I have a issue with a bike of mine. A NSR250 R SP

When I was driving suddenly the engine started to detonate. Luckely I was very close to home.

Now as I was inspecting the sparkplugs i saw that they were all wet and black. I checked the spark and it was weak an uneven.

I took away the ignitioncoils and checked the electric pase in the cables with a multimeter while kickstarting the bike.
Checked both front and rear cables.

The result was that the pase was uneven on both front and rear cables but it was slightly better on the rear cables.

What part can be wrong? Someone else has experienced the same problem?

Regards David
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Honda for Life