7.4 bravo one dies after several miles over 4000 rpms

[kschnebly]

My 7.4 mercruiser has left me needing a tow three trips in a row. The boat runs amazing as long as we cruise under 4000 rpms. We can go full throttle at takeoff and it runs perfect. If we cruise at over 4000 rpms it will bog down and die after a few miles. Only takes a few seconds between running great and dead in the water. Won't restart for 4-5 hours. Smells like gas. I've cleaned the distributor contacts, replaced the coil and checked all other connections. The boat was in storage for three seasons. I emptied fuel tank and ran lots of sea foam. Tonight I pulled the carb and fuel pump to check them out. I noticed I had some fuel in my crank case so I'm not sure if it's from it being flooded so much. Lots if cranking without it starting. I felt I should rebuild the carb anyways since it sat. Just looking for something I may be missing. It's a big boat and dealing with it dead in the water is a bummer.

 

[Pete104]

You should have ran on a remote fuel tank too see if it still happens before you disturbed anything! If it did, then check for spark. Chances are you've junk in the tank clogging the fuel system.

 

[alldodge]

had some fuel in my crank case

What is the engine serial number or what year is the boat?

Having fuel in the crank case and it won't start for 4 to 5 hours sounds like you flooding bad. The same fuel in the crankcase is a bad pump if your pump is mounted to the block

 

[kschnebly]

No Title

The fuel pump is a sea water pump and fuel pump combo that is run off of a belt drive. I do have some fuel in the sight tube so I am planning on replacing the fuel pump in case there is a diaphragm tear. When I pulled everything apart I had fuel in the lines all the way up to the carb. I don't think I have a fuel problem. My coil is getting really hot so I am still thinking its an ignition issue of some sort.

 

[alldodge]

SN 0D852083 is the same as my Rinker

Guess all the fuel that is getting in the crank case is from flooding and washing down the cylinder walls.

Are you seeing any fuel in the clear tube coming from the pump to the carb, or is it the carb just leaking?

Since you have the belt driven water/fuel pump combo, remember to change the gear lube in the housing. There is a link in my Sig for that type of pump maintenance

 

[NHGuy]

You do need to address the pump.
But also... Run in a circle near home til it quits. Then do the ignition checks, fuel checks.

 

[kschnebly]

I am not getting any leaking at the carb, I am getting some fuel/gear oil leaking from the pump and the site tube has some fuel in it. The tube is pretty yellow and full of gunk. So is the tube fitting on the fuel pump. I'm ordering a new one today and replacing the site tube with one that actually can be seen through.

 

[alldodge]

With the tube having fuel in it, then your diaphragm has a hole in it

Look for the Airtex 60932

 

[kschnebly]

I ordered the new fuel pump and a carb kit. Should get them in later this week. I thought about getting a new ignition sensor kit but figured I would get the fuel system worked out, change the oil and filter, and check the fuel water separator before I go changing out any more ignition components. Wish I would have read you response about the Airtex before ordering. I could have saved $60 bucks.

 

[kschnebly]

As an update I've replaced the fuel pump, rebuilt the carb, and installed an auxiliary blower that blows air on the thunderbolt ignition module. It runs when the compartment blower Runs. New seawater/fuel pump oil and also new engine oil and filter. I left it a half quart low so I can keep an eye on the crank case level better. Tomorrow I hit the lake and hope for the best. I'm bringing my voltage meter to trouble shoot if it does die.

 

[alldodge]

Sounds like a good plan, will be listening

 

[JaCrispy]

I had a similar issue with my 5.7 alpha one. Boat ran fine, at high RPM it would stall out. I replaced everything in the fuel system, rerouted the vent hose and rebuilt the carb. Still did it. Only thing I didn't replace, but kept replacing with the same part was the filter. I was using Napa filters (have business account) instead of Merc/Sierra filters. I bought a Sierra filter and kept it on board as a spare. Next trip out, ran the boat up and it stalled out like normal, cracked the filter loose and heard a 'hiss'. Replaced the Napa filter with the Sierra and haven't had a problem since. I've been around cars/motorcycles, boats for almost 30 years and this still baffles me. All I can think is the marine fuel system with the check valve may not be making friends with the Napa filter.

 

[kschnebly]

I had a similar issue with my 5.7 alpha one. Boat ran fine, at high RPM it would stall out. I replaced everything in the fuel system, rerouted the vent hose and rebuilt the carb. Still did it. Only thing I didn't replace, but kept replacing with the same part was the filter. I was using Napa filters (have business account) instead of Merc/Sierra filters. I bought a Sierra filter and kept it on board as a spare. Next trip out, ran the boat up and it stalled out like normal, cracked the filter loose and heard a 'hiss'. Replaced the Napa filter with the Sierra and haven't had a problem since. I've been around cars/motorcycles, boats for almost 30 years and this still baffles me. All I can think is the marine fuel system with the check valve may not be making friends with the Napa filter.

Thanks for the info on this. I am running a quicksilver filter on my fuel water separator but it has not been changed out. I have a replacement filter in the boat and will keep that in mind if and when it does this again. Sometimes you just want to understand why the heck these things do this but I gave up on that ages ago. Things that make sense on cars and bikes dont seem to apply to boats even though at there core they are not that different. Thanks again.

 

[kschnebly]

No Title

Heres the auxiliary fan that I installed to cool the ignition module as a precaution. Also a picture of some of the crud in the bottom of the carb. I've seen worse but that was in a motor that wouldn't run.

 

[JaCrispy]

Thanks for the info on this. I am running a quicksilver filter on my fuel water separator but it has not been changed out. I have a replacement filter in the boat and will keep that in mind if and when it does this again. Sometimes you just want to understand why the heck these things do this but I gave up on that ages ago. Things that make sense on cars and bikes dont seem to apply to boats even though at there core they are not that different. Thanks again.

When you said it wouldn't start up for 4/5 hours that's what made me think it was similar to my issue. My fuel system would generate a vacuum and until it equalized it wouldn't run.

Also make sure your tank vent hose has an upside down "U" and the breather/fitting on the side of the hull is clean. I took the hose right off the tank and blew through it.

 

[kschnebly]

Update, ran the boat on the water for several hours. I pushed it to 4500 rpm constant which was 35 on gps and didn't have any problems. Oil is clean and no signs of flooding. Only thing I noticed was it hard starts once the engin is warm. I'll need to adjust the pilot screws.