Hello: I'm confused you say the engine was freshly rebuilt 4 years ago and has only been run a few times but not enough to really warm it up. What makes you think it is so dirty you need to do something so drastic? How did it get that bad? The run on you describe is probly due to not being properly tuned up. Change oil and filter (before starting engine) Tune it up / drain tank / carb / and lines. Then see how it runs. MY HUMBLE OPINION
