90MirageTrovare

1990 Merc 7.4 magnum bravo
Runs great for about 10-15 minutes then when running at high rpm- wot starts to surge a little and will start intermittently what I can only describe as a “burst of compressed air sound” like the opposite sound of a backfire. Does kinda clear up when you back off a bit and then will idle fine and start running again and then will do it again. HELP PLEASE!!! Do I have a sticky exhaust valve?! I can’t really tell if it’s ignition or fuel problem. Any input is appreciated!

xlint89

Opposite of a backfire out the exhaust sound? (Rich fuel condition)

Or backfire out the carb sound? (lean fuel condition)

My first guess would be it backfiring out the carb (lean) because you're not getting proper fuel flow.

Do you have a fuel pressure gauge installed and can watch to see how many PSI you're getting at WOT?

j272

check filter at carb fuel inlet

90MirageTrovare

I don’t have a fuel pressure gauge, how likely is it that the mechanical fuel pump is starting to go?

j272

I have seen these filters on the carb inlet clogged.often ignored

compedgemarine

also could be fuel hose from the tank collapsing or anti siphon valve sticking clogged

getrdunn

iTrader: (7)

Just a guess also but what about the fuel tank vent hose clogged or pinched. A long shot but easy to check with loosening fuel cap next trip out. I had mud bees build a condo in my water pick up hose I recently found out.

90MirageTrovare

Changed the carb filter(screen), didn’t really do the “opposite of a backfire” thing but definitely after running for a bit at high rpm to wot, it started just running ****ty, definitely seems like starving for fuel. It only starts acting up after running for about 15-20 minutes. Any more advice is much appreciated.

Griff

You need to check the fuel pressure while running the boat.