While getting the boat ready to go out yesterday, I fired up the port engine. Well, tried to is more like it. It coughed a couple times and wouldn’t start. inhad to hold the throttle at half way to get her started and the. was really rough with a dead miss and serious valve train tapping. First I thought maybe I had spun a beating, but oil pressure was 40 psi (where it always is)). While running with the dead miss, I was getting a lot of fuel in the air (assuming map saw low vacuum and started dumping fuel). I shut her down and started her back up and the tapping cleared up. It seems like a lifter or two may have bled down. That’s my thinking anyway.

Interestingly, idle quality isn’t the same as the starboard engine, though it’s snappy on the throttle. It’s sounded like this since redoing heads last year and replacing injectors (went a touch fatter based on BUP’s suggestion). The heads have been done a few times so I’m thinking there may be a slight issue with the non-adjustable valvetrain and the decking of the heads (three rebuilds... I know we took off .006 to clean them up last year... no info on previous work but I have receipts from both jobs).

I checked the oil and it’s pristine. Mobil 15w50. Last time out a few weekends ago she ran great. No issues on the hose flushing her out either. This was a head scratcher.

Any ideas? I have a complete AFR set up and associates valvetrain parts that will go on these engines after the season is out. I’m just wondering if any of you have experienced something this odd.

Sounds to be a fuel related issue. I'm not sure of what EFI system but often by advancing the throttle it will enable flood clear modes (shuts down injectors). Perhaps you have an injector hung open?

Usually throttle has to be 80-90% for the injectors to turn off.

Thanks guys. It fired up without any issues today. The valvetrain is also quiet Maybe she just didn’t want to cooperate yesterday.

Engine is a 1998 502 MPI.

To look at issue of the different idle sounds between the two, I checked timing (both are at 8 degrees in service mode) and both were in a half an inch of vacuum via the Rinda (17.8-18.2 on starboard and 17.5-18 on port). I was thinking that maybe the valvetrain was tight on the port engine but vacuum reading seem close enough to indicate good ceiling. Throttle is just as snappy as the other. Maybe an injector is having an issue at idle?

Depends on the EFI system, hence why I asked. MEFIs are 50% with a few other caveats.

Yep, 50%+ for shutting the injectors down on the MEFI.