I thought they(500hps) came with 800s? I just bought 2 from KLAW.

If Dennis Moore was still around , he'd probably tell you to stay with the 750's. He was always conservative. The Merc 454/420hp used a 800cfm. I don't think you'd see much of a difference with the extra 50 cfm. Like Formula said, I'd look at your plugs.

The 500 hp probably did, but my problem with this boat (and the engines) "was" that the previous owner(s) went way out of their way to get as many mis-matched components on it they could, including Holley 4150 automotive carbs instead of marine carbs; not a big deal but just one more thing they got wrong. Believe I'll run it a bit and watch the plus. Like almost everyone has said, "I ain't going to hurt it running the 88/90's.