1996 Mercury 200 Problem

Hi all,

I am looking for some advice regarding an issue I have with my 1996 EFI Mercury 200 outboard. I currently run a 19" High 5 prop. I have also previously used a 19" and a 21" laser 2.

Growing up, my parents had a 1990 Mercury 200 non EFI on a very similar boat, so I consider myself reasonably familiar with how the motor should perform.

The motor starts and idles well. Acceleration is very good and as I believe it should be. Top end is strong and rev's to approx. 6500rpm (tacho only goes to 6000). The motor will also run at approx. 2000 and 4000 rpm with no issue.

My problem is at approx. 3000rpm. The motor with hold a reasonably constant rpm, however, every 4 to 5 seconds the boat momentarily slows with the sound of the motor temporarily reducing. A car mechanic who happened to be in the boat on the weekend commented that it felt/sounded like 1 of the cylinders was either switching off or receiving incorrect or poor fuel mixture.

This problem has been since I have had the boat, approx. 4 months.
I have just had the boat serviced. Spark plugs were checked and are in new condition, water trap filter, EFI fuel filter and oil all replaced. Motor was compression tested with cylinders all similar and in good condition. He was unable to locate anything that could cause my problem.

The mechanic also tested the coils packs and said they were putting out the correct voltages and operating as they should.

Many thanks,
Andrew
Melbourne
Australia

Thanks for the reply kimcrwbr1.

The guy who serviced the motor said the plugs looked in good condition. I can drive along at 2000 revs with no problem, accelerate up to 3000 and problem occurs, then increase speed to 4000 revs and the problem will go away.

I'm thinking the engine temp would not change that quickly but I will certainly pay attention to it this weekend.

Please excuse my ignorance, but what is the impeller you mention?

Running lean is a possibility, but what would cause this to occur, and if this is the case, would increasing RPM increase the problem.

I also forgot to mention, the unit that attaches to the plugs was replaced not long before I got the boat.

The mechanic thought it may be the computer, however, he spoke to a fellow Mercury dealer mechanic who said it is very rare for there to be a computer problem.

I'm running a small prop as we are skiing behind the boat and are looking for a very strong pull and good constant slow speeds. It's very rare for me to go over approx 4500rpm.

I'm using it in fresh water only, although I have not idea of the motor history. I'm thinking it has seen some salt at some stage as there are small signs of corrosion on the boat and motor.

I have not checked the thermostat and poppet valve. I generally keep an eye on the temp gauge. It always sits below half. Would a faulty thermostat or poppet valve cause potential overheating that may result in running lean?

Serial number OC110281