'89 Kdx 200

Hi there, I need some advice on how to check the Exhaust advancer spring... I've just taken the exhaust valves and operating rod out and serviced them, the bike however still doesnt operate very well at low revs and im not sure its hitting the powerband fully. The compresion is good and checked most other possible causes.
How do i check if its the advancer spring, or any other idea's on the cause of the problem?

I've only had the bike for about a month now. So do you think I should change the jets or try retune the carb?