I run my timing a bit advanced and it seems to cure the stumble problem I was having. Since modern gasoline has a lot more octane than the compression ratio of the 235/261 calls for, its pretty safe to do- just turn it down a bit if it does knock.
Also, it seems to happen more commonly with ethanol blend fuels. Oregon is using 10% ethanol all year round and you actually have to richen up the mixture sometimes to get rid of the stumble.
