Model 66sv has an opposed twin-cylinder engine 61 cc (3.7 cu. in.)
Uses a Walbro HDA carb, should be able to get a carb kit at a small engine shop. Initial settings on both carb adjusting screws is 1 and 1/4 turns out. Champion plug CJ-6Y set at .024 to . 028".
Looks like it is the same as an Echo CST_610EVL
Ray
Mange, Opposed means that the cylinders were opposite one another. -In other words the pistons travel 180 degrees to one another.
The John Deere was indeed an Echo in that era. I almost bought one of those saws new 17-18 years ago. Reportedly it was a real torque monster-I don't know how true that was.
A rebuild kit may or may not help. Some of these had an accelerator pump in the carb and they have been known to go bad. Be sure to replace the pump when rebuilding.
These saws are known for ignition failures which mimic a carb issue. One cylinder runs fine and the other is out of "time".
So many saws, so little time. Larry