CONTINUOUSWAVE

My 1968 16-footer is powered by a 1996 Johnson 88SPL that runs beautifully for ten miles, or so, say upriver to the fishing hole. Then on the way back, the 88SPL engine begins to bog down. The engine won't accelerate to normal wide-open-throttle of 5000-RPM, but is down to 4000-RPM. [During] acceleration from a standing start [the engine] stutters. Idle [is poor].

The water pump [overboard indicator stream is] cool.

I have rebuilt carburetors and installed a new fuel pump.I have switched out fuel lines, tanks, opened cap to vent.

I'm think something electrical must be heating up, like a [spark] coil or PowerPack. The 88SPL just runs too well for the first 30-minutes or so.

Any experience with this?

I know I might reach a bigger audience on an OMC forum, but I know many of you guys have had these engines, so I'm trying here first.

On my 1992 Evinrude V6 engine, I had problems related to the engine losing engine speed that I eventually diagnosed as being caused by the PowerPack becoming intermittent in supplying spark to one cylinder. Because the engine was a six-cylinder, the loss of spark in one cylinder caused the engine speed to decrease by about 200 to 300-RPM, depending on throttle setting. This was a rather subtle change in engine behavior. I was actually able to more definitely detect the loss of spark by monitoring the instantaneous fuel economy displayed on a fuel management accessory instrument I had installed. When one cylinder lost spark, the fuel economy was significantly affected, and this signalled clearly that something was wrong.

Since the condition exhibited mostly when the engine had warmed to operating temperature, the condition would persist for a while after the engine was shut down. I was able to remove the cowling and attach an in-line spark gap test device to individual cylinders, one at a time. This allowed me to find the cylinder that was loosing spark.

Once I identified the cause of the engine behavior was loss of spark and identified the individual cylinder, I began further testing to isolate the cause. My test method approached the problem by using careful inspection or complete replacement of particular components in the spark circuit, based on the cost of replacement, starting with the least expensive components and progressing to more expensive. As a result, I inspected and in some cases replaced the following components in this order:

1. the spark plug wire leads
2. the spark plug
3. the spark coil connections, particularly the ground connection
4. the spark coil
5. and when none of those measures gave a remedy, finally the PowerPack

Because the PowerPack was expensive, about $300, I was reluctant to replace it until I had become reasonably sure the other components were not the cause.

The correlation between engine temperature (warmed to operating temperature) and onset of the problem (loss of spark) was also a good indicator that perhaps an assembly of electrical components such as the PowerPack was failing once its operating temperature rose. The problem was never manifested when the engine was cold; it only occured when the engine came to operating temperature and had a chance to heat soak. This suggested that the PowerPack assembly was warmed to the same elevated temperatures and was then failing. For older electronic devices to fail when their operating temperature rises is a common behavior, perhaps particularly for OMC electrical components of the 1990's.

In my experience with OMC engines, the behavior of an engine having stuttering under hard acceleration is often a symptom of excess spark gap in the spark plugs; or, perhaps low spark voltage. For a lengthy discussion on the effect of spark plug gap on engine behavior, see this archived thread:

OMC V6 Spark Plug Gap
http://continuouswave.com/ubb/Forum3/HTML/013926.html

Poor engine behavior at idle would be hard to notice on a six-cylinder engine running on only five. Even a three-cylinder two-stroke-power-cycle outboard engine seems quite happy to run on only two of three cylinders at idle.

I once experienced problems with a three-cylinder outboard engine that would not accelerate to normal speeds at wide-open-throttle. The initial diagnostic procedure was a close visual inspection of all components of the engine spark system. This inspection led to the immediate discovery of the cause: a spark plug wire had become disconnected from the spark plug electrode due to the rubber in the spark plug boot becoming stiff. The remedy was to reattach the spark plug wire and boot to the spark plug, and use a nylon cable tie to increase the pressure of the boot onto the plug porcelain insulator.

For a more detailed description of my experience in resolving the spark loss problem, see my long discussion at the time of the diagnosis:

The 2001 Space Odyssey Dilemma
http://continuouswave.com/ubb/Forum3/HTML/011635.html

Heat soak is what I had in mind. Thanks very much for the detailed information and links.

As luck would have it, I was able to pull the coils AND power pack off a friend's 110 today, for testing. After reading your story, maybe it will be successful. Will report back.

I suppose replacing plugs and wires would be a smart move either way, and I'll pay attention to the gap.

The manual gives a different plug for sustained low or high speed use. I chose the high speed plug before. Let me know if any readers have settled on a plug for the OMC V4 engine. Many thanks.

On my c.1992 V6 Evinrude 225 engine, I just used the spark plugs recommended by the factory, and I used the exact brand and model number they recommended, not some equivalent product made in by another manufacturer.