1700 RPM is very low. 2000 is average. This is for mag. ign. Batt ign will go down to 1000 RPM. This would make it difficult to get a prolonged idle too. Carb diaphrams should last for 3 or 4 years before replacing unless there is something visibly wrong with them. What colour is the centre electrode of the plug? What is the pop off pressure.? Needles become more sensitive as you go to a bigger carb, as the pressure in the throat is lower. Ran a small motor shop for 35 yrs before I retired so I know Walbro and other chainsaw carbs inside out and backwards. Still have quite a few mod. motors of all different makes.