Engine runs up and down

[Berninger349]

I have a 90 HP (1993) Mariner.

Since the boat went in the water the engine has been running irregular. That is the rpms are up or down varying 200 rpm. In one minute it will change rpms back and forth several times without touching the throttle. But the motor never stalls and always starts. Some think I have water in fuel but I put stabil in last Fall and by now I've added 10 gallons of new fuel. I've drained the water/fuel separator as well. What do you think?

Steven

 

[CharlieB]

Re: Engine runs up and down

Generally, water in the fuel will clog a jet, the bead/droplet of water has much more surface tension than fuel and will not pass thru, blocking off fuel flow and killing that cyl so engine speed will not vary, if it runs at all, it will be a bit slower than normal.

Intermittant spark on one cyll will vary engine speed. Finding and proving the cause may take a bit of testing as there is NO RETURN on electrical parts and you DO NOT want to start changing expensive parts that are not the problem.


As with any motor problem, test the basics

Compression test

Spark test each cyl with an adjustable gap tester set to 7/16, looking for a hot blue snap, anything less is cause for further testing to determine why it is not hot enough. I suspect that you are going to find that one cyl is firing intermittently or has a weak spark. If so then download the free CDI Ignition Troubleshooting Guide from the links found at Rapair.com Follow the directions for testing you system to determine the problem.

Once you have a CONSISTANTLY hot spark on all cyl's, then ground all spark plug wires, remove then plugs, connect a timing light and remote starter button, set the throttle to WOT and check ignition timing. Adjust only if necessary following the factory service manual.

Retest EVERYTHING again to ensure all's well (It's great practice and then you KNOW it's right), then time for sea trials.