I bought a nice 2nd hand 97k .177 six months ago (off this forum) which had been tuned - looks like a V-Mach kit.

When I got it home & chrono'd it, it was over the limit. So I stripped it down and removed about 6mm of pre-compression washers (3 washers), cleaned & re-lubed.

Power then went way, way down to around 7-8 lbs sq ft! So I re-installed the washers one by one. It took all 3 washers / 6mm to get to around 11.5 lbs. So I then left it - job done.

I re-chono'd the rifle today to find that it was once again over the limit by 1-2 lbs. So I immediately stripped it and once again removed the 6mm of washers, cleaned and re-lubed.

Power is now once again down to average 7 lbs sq ft.

This is my 1st HW97 but not my first strip/clean/lube so I was wondering what might be causing this?

Could it be dieseling? After a few shots today there was no 'smoke' coming out the barrel. And I've not noticed smoke or smell when using the rifle since my initial strip down.

I know they can creep after a while but it has not been used excessively over the 6 months of ownership - a few tins

Or is there another likely cause?

Thanks in advance!