i'm running two 600's on a ch28 and they work great. the 500's have a very restrictive primary booster, and you could be getting too strong of a vacuum signal to them. you might try a 68x52 rod; also do a fuel pressure and float level check. long cams will screw up the metering system too. in my opinion those 500's belong on small engines only. that primary cluster is just too screwed up to put on anything that will flow air. back in the day carter had a properly done small afb. instead of using the carter model, that required more part changes, edelbrock got cheap and just restricted the front half. I believe it is possible to put 600 clusters in those carbs. be careful of bad advice!!!!!!!!!