OMC 3.0 Runs on muffs, not on lake...

[90sunbirdcorsica]

I'm new here and new to boating. I recently purchased a 90 Sunbird with the 3.0 Cobra Sterndrive. I'm now the second owner. Boat seems to have been very well maintained though I can tell it has spend some time sitting in the garage over the years. My problem is I had the boat running on muffs in the yard for 30 mins yesterday with no issues. I shifted through the gears, took it to fast idle, everything checked out fine. I decided to lake test it today. I got off the trailer, it fires right up, I am able to move over to the docking area, tie the boat up and leave it idling for maybe 3 minutes. Then it knocks off. Front that point on it would not fire unless I advanced the throttle. I had to continually play with the throttle and even then it would barely stay running, would not idle at all. To me it seems like a fuel problem but I don't really know where to start. I was so disgusted I came home and have not looked at it since. Why would it run fine on muffs then quit on me at the lake? Water in tank? Bad gas? Couldn't be the difference in how the tank sits while on water? Trash? Anyone else had this issue? Sorry for being so long...

 

[Howard Sterndrive]

start with a compression test
then look in the fuel filter to see what has been coming from the tank

 

[superbenk]

Maybe something is wrong with the float in the carb.

 

[bruceb58]

I would run it on a portable tank just to rule out the main tank.

 

[skippy2235]

I would check the fuel pump pressure.
Sounds like it is running lean. When you run on the trailer there no pressure on the exhaust so engine runs.
When in the water, you have to overcome the water pressure on the exhaust and fuel mixture is to lean and engine dies.
Items to look for:
Vacuum leak.
Fuel pump for carb should be 5-8 PSI,
water in fuel / bad gas
clog in carb / float / jets / fuel line.

 

[Boat Doctor1]

ok =what id look into= disconnect the tack wire on coil!! an bad/ intermittent tack will cause this condition! next as i believe that 3.0 l engins is (had one here for repairs) is nothing but an old chev 4 banger with updated non=point ignition system!! Remove dizzy cap, rotor, and change the "Module " in the dizzy!! along with a new cap & rotor!! Get all from napa - as their good caps & rotors are brass contacts & a quality part!! DO NOT USE an Aluminum contact dizzy cap!!!!!!!!!! The boat here i repaired in mention came in with owner complaint =it misses out @ speed. I changed all that along with a new set of spark plug wires & plugs - owner called after run on lake & stated="Best its ever run since new!!" in the old point type systems before the Designation 3.0 l the points would corrode when sitting in non use times also!!