ORIGINAL: w8ye
Try the fuel in the other engine
Just for the record, when you advance the cam timing, the idle will be great but the top end will be poor. With retarded timing, the top end will be great but the idle will be problematic.
If the fuel runs good in your 61, there is a possibility that your cam is retarded just one tooth.
Okay here is what happened. The fuel that I was using on the Saito 100 was 15% nitro. I had some 10% nitro fuel that I was using on the OS 61FX.
1 - I fueled the 61FX with the 15% fuel and it ran about 2K rpm faster (12.5K) top end and after running it a while I could not get it to idle very well. I didn't play with it much but noticed that it idled rough and similarly had to increase the idle, but only slightly.
2 - I defueled the 61FX and just to check ran it on the 10% fuel and it would idle just fine, but of course was 2K rpm lower top end.
3 - I fueled the Saito 100 with the 10% fuel and top end was almost 8500 rpm and it idle okay
4 - I defueled the Saito 100 and refueled w/ the 15% and same result, started sputtering and idle was very rough.
I think the OS 61FX liked the 15% on top end and seemed to tolerate it at idle, but did notice a difference. I am wondering if the fuel has absobed some water and the 2S FX is more tolerate to that than the 4S 100? I am going to try using a new batch of 15% fuel and see what happens, but it looks like that you might have been right.