Weak Spark on WR400?


Purchased a 2000 WR400 last year and rode it a few times before getting sidetracked by work and other mundane life necessities!

Pulled her out of the garage last month and could not start her up for love nor money (I offered £20 but it was a flat refusal).

She went down to my local bike garage and they noticed the spark was weak, we have replaced the ignition coil pack but this doesn't seem to have solved the problem. All the usuals have been checked, valves, fuel through the system etc and these all seem fine.

Just wondered if anyone had a suggestion to try next...summer is coming and some decent holiday time!



How did they test for a weak spark?


The spark should jump a gap of at 8mm at atmospheric pressure

Just looking at it is not a good test


Close the plug gap down to about 0.2mm and see if it starts - this is a good test for a weak spark (will not run good but will start)


I presume you have tried a new plug, fresh petrol etc


If it has been left sitting esp if the petrol was left on, then the pilot jet might be blocked

Unleaded petrol evaporates and leaves behind a varnish which blocks the very small pilot jet

Don't bother trying to clean it if it is blocked, just buy a new one

Edited by GuyGraham

Thanks for the reply GuyGraham.

Mechanic has gone through all the fuel side and everything is ok on that side of things, pilot jet is clear. Has adjusted plug gap. Tried new plug, fresh fuel etc.

He thinks is might be CDI or generator...is this a likely cause?

Thanks again


Find a new mechanic!


He sounds like a 'guesser' which is going to cost you.





CDI's tend to either work or not work with no inbetween, so if its sparking then its OK


Test the stator for its resistance values, but again if you have got a spark then its more than likely OK

When they start to fail, you get a misfire when its warm which gradually gets worse, or a complete failure


I'm guessing its fuel related then


Have you tried the 0.2mm plug gap trick?

Because it is a 2000 (manual compression release)  make certain the compression lease is not partially engaging.


 On my '99 one day after having started AND rode it!...... I went to re-start it and it was dead. I kicked in it till I was blue in the face, checked for gas, spark, and was at a loss ESPECIALLY as it had just run around the block great! I discovered I had somehow bumped and turned the compression release lever on the bar up a little bit, and that was all it took. The cable is tight enough that by slightly rotating the lever it cannot be started.


 I tend to leave the levers and compression release so they can be rotated with a little effort....old habit that I learned while racing, cause that way if you take a spill sometimes they will rotate instead of breaking a lever or perch. 

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