9.8 Merc Won't go to high speed

[John B.]

I've got an an older 9.8 hp Mercury 110 (about a 1967/68). Last year, the motor started giving me problems. 1st the fuel lines corroded. I couldn't find original parts so I found a couple of brass adaptors and the right size fuel line at an auto parts store. Then, it would not go into high speed, the best it would do was just putter in spite of how much I turned the throttle handle. I rebuilt the fuel pump but that had no effect. Both cylinders are sparking so ignition does not seem to be a problem. I was concerned that maybe I reduced the cross sectional area of the fuel line with the fittings I used but there does not seem to be a major reduction of area. I also checked engine compression, I get about 100 psi dry (without oil or fuel in the cylinders). I don't know if that is good or not, I can't find spec on compression for this engine.......I'm stumped!!!!

 

[Motor Boater Bill]

Re: 9.8 Merc Won't go to high speed

If compression is 100 and even (within 10% of each other) that's not likely your problem. Coils can break down at higher speed in spite of spark at idle, so don't rule that out. Try squeezing the pimer bulb to see if that helps--if so, this points to fuel delivery. Have you torn down the carb? Could be a float issue.

 

[John B.]

Re: 9.8 Merc Won't go to high speed

If compression is 100 and even (within 10% of each other) that's not likely your problem. Coils can break down at higher speed in spite of spark at idle, so don't rule that out. Try squeezing the pimer bulb to see if that helps--if so, this points to fuel delivery. Have you torn down the carb? Could be a float issue.

 

[John B.]

Re: 9.8 Merc Won't go to high speed

Thanks Bill!
I've had the carb apart 2 or 3 times and cleaned it thoroughly. How do I know if the float is not performing?

Also, how do I test the coils to see if they are performing at higher speeds? Or is this simply a visual inspection to see if the coil insulation is intact?

Thanks again!

 

[Motor Boater Bill]

Re: 9.8 Merc Won't go to high speed

Let's go step by step. First, what happens if you squeeze the primer bulb? If it helps, suspect the fuel pump. If it makes it worse, suspect the float. No effect, we'll go on to the next step.

When it won't go fast, is it uneven, or just smooth like still idling? Visually observe the linkage to make sure the carb throttle butterfly is opening completely, and the magneto is turning freely.

How do the plugs look after you have tried to run it wide open? Black and wet would suggest a float problem.