Mercruiser 4.3 dies at WOT

I?ve been having a problem with one of my 4.3 Mercruiser engines (4bbl carb). Here are the symptoms:

It starts fine when cold and runs great at idle or until I want to take the boat up on plane. Once I punch it to get up on plane it will rev up fine for about 30 seconds and then bog and almost die. It use to catch itself before it would die and the RPMs would climb back up to 4000 and run fine. Once up on plane it runs fine between 3500-4200 RPM. More likely than not it would only happen once in any given trip. Even if I came off plane and went back up it wouldn't happen again. It has been doing this for a month or so, but the engine would always recover before it would actually stall and then would run fine.

Now, the engine actually dies when it bogs during a WOT run and I can?t get it started again for 20-30 minutes. Then it starts right up like nothing happened.

I?m suspecting a fuel problem which may also be causing the engine to flood (which is why I can?t start it again). But I guess it could be a coil or electrical problem as well?

I'm going to replace the fuel filter, but wondering if it might sound like a bad coil or something electrical.

The engine is a new GM base motor with about 40 hours on it, but the carb and other bolt on parts are from the old engine. It has the thunderbolt ignition and the wires are new. The boat is an 88 Sea Ray Sundancer with twin 4.3l Mercurisers.

Thanks for any advice.

Re: Mercruiser 4.3 dies at WOT

I would be thinking fuel also. I think it is "starving" rather than flooding. I would think that if it were electrical it would shut off just as if you turned off the key. Just my $.02.. Best of luck...

Re: Mercruiser 4.3 dies at WOT

Was hoping to hear what you found. I have a 1988 4.3 liter Sea Ray Seville. Has been doing the same for years. No has gotton worse. Has spark and gas. Starts back up after a min then dies or bogs out.

Re: Mercruiser 4.3 dies at WOT

Sounds like a bad float level to me. Explains why it's only failing on high demand for fuel. If your not running it hard is the driveability fine?

Re: Mercruiser 4.3 dies at WOT

Similar symptoms to what I had last season in my '86 4.3 4 barrel. 1. Check your oil level (is it high does it sell like gas) 2. check your carb inlet fuel filter (are there little bits of unexplainable black debris). Mined end up being the mechanical fuel pump. The motor would start and run out fine until WOT then it was basically staving itself. I though it was the ignitin coil failing at high temps but in the end the fuel pump was the problem. If it ends upo being your fuel pump I suggest you change your oil and filter as the oil is likely diluted from the gas and therefore is not good from lubrication and if enough gas is present catistrophic failure is possible (ie BOOM!)

The other thing it could be is water in your fuel. A table spoon of water in your carb can create the same symtoms. If you you pull the top of your carb off you should be able to see if ther are any fouling issues there.

Re: Mercruiser 4.3 dies at WOT

Also make sure your choke plate is not shutting down at WOT. Not only would the boat shut down, but you will have flooded the engine. Might explain why after sitting it starts due to the rich condition already present in the combustion chamber. Bad choke springs can be a real pain and looked over many times.

Re: Mercruiser 4.3 dies at WOT

At times it runs great under load as well. It is an intermittent type problem. No sputtering just sudden loss of power and dies. The problem has just got worse over the years. I was thinking the float might be sticking as well. Once after it restarted at high RPM there was too much gas shooting into the carb. Also I just noticed the fuel filter in the fuel pump (stone) had never been cleaned or replaced. Still had original wire and factory paint. I also plan on checking the anti siphon valve. You help in this procedure is appreciated.

Re: Mercruiser 4.3 dies at WOT

had the same problem on mine and it was starving for fuel. turned out it was a blocked fuel filter. both the filter on the pump and the one right before it goes into the carb. you need a 1" wrench to get this off and be prepared to snap the line. i have twins and snapped the line on both because even while using a separate wrench on the smaller nut the line still won't break free. good luck.