Mercruiser 454 Magnum Performance Issues

BIGDSB

Cadet
Joined
Aug 30, 2008
Messages
9
I am hoping someone can help. I have a 24 ft sports boat fitted with a Mercruiser 454 Mag (carb) 365hp, Bravo 1 leg with 4 blade 24 pitch bravo stainless prop. The boat is reputed to run at 70mph, I am the second owner and not seen anything like that sort of speed. I ran the first season and hads lots of fun, when running 3/4 throttle you had a real surge of power when playing with the last 1/4. I have no idea of what speed she was running, the gauge was indicating near 70mph. When the season was over I had someone refresh the motor, new rings and a few other bits and pieces, (purly for peace of mind rather than something wrong with the motor)

Second season the boat does not seem as fast, I have now got a gps, the speedo is reading 65mph but the gps is showing 58mph at wot, 4200/4300 rpm; the same last year and no different this year. I have had my engineer look at the motor and he found; timing out 1 degree and plug gaps not set, the compression was all good and all else seemed ok. Launched last weekend and the boat is pulling more revs but when it reached 4500, the motor cut out, the revs shot to zero for a second then back to 4500 the straight back down again to zero. It was not till I backed off the motor would run.

Please does anyone have any ideas as to what this could be? my mechanic says it is a rev limiter, do I have one on my motor? could this be an electrical fault? fuel pressure? where do I start? HELP!
 
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