sputter at high speed

[John O.]

hello all, i have a 91' force 50hp 508F91C. I have a problem with the motor sputtering at high speed. new plugs, coils, timing is set, linkage is good. carb has been removed and cleaned, new fuel lines, new primer bulb. If this has happened to anyone out there, please explain what it took to fix the problem.

 

[pnwboat]

Re: sputter at high speed

If you haven't already done so, do a compression check.

You can verify if the fuel pump is working the way it should by squeezing the primer bulb to see if that temporarily fixes the problem. If it does, then you probably have a fuel supply problem. Things to check would be fuel pump diaphragm, plugged fuel filter, make sure the gas tank vent is open.

If squeezing the primer bulb has no affect, check for loose bolts or air leaks around the carburetor adapter plate, make sure the gasket is in good shape. Sometimes the bolts holding the adapter plate to the block work loose and cause an air leak. Make sure the by-pass port gaskets aren't leaking, and the 4 bolts that hold them in place are tight. Check for any vacuum leaks on any of the hoses for the fuel recirculation system, and going to the fuel pump itself.

 

[John O.]

Re: sputter at high speed

If you haven't already done so, do a compression check.

You can verify if the fuel pump is working the way it should by squeezing the primer bulb to see if that temporarily fixes the problem. If it does, then you probably have a fuel supply problem. Things to check would be fuel pump diaphragm, plugged fuel filter, make sure the gas tank vent is open.

If squeezing the primer bulb has no affect, check for loose bolts or air leaks around the carburetor adapter plate, make sure the gasket is in good shape. Sometimes the bolts holding the adapter plate to the block work loose and cause an air leak. Make sure the by-pass port gaskets aren't leaking, and the 4 bolts that hold them in place are tight. Check for any vacuum leaks on any of the hoses for the fuel recirculation system, and going to the fuel pump itself.

that was my next question. If I squeeze the primer bulb while im sputtering at high speed, would that force fuel past the diaphragm. if it speeds up when I do that, would that prove to be a fuel pump problem.

 

[pnwboat]

Re: sputter at high speed

The fuel pump would be the first thing to check if squeezing the primer bulb gets rid of the sputter. Could also be some crud partially clogging the pickup in the tank or some other restriction too.

 

[John O.]

Re: sputter at high speed

thanks for your help. i think we will have it out this weekend and I will see what happens.

 

[John O.]

Re: sputter at high speed

Ok, I had the boat out this weekend and it was still sputtering at high speed. I tried squeezing the primer ball to test if it might be the fuel pump and it made no difference. Crazy thing is when I went the trailer it, because the ramp is pretty shallow, I had the gas it to get it on, and I had no sputter. could it be that my motor should be sitting lower in the water. I notice that when turned off and just floating the prop is probably no more than 6-8 inches in the water. i am probably grasping at straws but it is a thought. thank you in advance for your thoughts on this matter.

 

[pnwboat]

Re: sputter at high speed

The motor should be mounted so that the cavitation plate (red arrow) is pretty much even with the bottom of the boat.



Do you have a tach to see what your RPM's are?

 

[John O.]

Re: sputter at high speed

i do not have a tach. that and other guages are are on my list. i have heard the plate need to be even with the bottom of the boat, does that mean even with the bottom of the logs on a pontoon. thanks again for your help.

 

[pnwboat]

Re: sputter at high speed

I don't know about a pontoon. I would guess that at full speed, the cavitation plate should be just barely be out of the water. At least that's the general idea on most boats. I may be wrong but maybe someone else can add their 2?. Most boats with outboards, when sitting still in the water, the cavitation plate is well below the surface of the water. Maybe it is mounted too high??

Most pontoons generally are more of a load on the motor, so it's even more critical to be sure the motor is running in the right RPM range. Prop selection is important. But before you start messing around with the prop, you need to know what the WOT RPMs on the motor are.