1984 MONTAUK 90-HP Struggles to Accelerate Onto Plane

[MichiganMontauk]

Q1: should a 1984 MONTAUK 17 boat with a 1984 Mercury 90-HP in-line six-cylinder engine be able to accelerate onto plane easily with just two people aboard?

BACKSTORY

I recently bought 1984 Montauk 17 with a 1984 Mariner 90 two-stroke-power-cycle six-cylinder outboard engine.

After sunset on September 9, 2024 I was operating the 1984 MONTAUK 17 with just me and my wife aboard. The boat could not accelerate onto plane unless one of us went to the boat. Once the boat was on play the maximum boat speed was 29-MPH, which was achieved by careful trimming of the outboard engine. While on plane both my wife and I could sit at the console without causing the boat to come off plane. Besides my wife and I there nothing in the boat other than life jackets and a six-pack. Neither I or my wife are especially big people.

I was expecting there would be no concern with regard to being able to get the 1984 MONTAUK 17 with 1984 MARINER 90-HP, which by ear sounded like it was operating "okay."

[jimh]

Q1: should a 1984 MONTAUK 17 boat with a 1984 Mercury 90-HP in-line six-cylinder- engine be able to accelerate onto plane easily with just two people aboard?

There should be no problem for the MONTAUK 17 boat with just two normal-sized adults aboard to accelerate onto plane with a 90-HP engine. The Boston Whaler specifications for the classic 17-foot hull indicate that the minimum engine horsepower to reach plane in light-load conditions is only 35-HP. The maximum permitted power is 100-HP. With the classic Mercury in-line six-cylinder engine rated at 90-HP, accelerating onto plane with a light load of two normal size adults should be very fast and without any need to re-distribute weight in the boat.

If this cannot be done, there is probably some problem in the engine that limits its power output, or there is some problem with the propeller that is causing the thrust produced to be insufficient, or there is a problem in the hull with a huge increase in the hull weight due to retained water.

Be sure the engine is operating on all six cylinders.

Give the engine crankshaft rotation speed in RPM.

Give details of the propeller, including material, number of blades, diameter, pitch, and model designator.

[MichiganMontauk]

Thank you

I’m guessing the [Mercury 90-HP outboard engine] is the [cause of the poor performance of the boat in reaching planing speed].

I don’t hear the missing cylinders.

I will post a follow-up after I gather more information.

[Phil T]

With today's technology, you can provide so much more information.

Use your phone and take videos (1-3 minute) showing:

Engine cold start and idle.
Engine mounting position on the transom
Propeller size and condition.
Hard acceleration from no wake speed to 30 mph. (Engine trimmed in)

Post videos to Youtube, Google drive etc and drop links into this thread.

[jimh]

Regarding the ability to detect by ear that one cylinder of a six-cylinder engine is not operating: that is really essentially impossible. I will cite examples from my own experience involving six-cylinder engines.

At one point I owned a conventional loop-charged V6 outboard engine, and I discovered that one cylinder was often loosing spark. The actual way I discovered this was by monitoring the fuel economy (MPG) while operating the engine. When the one cylinder would lose spark, the fuel economy would decrease slightly, due to the fuel in that cylinder not being combusted and not producing any power output. Once I became aware of this change, I could see that the engine speed would also drop about 200-RPM. The tachometer for that engine was an analog dial, and 200-RPM was about the width of the dial pointer, so it was not very obvious if only casually glancing at the tachometer.

The ONLY reason I could find this intermittent loss of spark in one cylinder was because it was intermittent, that is, it would come and go randomly while the engine was running at a steady throttle setting. If the engine had always been running on five cylinders, I would have NEVER been able to detect that by just listening to the engine running.

In another situation, this time involving an automobile with a V6 engine, I was trying to find the cause of an intermittent no-start condition. In the process I found that the engine had been running on only five cylinders for a very long time, as the spark coil for one cylinder had a bad connection. Again, at no time during owning and driving this car did I ever detect by ear that it was running on only five cylinders. Once I replaced the bad spark coil the engine produced more power and had better fuel economy.

In another case involving an outboard engine, I did detect that the engine misfired, which for this engine (a modern, computer controlled, direct-fuel-injection ultra-low emission two-stroke-power-cycle 225-HP engine) had never occurred in the ten prior years of running the engine. The misfire existing for a very short duration, maybe a few seconds, and then the engine resumed normal operation. My initial thought was the fuel I had recently purchased at a marine fuel dock in a very remote location was the cause, but eventually--and only because the modern engine recorded in its engine history log the actual misfire event as being caused by a bad spark coil--the actual cause of the misfire was found to be related to loss of spark in one cylinder. And perhaps helping me to identify the misfire by ear was it occurred when the engine was being asked to accelerate under a heavy load.

The take-away from these three experiences is that unless the misfire is coming-and-going and NOT a continuous misfire, you might be able to detect it. But a multi-cylinder engine like a six-cylinder will be quite happy to run on five cylinders and not give you any obvious signal about that problem.

A good way to detect that loss of spark is the cause of a misfire is to insert an in-line spark gap tester in the high-voltage lead at the spark plug, and observe the presence of spark jumping the gap in the tester. This test is best done in subdued lighting, so don't try to do it noon on a brilliant sunlit day.

By using the spark gap tester as described, I was able to confirm the loss of spark in a cylinder. The tester cost was (at that time) about $12.

[jimh]

Another comment about your older Mercury six-cylinder in-line 90-HP engine and possible loss of spark: in that epoch of Mercury products their spark ignition system was rather (somewhat famously) prone to problems. I don't recall the details, but I believe that the excitation voltage for the spark coils was generated by two different permanent magnet alternators, referred to as "low-speed" and""high-speed" "switchboxes." The implication of this dual system is the engine might run perfectly at idle speeds (operating on the "low speed switchbox”) and run poorly at higher engine speeds (operating on the "high speed switchbox").

These in-line six-cylinder Mercury engines are still being restored and used, and you should be able to find websites where the entire focus is on the Mercury in-line six-cylinder engines and their repair.

You can also find articles in the archives of continuousWave.com about these engines by searching. Here are a few archived articles that discuss the Mercury in-line six-cylinder engine:

Mercury 115 In-line; No Spark
https://continuouswave.com/ubb/Forum3/HTML/020581.html
See comments about "switchboxes"

Older Mercury 90-HP
https://continuouswave.com/ubb/Forum1/HTML/002744.html
See comments about "I can't seem to get it running right."

16-footer, c.1970 Mercury 115-HP
https://continuouswave.com/ubb/Forum4/HTML/005413.html
See comments about "...they are also famous for having carb sync problems, distributor problems, and for burning down the top two cylinders from leaning out."

Regarding that last remark,you should measure the peak cylinder pressure of all six cylinder. The variation among the cylinders should not be more than about 10-PSI. A cylinder with damage to the piston, the piston rings, or the cylinder wall can be detected by this test.

For more advice about assessing a used outboard engine, see the REFERENCE article at

Assessing Used Outboard Engines
https://continuouswave.com/whaler/refer ... Check.html