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Engine sputters but does not die under load

C

Cruisers Man

New member
Hi guys, I am new to the forum but not new to boating. I am need of some expert help with a problem I am having with a 5.0 carb motor. The boat is a 2001 Cruisers 30 footer with twin 5.0 liter mercruisers. The problem is this, when I come up on plane and am running WOT after a minute or so the starboard engine will sputter but not die so I trim the throttle back to 3400RPM and it will run fine. I have changed the fuel filters multiple times and replaced the spark plugs. Timing seems to be right, but could something be up with the spark advance? This problem has baffled myself and my mechanic, any suggestions would be greatly appreciated.
 
1 First, I would disconnect the tach lead at the coil, Grey wire. If the tack is faulty it can influence the ignition.

2 second, when this occurs if you activate the throttle lever several times very rapidly like pumping a gas peddle several times quickly,does this help?
Also do it to the good running motor for comparison, it should choke it out. If it does help the problem motor then it is a fuel problem. if so do step 3.

3 third, swap fuel lines and see if the issue goes to other motor. If it transfers then it is a fule supply problem.

4 fourth, swap ignition mudule from bad running motor and see if issue transfers to other motor,

Do any of these in any order execpt number one. do that first.

Remember you have two motors so what ever you suspect as the problem just swap it out on the other and see if the issue transfers........

To me it would seem that either it is a carb issue or a ignition issue (either timing control module or tach)

also listen carefully to the bad running motor, if it is having detonation issues/lean running it will reak havock with the timing because of the knock sensor, which should be just forward of the starter motor screwed into the block, which could be causing the timing to retard at wide open throttle.
(Not likely but possible)

Most wide open running issues are typically fuel related, supply or carb. sometimes ignition, also a dirty flame arrestor (lack of air)

also a broken choke/linkage allowing the butterfly to possibly close.........

slightly clogged gas tank vent line???

these are my suggestions...........
 
Last edited:
kghost said:
1 First, I would disconnect the tach lead at the coil, Grey wire. If the tack is faulty it can influence the ignition.

2 second, when this occurs if you activate the throttle lever several times very rapidly like pumping a gas peddle several times quickly,does this help?
Also do it to the good running motor for comparison, it should choke it out. If it does help the problem motor then it is a fuel problem. if so do step 3.

3 third, swap fuel lines and see if the issue goes to other motor. If it transfers then it is a fule supply problem.

4 fourth, swap ignition mudule from bad running motor and see if issue transfers to other motor,

Do any of these in any order execpt number one. do that first.

Remember you have two motors so what ever you suspect as the problem just swap it out on the other and see if the issue transfers........

To me it would seem that either it is a carb issue or a ignition issue (either timing control module or tach)

also listen carefully to the bad running motor, if it is having detonation issues/lean running it will reak havock with the timing because of the knock sensor, which should be just forward of the starter motor screwed into the block, which could be causing the timing to retard at wide open throttle.
(Not likely but possible)

Most wide open running issues are typically fuel related, supply or carb. sometimes ignition, also a dirty flame arrestor (lack of air)

also a broken choke/linkage allowing the butterfly to possibly close.........

slightly clogged gas tank vent line???

these are my suggestions...........
Click to expand...

Great ideas, I have checked the vent line, no problems. I have checked the fuel check valve, no problems. I did not know that the tach in the dash could cause problems, but by chance I am changing all the instruments in the dash this weekend. I really think it could be the knock sensor or in the timing somehow. When it is doing this it is only after a minute or so of WOT. Thanks I will start with your suggestions this weekend.
 
Replacing the tach is not trouble shooting

removing the entire wiring from the ignition is!!!! (grey wire)

only remove the grey wire from the coil, if it is not attached to the coil then it is attached somehwere in the wire harness near the coil/ignition control module. If this reveals no change then guage is most likely good!!

Your two control modules should be stacked on or near the disrtibutor. I believe the small one is the knock control and the large one is the timing control.
 
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