This blaster does around 350fps, right on the limit for Nuketown. To resolve that issue I’ve replaced the OG spring with an unequal X-force M90 spring, this lowered the FPS down to 315 - good result.
However, after firing one mag with the new M90 spring the gearbox started making some awful grinding noises. As an experiment I put the OG spring back in and within 5 dry fires the noise had gone.
#notanexpert but I would not have expected gearbox grinding from lowering the power of the spring.
Sounds like the heavier spring’s possibly caused wear on the gearing and piston rack. You don’t notice it with the heavy spring in but you can hear it with the lighter spring in due to the change in gear tooth loading.
Possibly also worn your gear bushings and the change in load’s allowing a bit more movement and affecting gear meshing.
New gears, new piston, new bushes maybe. At least that’s what I’d do… and don’t forget the motor pinion gear too.
Again - not an expert, the noise was so noticeable that I can say with confidence that it wasn’t there with the original spring. This was a “there’s something significantly wrong sound” emanating from the gearbox with the lighter Xforce M90 spring.
I found an older tac edge M90 spring and gave that a try, dropped the FPS down to 330 and ran smoothly without the sound, so that’s in there now…
Ruling out the possibility of something going back in squiffy the first assembly, I have no idea.
Are the two M90 springs the same diameter? Is there a chance the first M90 was slightly larger in diameter and binding up on the inside of the piston? Or possibly flexing like DChapo suggests?
I know if you flex an unequal spring out of the box the curve’s not even like a regular spring, so… maybe.
That is weird indeed.
I noticed in one of my double bell AKs, power is 360fps in semi and drops and gets very inconsistent in auto as they are under volumed, so changed to a full cylinder and went up to 390fps. Wanted to drop it to more reasonable level to get some accuracy and make it more field legal so dropped an m90 spring in. The double o-ring piston head is so tight it could not get up to power, and also pre-engaged like a MF! Sounded awful too because of this and broke some teeth
not sure what piston head they are running in SRC blasters but might have done a similar thing with pre-engaging?
Either that or the spring is diving, otherwise it is quite odd to get horrendous noises from spring changes.
