mercury 525EFI backfiring loss of power
 

Hello Everyone,
I hae read through different related posts of what could cause my 525EFI with approx 104hrs suffer from backfiring.
both Mechical and Electrical Fuel pump have been changed.
Spark Plugs changed,

Engine runs normal for a few minutes then starts backfiring and will run on 950/1100rpms only as soon as i puch the throttle it chokes... high smell of unburned gas,
can hear the exhaust on portside not exhausting normally... like if engine was running on fewer cylinders...

Any help is highly appreciated.

Start with the basics:
#1 fuel psi test
#2 compression test
#3 scan for codes

A friend of mine had this exact problem on his 500EFIs. He changed and checked EVERYTHING and still no results. Turned out to be that his ground wire for the engine was loose. It would break contact intermittently while underway causing the same symptoms you describe. Might be something to check.

Sounds like a ground issue or a header sprung a leak . Once she heats up the crack opens up and aqua puts out the fire .

Possibly a bad coil pack. Has the engine been scanned for codes?? Without being scanned, you are just guessing.

Recently had same issue on my 525s, changed mechanical fuel pump, filters, etc. Still no pressure at the rail.

Problem ended up being fuel line from tank to mechanical fuel pump. Inner lining was separated and clogging fuel flow like a clogged artery.

After changing fuel line, my 525s never sounded so healthy and smooth running since Ive owned her. They are thirsty engines!

Roughly……
500hp .500 bsfc (hp/lb/hr ) fuel is 6lb/gal
500 X .5 = 250lb
250/6 = 42gal/hr. Yum yum. I need more fuel. :)

So when initial start up when cold it seems normal until it heats up after a few minutes. Then smell unburned fuel? I’d start adding additional grounds… one at a time. Perhaps the heat is causing an unwanted expansion of ground bolt etc.

Sounds like a typical magneto issue with sleds huh ? Lol.

IAC might be choking it to run fat