Kris, Sounds like your doing everything about right. An 1800 RPM idle is sort of low, I haven't been able to get that low an idle with the stock plug normally. I set mine for a 2000-2100 RPM idle and the planes will not roll on the short grass where I fly. When the glow plug goes get an OS F plug. I use the APC 14X6 and 14X7[special order item normally] on my Saito 91's. Your top end RPM is about the same as I get on the two 91's I'm currently using.
Fred B