BIGBREW

I have a 03 496 H.O. I think it has less than 300 hours on it I cant remember what clock said, Just bought the boat last year, took it out the other day for the second time. Started fine, my batteries were almost dead so I sat idling for about 15 minutes while I waited for my buddy to get to the boat launch (needed to get beer) when I took off it was backfiring and popping really bad between probably 2-3k rpm......after that it cleared up and ran great all day. The next morning that same buddy I idled 15 minutes for was stranded on the lake because he had too many beers and had run out of gas so I had to go get him, same thing it popped and farted till i got above 3k and ran fine, even while i towed him about 8 miles. Get the boat home and start looking the motor over and find the vaccum line between the back of the motor and the intake completely loose on both sides, (so much for the marina that did the winterization on it last fall) did get some new plugs for the motor that ill put in this week..........Thank you for reading my rant, anyone that read the whole thing have any ideas on what may be going on?

BIGBREW

Bump

Raylar

There are two lines that come of the top of each dump valve bodies on the raw water system on each side of the engine that are simply vents so when the small hand pressure pump is pluggged in and pumped up to drain the raw system when winterizing to stop raw system freeze ups these small vent lines allow the chambers in these pressure activated valves to adjust. There is only one vacuum line on the engine and it runs from a fitting on the intake manifold and goes down to the fuel pressure regulator on the top of the CoolFuel unit under its plastic cover on the starboard side of the engine if the engine is a 2003 model 496MAG.

What you are describing could be either sparkplugs that need replacing or a fuel system, ie. fuel water seperator or filters that need changing or it could be again internally rusted injector fuel rails and clogged or patial clogged injectors and debis in those areas.

Best Regards,
Ray @ Raylar

bigredbaja

I have a 03 496 MAG and mine was doing the same thing but at higher rpm's i replaced the plugs and that fixed the problem. In my case i think a plug was cracked or plug wire was loose, i had a leak down done befoe i bought the boat last fall and i think a plug was damaged int he process. Start with the plugs first. I did pour in some Lucas Oil Fuel treatment just in case.

BIGBREW

i replaced the plugs today, have not had a chance to run it under load yet, fired right up and sounded perfect, that vaccum line that i found unhooked ran from the port side of the throttle body and ran to the back of the intake plenum, so i hooked that all back up and im going to give it a try on sunday. I need to get an owners manual for the motor. I know motors fairley well but it has been a long time since i wrenched on one myself. Small reservoir on the port side of the intake, is that for the drive fluid? sure smells like it but its low and im getting an alarm. Did have it winterized last year by a local marina dont know if they checked it, but im guessing thats where the alarm is coming from. Will top it off tomorrow and give it a start on the hose, ill take it out on the lake on sunday and see if I still get the back fire. Thank You again guys for taking your time to answer my questions, wish i could meet some of you and buy you a beer.

gotstreetracin

i'm having a similar issue and i think mine is the fuel pickup in the tank

BIGBREW

ill run mine tomorrow and give an update.

BIGBREW

Ran great today, not sure if it was that vaccum line or the spark plugs I put in AC-Delco 41-983 and gapped them at .050 like Raylar said.