and possibly the firing pin being slightly shorter than new from years of use. I'm thinking the combination would mean the hammer cannot strike the pin enough to open the valve consistently. Just wondering if anyone has seen this sort of behaviour before? And does anyone know the overall length of the firing pin from new, currently measures 7.54mm?

[hawkdriver]

I have a feeling your hammer spring might be the cause, i've never stripped any umarex but had lots of the guns and some seem to develop a weaker hammer impact more with age than use, as i've had very little used guns do the same with no wear on firing pin or hammer. only a thought

[charliemow13]

Thanks hawkdriver but the way it seems to be arranged in this case is the hammer spring tension is actually keeping the valve stem closed. I stripped down the valve assembly so I could see what goes on inside the pistol when you pull the trigger. As far as I can see a weakened hammer spring in a CP88 would mean it was actually easier to discharge. When the hammer strikes the firing pin this in turn acts upon the valve stem against the hammer spring to compress it to open the valve momentarily to let a volume of CO2 by to discharge the pellet. Kind of the reverse to what I've seen in a pcp mechanism where when the rifle / pistol is cocked the striker is held back under hammer spring tension to be released to strike and open the valve and discharge the pellet. In pcp a weaker hammer spring would open the valve less as its not flinging the striker towards the valve with such force. In the CP88 a weaker spring would make it easier to open the valve. Hope I've explained that clearly, its fairly simple to understand with the parts in front of me but not as easy to articulate in words.

[charliemow13]

Well I went with my gut feeling and pulled it all to pieces, put in a new hammer, firing pin and a couple of seals. The CP88 lives again! 😁
http://imgur.com/a/IWROTQl

[Jimny4x4]

Glad to see its up together again .
I would have suggested the internal hammer spring , as somebody else has done too .
If any gunk from the bulb works its way up into the valve . A piece of metal from the bulb perhaps ?

[charliemow13]

I did strip the valve assembly but it was as as clean as a whistle inside and no evidence of any gunk or debris. The spring still seemed very firm. First time I've ever done a CO2 pistol or something as fiddly as a CP88 so pleased it's all worked out ok. Had 8 magazines through it this afternoon and no problems, also quite impressed with accuracy shooting it single action.