Engine not getting enough rpm's

[TinLizzy]

This past weekend we took a trip out to Isle Royale. It is about a 75 mile run from dock to dock with around 55 miles of open Lake Superior to cross. The winds were a little brisk on the way there and it was very foggy, around 100 foot of visibility. The boat ran great on the run out there, cruising at 23 - 24 mph at around 2,700 to 2,800 rpm with a fair amount of tab. It took around 60 gallons of fuel when we got there. On the way home we ended up with a few more passengers as there boat had some major issues and had to be left there until repairs can be made. While it was mostly calm, my port engine would not run up the same way the starboard engine would. The engine sync was right on, but the port throttle was way ahead of the other one and the tach's read port 3,000, stb 3,400. When it is calm I usually try to run both engine around 3,000, which is in between 27-29 mph. I tried to give the port more throttle to see what was going on and it would not go over 3,100 rpm. The stb would spool up as usual. SO, this week I intend to take a ride to see if the problem persists, or if it was just not liking the extra weight. My initial guess is that I have a plug wire that is starting to go bad. I will also do a compression test while I am at it, but seeing that It idles great and was running perfect right up to that point I don't think that is an issue. I also plan to change out the plug wires with the chevy big block wires. Wish me luck

[barkleydave]

Additional passengers = more load especially if weight is not evenly distributed.

Take her out again without the extra passengers with same fuel and water load and see how it works.


Too many factors to even make a guess without looking at it.

Been up in that area several times.. BEAUTIFUL WATER! (2 months per year...LOL)
Burrrrrrrrrrrrrrrr


dave

[tundrarules]

Tinlizzy, you are right on the money! The plug wire or plug may have to get hot to start faulting. Take the boat for a run then pull one wire of at a time to see which cylinder is the problem. When you pull the one that's bad you will see no change in engine sound. Change that plug first, then change wire if the plug didn't fix it. Like you said... Maybe time to give the ole' girl a full tune up anyway.

[ericinga]

It's either spark, air or fuel. This sounds like spark.

If it were me, I would do the following:
- warm up the motor. Verify initial advance of 3-5 degrees at idle with a timing light.
- increase engine speed to 3k rpm and verify total advance of 23-25 degrees.
- If ignition is not advancing to 23-25 degrees, check your advance weights (or vacuum motor if vacuum advance).
- if ignition timing is good, check the plugs and wires. I do this by running the engine at idle and pull the wires off of the distributor one wire at a time. The problem wire will not cause a change in engine speed.

[TinLizzy]

So, after taking the boat out by myself, it ran great. No problems at all. I did change out the port engine spark plug wires with some Mercury wires for conventional ign. and they fit nicely. Then we left for another long trip, and about 20 miles out the port engine started to drop about 3-400 rpms, then back up to speed immediately when digging into the backside of a following sea wave, and get progressively worse. We shut off the engine, I replaced the coil and all seemed well, another 30 miles and the problem came back. So the next morning I investigated a little harder and noticed that the brand new plugs wires had a few cracks in them, the wire from the coil to distributor and #5 cyl. Replaced both wires with old good wires and also replaced the ign switch. Ran for 2 days and probably 90 miles, and no problems until almost back at port when I tried to open it up to full rpm's and it immediately started to miss and fart and pop until I got under 2,900 rpm, and then it would still do it at cruising speed after that, but not as bad. I know I still have to do a compression test, but now I am looking at a few possibilties. Either a bad tach or engine sync, or possibly a fuel issue. At this point I still don't think it is a fuel problem.

[jralbert]

This is a very close scenario to a fuel filter issue, the little CG-11 filter at the point where the fuel line enters the carb. When it gets clogged, it makes the engine capable of only a cranky 1800 or so rpm. The filter is cheap and easy to remove, tricky to replace (because the threads don't want to line up correctly).

[TinLizzy]

After taking it out tonite for a quick ride, and it ran great, I noticed that the wire off the back of the alternator was rubbing on the exhaust manifold, right down to bare wire. Hopefully that was the issue, but I am not confident yet that it was.