Idle too high. How do I adjust

[ZZZ Temp-Gold]

I have repowered my boat with a '90 rv tbi 7.4. It has a 747 ecu and works
nicely, other than the idle. The boat likes a 500+/- rpm idle and it's
currently idling at 800. What can be done to adjust it? (Ive tried backing
off the butterfly, and this seems to help, until I restart the motor and it
goes back up to 800 again.) If reprogramming is the only solution, Id sure
buy an 'adjusted' chip. 

Also, it pulls up to 4200 rpm. Is this the upper limit of the motor? It
seems reasonable, just curious. 

Thanks,

Greg Gold 
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