[NSRCA-discussion] OS 140RX rough and inconsistent idle

[Ed Miller]

Have an OS 140RX that has developed a rough and inconsistent idle. Engine 
runs perfectly except for idle. It has a Mintor head with either an OS F or 
K&B 1L in center plug and an OS A5 in the angled plug position, a Perf Spec 
AAC piston/liner assembly and a Boca sealed SS rear bearing. I have the head 
mounted with angled plug towards the front or carb side of engine. Fuel is 
S&W 20 nitro, 16 synthetic and 2 castor. Engine has 7 gallons of fuel run 
through it. Prop is an APC 15.75 x 11 3 blade by Dave Lockhart.  The lowest 
I can get the idle before it quits is 1900 ~ 2000 rpm and it is rough, 
sounds like it is firing every other revolution. Idle mixture screw is 
working, I can richen idle to point where engine will barely transition from 
being rich and I can lean mixture so the transition is fine but engine slows 
at idle like it is too rich. While it is slowing with either the idle 
mixture screw lean or rich, engine is rough, again like it often fires every 
other revolution at times. I used to be able to reliably idle this engine at 
1300 rpm. I have a second OS 140RX set up exactly the same except it doesn't 
have the AAC piston/liner and it runs/idles perfectly. Tried cleaning the 
carb/spray bar with no change. I once had a Super Tiger that exhibited 
similar characteristics due to a worn out con rod.  Is it possible the con 
rod on this OS could cause this issue ?? Any suggestions will be greatly 
appreciated.
Ed M.