Being an English product order, your Leo may be designed for low or no Nitro methane. In other words very high compression like FAI Pylon Racers and CL speed. Nitro is almost non-available in Europe, or at least very expensive. FAI competition requires 80% methanol and 20% oil. NO nitro.

The early ASP engines (O.S. Clones) were like that. More than 5% nitro and they may not run at all, especially the larger ones.

Grab a soft-drink can and cut yourself several head-gaskets. Add 1, then maybe 2. See what happens. My Asp .75 and .90 had to have them to run on even 10% fuel until they had several hours flying time.

In addition the K&B plugs are rather old technology for high performance engines. Try the McCoy 59 or OS/Enya # 3 or 8.

Back in the '50s most glow fliers used the CHAMPIONS. Yep those were cute little buggers. Then the first imported OS High Compression engines arrived on scene and "Whamo" those white elements simply aimed for the eyeballs. Change happened.