High speed misfire on 150hp XR6

[rayhawk]

Hello, I am having a hard time figuring out what is wrong with my motor. I have a 1995 150hp Mercury XR6 motor on my 18' seacraft. Basically, I can no longer get it to go above about 3600 rpm consistently. I can plane off easily, and even get the motor up to close to 5000 rpm for about 10 seconds, then it will start to cut out until I leave it below 3600 rpm. I can cruise along no problem, but if I try to go above 3600 again, the motor starts to cut out.

I tried choking it while this was going on, thinking that it could be a lack of sufficient fuel, but that only slowed the engine down more. Not sure if that means it is definitely not a fuel problem, but that is how it seems to me.

Now to the ignition system. A little history first. I was having some issues with starting while hot, and at one point it was clearly running on one bank. I did not have any high speed issues whatsoever. It ran perfect at high speeds. Anyway, I tested the switch boxes per the Clymer manual. It seemed to be an intermittent problem with weak spark at low speed, so I replaced one switch box. Next time I went out, everything ran well, but I had the hot start issue again. So I replaced the other switch box.

The last time I went out, I still had the hot start issue (no spark at all), and a lovely, new problem, the high speed misfire mentioned above. So I decided to replace my ignition switch, thinking that may be the (intermittent) hot start issue. Unfortunately, the high speed misfire still remains. I believe I solved the hot start issue (although I need some more time to be sure).

So I am starting to suspect the stator has a problem. So today I checked the DVA for a sudden drop on the red, red/white, blue, blue/white stator leads per the CDI troubleshooting guide. The misfire came, and no drastic DVA drop. It did drop a bit, but that seemed to be due to the rpm drop. No really sure what they mean by a sharp drop though. I know there was good voltage, as it pegged my 250V scale, and I had to go to the 1000V scale on my meter. I can re-run the other stator tests again, although it was in range before.

I also disconnected the yellow rectifier/regulator leads, the misfire was still there. I replaced those about 8 years ago. The next step is the inductive tachometer connected to each spark plug lead to see which ones are dropping out, but just curious if I have enough information to replace the stator at this point.

Sorry for the long story, any idea what I should look at next? Thanks.

 

[guyjg]

The switchboxes are interconnected w/a bias circuit. When you installed the 1st switchbox it could have been damaged if the other switchbox was defective. Let's hope that did not happen. You stated it has a hot start issue. That could be related to the stator failing due to heat. Retest the stator and trigger coil cold and record the ohms for each. If the ohm readings are w/in range then you'll need to pull the flywheel to add heat to the stator to see if the ohm reading goes out of range. Use a hair dryer and give it the heat w/the ohmmeter attached so you can see any abrupt change. Heat will increase resistance in the stator a little but not out of range unless the stator is defective. You may just find some corroded wires under the flywheel.

 

[Docksidemarineservices]

http://issuu.com/cdielectronics/doc...t.xml&backgroundColor=000000&showFlipBtn=true

This will help.

 

[faztbullet]

Does it miss intermittently(bucks) or act like a rev limiter(stutters) when you get to this rpm and starts to cut as you say? What is the charging voltage when engine is running? Are you using the DVA adapter on your meter or just reading it with meter? DVA voltage is the converted(slowed down) reading from the adapter to your meter....From what you have posted it sounds like stator is going south.

 

[rayhawk]

Thanks for the replies guys. I am having to make a flywheel removal tool, the one I have doesn't match the bolt pattern. I will let you know what I find as soon as I get the flywheel off.

And it acts just like a rev limiter, as you said. I am using the ESI DVA adapter on my analog volt meter. The charging voltage is good while the engine is running, the gauge was showing about 15V. I did try disconnecting the stator yellow leads and still had the misfire.