New to the forum and looking for some expertise.

I have an older Stihl fs66 brushcutter that will only prime with a shot of fuel/carb cleaner directly into the carb. New carb, fuel lines, gaskets. Impulse orifice is free and clear. When I do prime this way and get it running, it idles high. Exhaust is free and clear. I'm stymied.

This model doesn't have a vacuum bulb so I'm also wondering if I can retrofit with a carb that has one and run a return line to the fuel tank.

Did you try adjusting the low and high carb mixture screws?
 
CC richens the mix.  
 
How are the two diaphragms inside the carb, especially the fuel pumping side?
 
Also, I wouldn't start modifying something that did work (for years) and 
is obviously not correct at this time.  Fix what is currently broke.
 
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How'd the two diaphragms look inside the carb? 
The metering diaphram pliable, the pump diaphram FLAT and not concave.
 
Did you try adjusting the high and low speed screws on the side of the carb?
Turning counter clockwise richens the mix
 
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https://www.diyspareparts...ine/fs-66-brushcutter/

Brand new carb. The H/L settings that keep it running are leaner on L and richer on H. The 1.25 counter C from closed factory presets don't keep it idling.

I even swapped diaphragms and Reed valves from New carb back to old in case of some kind of issue with new carb, but get same problem no matter.

You know what the settings are now so try adjusting.
 
Often, one screw setting will affect the other...
 
As you have a high idle, that indicates a lean condition.
I'd open the low speed screw and close the High (if loading up).
 
If starting from scratch, I'll the high speed screw first, then come back to the low speed for a solid idle.
Don't be afraid to go out a full turn.  It won't hurt anything..
 
Might want to double check all gaskets.
 
Lastly, rare but does happen-  Check for leakage (wet oil) between the crankcase halves 
and tightness of the bolts holding the cylinder to the block.  Crank seals as well...
 
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