Will she lean out with the glow plug lit?
If so your fuel is old. Get some good fuel with 100% oil castor. The YS used an ABN P&L and as such need all the protection that 100% castor affords. I ran my YS short stroke 60 on 11x 7 to 11x 8 props on a full wave pipe with 5% nitro fuel.
for plug selection look here,
http://www.modeltechnics.com/glowplug/firepower.html.
!!!DO NOT USE PETROLEUM BASED AFTER RUN OILS!!!!
All the best,