First off I would say the 15-8 is way too much load for a 91 on only 20% nitro.

However it sounds to me like this engine is past due for some service work. Probably a ring

You may have an air leak somewhere but I'm not sure. Since the engine is pretty old I would send it in for service. Richard can fix you up. If there are worn out parts that need attention he can solve it for you. On engines that are old like this with lost of run time or just been sitting likely there is something going on.

I would say it might be rusty bearings. But rather than throw parts at it. I would let YS Parts and Service work it over. In the end they will be able to find if something else is wrong as well. Rusty bearings could so the signs you have with the tough to start and poor idle. bad bearings can kill a glow plug on the first try to start.

www.yspartsandservice.com

Troy Newman
Team YS