Mercruiser Throttle ? Increasing and decreasing

I have a 1994 5.0 Mercruiser ( 200 hp ) 2 bl carb with 485 hrs. The RPM?s will quickly increase and then decrease without touching the throttle. When the RPM increase and decrease so does the MPH. Since the increasing and decreasing is so quick I?m thinking it?s more electrical then a fuel issue. I changed the wires and cap. This didn?t solve the problem, but the boat starts much better.

Trashman83

Re: Mercruiser Throttle ? Increasing and decreasing

Trashman83 - Sounds more to me like a fuel issue - potential items might include (i) fuel filters need changed; (ii) getting air in the fuel line between the tank and the fuel pump; (iii) water/fuel separator filter (or water in fuel); (iv) fuel pump is going out.

Re: Mercruiser Throttle ? Increasing and decreasing

Dry gas and change two filters might be a good place to start.
Thanks for the advice

Re: Mercruiser Throttle ? Increasing and decreasing

Trashman83 - You might also try spraying some carb cleaner through the carb. Things can get gunked up in there over time and start sticking. I try to run Lucus gas treatment on a regular basis in my gas to help keep things clean.

Re: Mercruiser Throttle ? Increasing and decreasing

I'm having a similar problem with a Rochester 4brl carb, my rpm oscillation starts at 3200rpm WOT. At what RPM does your issue strart?

E

Re: Mercruiser Throttle ? Increasing and decreasing

It doesn't matter, low or high. You just notice it more at a high RPM becasue it jumps up 400 or 500 RPM really quick
trashman83

Re: Mercruiser Throttle ? Increasing and decreasing

trashman83 said:

It doesn't matter, low or high. You just notice it more at a high RPM becasue it jumps up 400 or 500 RPM really quick
trashman83

Click to expand...

This may or may not be relevant, but I had a similar issue many years ago with my first boat. This was an outboard, but the cause would apply to an I/O.

From the time I bought the old crate with her sixties vintage Evinrude, it ran on three of the four cylinders. And, as I was new to boats and to four cylinder engines, I didn't realize it until one day it began to quickly accelerate momentarily - off and on - as the fourth cylinder would occasionally fire for a few revolutions. Sounded different too at those moments.

I eventually figured out that I had one plug fouling almost all the time. Conversion to a different plug type and an electronic ignition cured it. It acted just as you describe here. Sudden accel and decel with no apparent cause.

If you don't have something going on with your fuel system (carb or injectors) I would look for a misfiring cylinder coming in and out.

Re: Mercruiser Throttle ? Increasing and decreasing

All - Thanks for all the help. I will post all results.

Re: Mercruiser Throttle ? Increasing and decreasing

I change the plugs today and my problem disappeared.

Thanks again to all who replied

Safe boating