Brew, are you running about a 12x8 or 13x6, the compression is not set from the factory, you need to set it according to Mr. Davis' instructions which are very well written. You need about a 30% greater prop load than as a glow engine. For example I ran a 12x8 two blade on my Fox .74 as a glow and 12x8 three blade as a Diesel. When you add more prop load you'll need to reduce the compression.