'98 Merc 115; Backs off at high speed

[Warncorp]

I have a 1998 Mercury 115. When running on plane at nearly full throtle (4,800 to 5,000 rpm) the rpm's suddenly go down about 500rpm's for 1-2 seconds, then it goes back up again. This began a few months ago and would hsppen only once in awhile. Now it is happening every 3-4 minutes while running on plane. The motor idles fine.
What should I look for?

 

[CharlieB]

Re: '98 Merc 115; Backs off at high speed

You need to determine if it is caused by the fuel system or ignition system.

An inductive timing light may show a lack of flash if the spark is dropping ona cyl, you will have to repeat the test on each cyl. Rig the light, run the boat til it happens and watch the flashing, change to the next cyl, repeat. If any failure then start testing igniton per directions in the CDIElectronics Ignitioin Troubleshooting Guide.

A failing fuel system, fuel pump dying, air leaking/sucking into the hoses between the pump and tank may cause iintermittant problems. Disconect the fuel line to the carbs, point it into a suitable container, crank the motor and observe the fuel stream, it should be a strong pulsed flow the diameter of the inside of the hose, pulsing out a couple of inches from the hose.

Temporarily install a clear hose between the puymp and carbs, run the motor and watch for any air bubbles passing thru, indicating air sucking into the system.

 

[Texasmark]

Re: '98 Merc 115; Backs off at high speed

Had the same engine and the same problem, but mine did it at 6000 rpm on a 17' Ranger fisherman with a 21P ported Quicksilver SS prop. Back off to 5000 and it ran fine; goose it and in a couple of minutes it just started shutting down. I had the oil premix tank configuration which they used before they quit making the tower when oil injection became popular in other engines.

I literally ran the engine out of gas. I tried an electric fuel pump but that was just a mess with the plumbing the way it had to be for the oil tank. I even thought about plumbing direct, scrapping the tank and just add oil to the gas when I filled up.

Solution came in rerouting the fuel line, making runs as short as possible with minimum bends and those I had to have were smooth and slow. Solved the problem.

HTH,

Mark

 

[Warncorp]

Re: '98 Merc 115; Backs off at high speed

Thanks to both responders. I've gotten similar advice over the last couple if days. I may wind up taking it to a shop because I'm leary of hanging off the back of my boat at 30mph with a stobe light. I do need to re-look at the fuel system first. I was also told that it could the high speed relay or even the stator because the problem occurs more frequently as the engine heats up. Any thoughts on this? Thanks again for taking the time to answer and for your excellant suggestions. Bryan

 

[Chris1956]

Re: '98 Merc 115; Backs off at high speed

First test the lack of fuel theory. Have someone pump the primer bulb while at WOT. If she stays at RPM, you have a restriction in the fuel line (filter, air vent or antisipon valve), or a weal fuel pump. Both are easily fixable, and much more common than loss of spark on one cylinder at high speed.