I have a 575 that doesn't want to idle sometimes. Once i get up on plane and try to hold a steady RPM (3000rpm) it might run 100 yards or a mile then it acts like its running lean, drops about 500 rpm and backfires and keeps running. When I'm at idle it will backfire and die. It has about 60 hours since complete rebuild. It is showing no codes on the scan tool. I have tried the following known good parts: distributer, distributer module, cap, rotor, wires, spark plugs, map sensor, fuel filter, water separator. I installed a key switch at the 10 pin connector and started the motor to eliminate the wiring in the rest of the boat. I have 2 gas tanks, tried them both. My fuel pressure gauge is 43 at static and around 38 in boost. My fuel gauge is bouncing at RPM from 34- 40. Thats the strange thing to me! I was going to try new IAC's but didn't think it would matter once i was off idle. When the motor was provisionally rebuilt i had the injectors cleaned and flow tested, the blower shop rebuilt the blower etc. The spark plugs look lean also. Any help would be appreciated!

Had a similar problem a couple of seasons ago. Did a complete top end rebuild and still kept jumping out of time. Turned out to be a $2 nylon bushing in the distributor that had elongated. Put in a new one - problem solved! Good luck man, that kind of shizzz will drive you crazy....

I would think valave train related , broken spring , cam lobe going away ?

I would think valave train related , broken spring , cam lobe going away ? maybe throttle body issue ?