Hitachi carb power valve

[David D]

Hi everyone

Can anyone explain to me what a power valve does and what the symptoms might be if it wasn't working properly?
I'm on the brink of taking my Brumby to get someone to have a look at it as I can't get to the bottom of an ongoing issue with the carb since I rebuilt it.
This is the original thread:

showthread.php?t=23687&highlight=hitachi+carb

It struggles with hard acceleration and at full throttle but the accelerator pump seems to be working OK. Idles good and starts good. OK generally driving but feels flat when I accelerate hard. I took the top off the air filter and saw the secondary open when I revved it. It feels fine revving stationary but not good under load.
I was just wondering if there was any way I could check the power valve to rule it out before I take it somewhere.
It's a 1987 EA81 Brumby - not sure what model Hitachi.

Thanks

David D