Back fire on start stock 496HO
 

Unless you want to just throw money at it, I would do some testing first. Compression and leak down test would be my first, then check fuel pressure

Do the above tests, pull the codes, check IAC, water separator filter. Having injectors tested, flowed, and rebuilt can never hurt and they are very suspect. And for what it costs from Bup, not expensive to do.

scan and knowing the parameters. fuel pressure testing is the first 2 steps for pin pointing issues or not. Then move on to comp and leak down. Easier testing first to rule out then move on. When was the last time you changed fuel filters and did fuel samples ? How many hours on the engine(s) ?

Have to jump in on this --- Pulling spark plug wires on DIS set ups is asking for huge trouble - whom told you to do that? thats 50 year old carb testing methods. And if you have gas fumes in the engine compartment - we will say good bye to you now. 4 years ago people working on a 496 in my area burned down a complete section of the docks and boats in the marina from pulling plug wires off and the spark / gas fumes caused a huge fire. I am posting this as a huge safety issue for everyone AND NOT TO DO THAT FORM of backyard testing plus possible ignition / ecm / pcm inflicted issues.

And for the record --- NOT A SALES promotion but injector testing and flow testing / cleaning IS PART OF MAINTENANCE along with fuel samples. I have never had a used set of all 8 injectors come in that were completely balance across the board. NEVER. And on a 496 regardless MERC - Volvo - Indmar - PCM - Marine Power - it needs to be done. NO way around it. Its really not the injectors themselves but the contamination issues are the factor here especially from improper fuel maintenance and storage.

Added info on top of --- Keep in mind internal fuel pump wear happens on all electric fuel pumps. Guess where that wear ends up ? Then you have the rubber fuel lines that no one ever changes. Currently the range for those are 4 to 7 years max for change out .

2 contamination issues right there that happen in every marine engine even carb engines, the same happens.

300 hrs on spark plugs and wires man I would start there as well --- How many hrs on the app totally ?

<div style="text-align:left;">describe runs fine after start up ? can you reach full rpms at WOT ? 5000 rpms HO mag and 4800 for mag 496 is WOT</div><div style="text-align:left;"></div><div style="text-align:left;"> I would not rule out CRANK SENSOR here either for starting issues. That signal and cam sensor have to sync for start up among other things</div>

Have no idea what happened here with that mess -- OSO ?

will fix in the next post