Mercruiser 4.3LX stalls after starting

[sundriver]

I have a Mercruiser 4.3LX 4bbl I/O. It recently developed a problem of starting right up and running great for about 30 seconds then dropping out. You can start it and set the throttle at 1500 rpm and it will hold steady there for 20-30 seconds, then the rpm will slowly drop until the motor stalls. You then have to pump the throttle while starting to get it to start again, but it will only run for a few seconds.
I have checked and cleaned the fuel screen at the carb, manual fuel pump (no splits in the diaphrams), breather tube from the carb to the bottom of the fuel pump. I had rebuilt the carb a few years ago so all of that should be OK. Would the float be sticking or should I check the coil or electrical system. Any help would be greatly appreciated. Thanks!

 

[Bondo]

Re: Mercruiser 4.3LX stalls after starting

Would the float be sticking or should I check the coil or electrical system.

Ayuh,... Sure sounds like a Fuel delivery issue,....

Could be the tank, lines, fittings, filters, or the carb...

 

[mia9516]

Re: Mercruiser 4.3LX stalls after starting

Have you checked your throttle linkage and idle adjustment screw? I have the same motor as yourself and was having similar problems. Turns out the previous owner changed the idle adjustment screw for whatever reason. The new one was not long enough. All I did was find a similar screw, that was longer, and fixed the problem right up. Now the only issue i'm having is that the spring isn't long enough to prevent the screw from backing out during normal ops. Needless to say, after running a while, she won't stay running because the idle rpm's are too low. According to the manual, we should be idling at 650-700rpm's. Good luck to you.

 

[Don S]

Re: Mercruiser 4.3LX stalls after starting

Without a year or a serial number for your engine, it's hard to say what your problem is. Could be a wire off an oil pressure switch that controls the electric fuel pump. But who know, you may have a mechanical pump.

 

[sundriver]

Re: Mercruiser 4.3LX stalls after starting

It does have a mechanical pump. I took it apart and cleaned some gunk out of the breather tube below the diaphram. Everything else looked great and I was sure it would work. No such luck.....
I am the original owner so I don't have to worry about getting into what the previous owner was thinking or doing. Would you suggest adding a treatment to the gas to break up any water? I didn't suspect water was an issue since it runs so well, even though for a short time. Would the coil break down in less than a minute of running?