I'm a firm believer in CO detectors, particularly those that will alert you to any CO in the cockpit. Over the first 200+ hours my CO readings have always been 0. However, beginning after my last condition inspection, I began to see low levels of CO, generally less than 10ppm during climb only. These resolved when cabin heat was turned off or air vents opened. Over the next 2-3 flights I also saw levels up to 15ppm during taxi operations. Given the change, and apparent worsening of the CO situation, I grounded the plane for further investigation.
Findings included the exhaust no longer centered in the lower cowl opening and the upper slip joint in the Cyl1 exhaust slipped out and jammed (see photos). A call into Van's support was helpful. They noted the slip joint needs to have anti-seize paste applied regularly to prevent jamming (there is motion in this joint on startup). If inadequately lubricated, the joint may slip out and jam. This distorts the relationship between the exhaust stacks and the muffler, allowing CO leaks. They recommended frequent lubrication and a spring like brace around the joint (ordered from Van's). I also identified several holes in the SCAT tube between the cowl NACA intake and the heat muff. These may have allowed additional CO into the system.
Lessons learned: 1) Any CO in the cabin, even if at "safe" levels may be a sign of problems with the exhaust or heat systems. 2) A change from never seeing any CO to seeing CO without obvious explanation is an indication to ground the plane and investigate. I should have done this sooner. 3) Van's support is outstanding.
Note: The spring like brace for the slip joint has subsequently been released as SB-00064