I had one of those model Reds. It was very difficult to tune. Really I never could get it down. I could get it to where it ran well, no pinging, running on, etc. But it would randomly flame out. Another Reds user at the time told me you have to let a Reds idle higher than say an OS or Nova. I feel that person may have been right. I was trying to get the same low idle like my OS. Maybe try raising the idle ?

When the 3 needle carburetors came out I recall watching a video on them by Elliot Boots. In that video he specifically said they allow you to get the lower idle like an OS. From what I see Reds users are having an easier time tuning with the 3 needle carb. If memory serves me correct the 521 has a 2 needle. I could be completely wrong on this. Hope this helps.