Yamaha C55, 2cyl, 1993 Double firing

[daselbee]

Hi guys, I have a 1993 C55 2 cyl 2 stroke that appears to be double firing on #1 only. I have come to that conclusion because the tach on my timing light reads double on #1 vs. #2. Engine bucks and pops and will not idle. I have replaced the CDI unit, and have exactly the same results. I have pulled the flywheel, and swapped the two pulser coils, same results. I am now wondering about the trigger coil itself. I cannot see that as the problem, because (I think) it only generates voltage, and the actual pulse to fire the pack comes from the two opposing pulser coils. Very funny thing...when cranking at starter speed, NOT RUNNING, both cylinders read about 500 RPM on my timing light tach. This engine only double fires #1 when it is running. The minute it starts up, tach reads twice the RPM on #1 vs #2. Anyone experience double firing on this style motor, and what was the cause? The more I think about this, it makes me want to examine the actual wiring for frays, shorts, etc. I will do that today. But, can this trigger coil cause double firing? And, is it possible I am way off and it is not double firing? The fact my tach reads double tells me it is double firing. Anyway, thanks for any input.

 

[99yam40]

Re: Yamaha C55, 2cyl, 1993 Double firing

My thoughts are you have carb problems from the sound of things, lean sneeze.

some motors use waist spark ignition systems,which fire on each stroke.
Exhaust stroke spark does not hurt anything
but this would not explain why both plugs are not doing the same thing

 

[daselbee]

Re: Yamaha C55, 2cyl, 1993 Double firing

I pulled the magneto plate off fully assembled with all the coils, harness, etc. I found the white/red wire from a pulser coil (leads directly to the pack input) and it is #1, by the way, found that wire all frayed and touching the hub of the flywheel. AAHA!!! Fixed the wire, re-started...same thing. I studied the magnets on the flywheel. One is different from the others. It is split down the middle, top to bottom, with a 90 degree "zee" in the split. It is made that way. Looks like a vertical line heading down, cuts 90*, and heads horizontal, then turns 90* down again ending on the other side of the magnet. Hard to describe. Anyway, that has got to be the mag that drives the trigger coil, since it apparently has some N/S pole shift nonsense going on with that split. All DVA volts look good. I have no way to "see" the trigger inputs to the pack. I wish I could look at them with a scope or something. No scope, though.

Been thru the carbs...they were a mess, but I will double check the idle circuits for clogs again. I also thought about the waste spark, and maybe it was supposed to do this, but that would make running a tach iffy at best. Unless you KNEW....you would be chasing tail on that double rpm. And #2 is what I would call normal RPM reading.

The bucking shuts the engine off, and it is violent...not a mild lean sneeze. I usually work with the steering link disconnected so I can swivel the motor easily, and this bucking will jerk the motor out of my grip when it happens.

Guy brought this engine to me, said it needed an impeller, that's all. Yes, uh huh, impeller was GONE, gearcase had fishing line in the propshaft seal, R/R the carbs..total disaster inside....finally get ready to start....primer bulb would not get firm...turned out to be a torn fuel pump diaphragm leaking into #2 CC, finally ready to start, and I have this ignition thing going on. What a P.O.S.

 

[daselbee]

Re: Yamaha C55, 2cyl, 1993 Double firing

Nobody else have any ideas? Two specific questions: 1. Is this a waste spark system? 2. Can a bad trigger coil cause double firing? Yamaha pros on here should know the answer to these. Thanks. On a new note, I have noticed a bit of rotational play in the magneto plate (that's what they call it), when engine is stopped. As you rotate the flywheel, the mag plate kinda moves/rotates back and forth about 3/8 of an inch with the pull of the magnets on the coils. You can see it move against the stop, and then away from the stop as the flywheel turns. I have tightened this up, and will try starting the engine again this morning. Will post the results, but I can't really see as this will do anything.