I cleaned out my mixing elbow a while ago by placing it in a bucket with a gallon of Marsolve, then using a small 12-V bilge pump in the bucket to circulate the Marsolve through the water injection port. I let the system run overnight and it seemed to do the job. Later several others pointed out to me that I probably could have taken the elbow to radiator repair shop and got them to chemically clean it out for me with less effort.
