Built a 18 hp onan in my 446 after a long talk with Boomer and following his recommendations used a John deer block with an oil filter. Everything went smoothly and it runs well. But it’s developed a strange issue. When running at speed mowing it will gain some RPM then drop back to normal. I’ve checked everything short of pulling the timing gear cover and checking the Governor ball/ cam asy. Any thoughts? Or do I tear into it?