Dropping a cylinder intermittently

[Curious]

I've got a 1996 Yamaha 200 SW(carb)motor.  Runs great most of the time but recently at wide open it started to intermittently cut out as if it was dropping a cylinder.  I cruised around today with no problems, got it up to about 3400 rpms and it ran great at all rpms.  Did a little tubing with the kids and under the load of pulling the tube the engine started to cut out again intermittently.  After that it would bog down a little when I initially throttled up then it would come up to speed then start cutting out again.  I had a similar problem last year that I thought I had tracked down to a loose plug wire but I checked them all today and they're tight. Can you guys give me a short list of where I should start to troubleshoot this?
Thanks, Dan

[fitz73222]

The low hanging fruit as they say would be a spark plug or coil. Can you determine which cylinder is misfiring? We'll go from there..

[GoneFission]

I'm with Fitz on this one - sounds like a coil problem.

[flounderpounder225]

also, the spark plug wire BOOTS have a set resistance in them, they are not permanently part of the wire, they screw off.  If you can use an Ohm meter, you should remove them one at a time, ohm the boot from the "screw" you will see in the wire end, to the clip that attaches to the tip of the plug, it should read about 4-7 Ohms, if its much higher than that, it is creating too much resistance in the secondary ignition circuit.

[GoneFission]

also, the spark plug wire BOOTS have a set resistance in them, they are not permanently part of the wire, they screw off.  If you can use an Ohm meter, you should remove them one at a time, ohm the boot from the "screw" you will see in the wire end, to the clip that attaches to the tip of the plug, it should read about 4-7 Ohms, if its much higher than that, it is creating too much resistance in the secondary ignition circuit.

Is it 4-7 ohms or 4K-7K ohms?    I've wondered why Yamaha did not go with solid wires/boots and spec a resistor plug?     Seems like that would be simpler and less chance for problems...  

[Curious]

Great thanks guys, I'll let you know.  What is the best way to determine which cylinder it is?  Take each wire off while it is running and listen for it to drop?  How do I do this without energizing myself each time, pliers with rubbah handles?
Thanks, Dan

[fitz73222]

I use plastic fuse puller pliers and they work fine for plug wires. I don't remember if your engine has coil packs or individual coils per each cylinder. One test you can do and alot less dangerous is try running the engine at night on a flush and pull each plug wire off and see if you can observe the coil trying to short to ground through an insulation crack. It will light up like christmas tree. Also pull the spark plugs and make sure they are clean and gapped. Make sure there is no water, aluminum or overfueling issues. If everything looks good, do a resistance test on the coils to see any high ohms readings. Is your fuel in good condition and fuel filter clean?

[Curious]

FP, the boots are fine, they all tested between 4.8 and 5, thanks for the tip.  The plugs appear to be fine and all six have really good, strong spark.  

Fitz, I ran it tonight and took each plug wire off, (it was definately not running on all six).  The top two on the port side had a little, weak spark jumping from the plug wire to motor.  The rest were fine.  I'm pretty sure it was too weak to make a huge difference, but maybe not.  To answer your question, there is a coil for each cylinder.  How do I do a resistance test on the coils?  Are the plug wires part of the coil or are they removable?  They appear to be part of the coil and not just plugged in.

I'm starting to wonder if it could be fuel related....For what it's worth, I replaced the tank and all hoses this winter, the fuel is new and treated, and the filter is new.  
Thanks again, Dan

[Curious]

Took apart cleaned, reassembled and reinstalled all three carbs today.  They had some little chunks of the ethanol goop from sitting but not too bad.  Started it up and it seems to pretty much run the same.  So I'm back to thinking ignition again.  Went out tonight when it was really dark and took a closer look while it was running.  When the engine is running the plugs glow.  Three were glowing steady and the other three were "flashing".  A couple were flashing a little and one, the bottom starboard side, was really bad.  At times it would stop glowing altogether for a second or so.  I also observed several weak arcs along a couple of the plug wires.

My plan is to start with replacing all the plugs even though they are brand new last year with only a few hours on them.  Then I guess I need to think about replacing the coils.  Should I just replace all of them (at about sixty bucks apiece) or just the ones that are suspect?  Also, are the plug wires able to be removed from the coil and replaced or are they permanent?  
Thanks, Dan

[dburr]

Dan there should be no stray ark from any of the plug wires or boots to any ground....  If you have arking then the spark at the gap in the plug will not be hot enough for good combustion and you will have a rough running motor..

Some wires are removable from the coils, they can be unscrewed from the female receptical of the coil after you pull the boot back, I am not sure what have, but if the boots are not sealed you may be lucky and can change them..

Good luck!

[Curious]

Turns out it was a bad coil.  It was intermittent at first but then dropped altogether. Top starbord side cylinder.  Haven't had a chance to test run it yet.  Hopefully this will be the end of my gremlins for a while.  Thanks again for all the advice.
Dan

[dburr]

    Good deal!

Now it's time for chasing some Blues!!!

[Curious]

Yeah Dave, blues everywhere.  Caught a bonito the other day too.  Check out the Ocearch shark tracker-  I was thinking of taking a ride today off Great Point towing a seal decoy behind the boat.