@ Dan, a remark on what you mentioned which is true off course,

If you need the cap to get the engine stable then there is something else wrong. It might be the front bearing has an issue or whatever but as I've said before the XRDs are not the most stable of engines anyway.

What I noticed with my onroad nitro engines in the 8th class, when I used a similar BUKU solution is the following, My discarded worn nitro engine(picco and nova) which didn`t idle anymore and had erratic tuning problems, after this modification was idling very good and best of all was holding its tune for ever!. Both engine`s had this result. I know its had to do with the front bearing sealing, but both bearings were relatively new and already replaced with an offroad, one rubber sealed.

Just sharing