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yamaha mark

weak orange spark

6 posts in this topic

hey how we all doing?

on my last outing in the bush my yz400f really gave me some hassles.riding it could almost be compared to riding a wild bull. hesitation, erratic throttle response and it felt like at any given time it was gonna throw me off bad! it was actually very scary to ride.when i got it home i found that the orange wire from the coil had been melted by rubbing on the exhaust ,so i have replaced the whole wire.i have found that i have a very weak orange spark now.i have tested the stator its within specs, the tps is good, i have checked for continuity throughout the whole wiring harness its good,put in a new cr8iex spark plug,and have had the carb apart more times that i really care to count.is there anyone who can help please its getting to be very frustrating.:smashpc:

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The coil is usually a good place to start.

If you're up for it, and have a source of used parts, coils from a Hayabusa (coil over plug, or "stick coils") can be used as an upgrade. Be sure to ask for a connector from the harness to go with it.

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thanks greyracer i will look into it.also today i put it all back together(started first kick as usual) and its idling poorly even with idle screw wound all the way in.any clues?

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The 400 and 426 had marginal ignitions at very low speed, and if the coil is weak, that could have some effect.

But the carb is where most of the trouble usually is. One thing that's always worth a look with these is the vacuum release plate on the engine side of the slide. It's easy to install upside down because it looks as if it should go that way. The square edge goes down, not the rounded edge. The plate is also prone to cracking or breaking after it gets worn, and may be missing a piece.

Beyond that, make sure the idle mixture adjusts in a normal manner, and track down the cause if it won't.

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thanks again greyracer you really know your stuff.i will deffinetly look at that like i said b4 have had the carb apart many many times.maybe i overlooked that.i am also sending the carb away for ultrasonic cleaning that'll make sure is is truly clean and going to rejet at the same time and may even weight the flywheel so i will run better at lower rpm in low gears.

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o.k here we go.i have stripped the carb once more and it was spotless and the vacuum plate was in fact installed correctly.it turns out that the diaphragm in the accelerator pump was well past its use by date,had a split in it.so thanks again greyracer for pointing me in the right direction once again. :smashpc:

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