air arms Tx200MK3 loss of power

[Wilfy100]

Hi,
I wonder if anyone could help me some info and perhaps some reassurance about my TX200 MK3 .177 (full length barrel)
I bought this brand new but I did not chrono it then. After a while I fitted a TBT drop in kit consisting of delrin spring guide and top hat plus delrin top hat washer and power washers. I tried my best to ensure with the kit fitted that the spring was compressed the same amount as with the original parts, (i.e. the new delrin parts were the same length as the parts they replaced). When I chrono'd it it was making 12.1 FT/lbs with 7 grain pellets. However like a complete idiot I recently left it cocked for a period of14 days. I chron'd it again and power was down to 11 Ft/lbs with the 7 grain pellets. My pellet of choice are arms diabolo field, with them power was 10.66 Ft/lbs (these being heavier at 8.4 grain).
I have measured the spring free length at 221mm, another correspondent on this forum gave a spring length of 220mm so it is possible that it has not settled in length.
I have now fitted two extra standard air arms steel power washers ( in addition to the one TBT Delrin one). Power is now up to 11.44 ft/lbs with the 7 grain pellets and 11.11 Ft/lbs with the AA pellets. While I know these figures are quite reasonable, would anyone say that leaving the rifle cocked for two weeks has done any harm. How many power washers have people used when controlling power? As standard the rifle had 3 steel washers fitted.
I am not chasing power for powers sake, the situation is that I have quite a few rifles but all of modest power (between 6 and 10 FT/lbs ), however I would like to have one at around 11.5 Ft/lbs with the AA pellets (provided the lighter pellets did not exceed 12Ft/lbs). this is so I can do paper target shooting at longer ranges. Has anyone got any thoughts on this at all, I suppose the only way forward is to get a new genuine AA spring and try that.
Also I have a TX200 Mk2 SR (semi recoiless) could anyone tell me if the piston, seals and spring are the same as a standard TX200 Mk2. This is an absolutely brilliant rifle, I highly recommend if the opportunity arises that you get one or at least try one out. The anti recoil system is the same as the Feinwerkbau 300S but I much prefer the TX200, the barrel and action seem to slop back and forth much more on the FWB300S.
Cheers Wilfy

[sherlock1963]

It sounds to me like you have done no harm at all. If the spring was compressed for 2 weeks and is still 220mm then that's fine. As it is making slightly more power with lighter pellets it may be slamming slightly if you have any metal washers in the piston you could try taking one out and adding the same dimension on the spring guide. Heavy and light pellets should, ideally, make the same power. I use FAp 7.3g and JSB 10.34g to test mine. I realise there are other factors involved but it is a benchmark I like.

TX200SRs and TXs have identical pistons, comp tubes, springs and seals. The sled 0-rings are the same as the breech seals too. A well sorted SR is an amazing bit of kit, well worth changing the sled nuts to Aerotight to stop any loosening and worth stripping and polishing the trigger sears (same as any TX, no matter what age)