Help troubleshooting weak/no spark

My son's 2003 XR70 was running a little rough at the end of last year, backfiring on deceleration and needing idle speed adjusted to keep it from racing as it warmed up. But it always started easily.

This year we had a lot of trouble getting it going and when it does start, it will start with the choke on, won't run at all with the choke off, dies after about 3 minutes and then won't restart for another 20 minutes, then repeats the routine.

I checked the spark and it is so weak I can rarely see it. There are times I swear it had no spark at all, but then it starts and runs for a few minutes, so it must have some.

When I check the alternator with a multimeter set to AC volts, I get around 200V peaks at the black/red wire going to the ignition module. When I check the pulse generator I get around 1V peaks on the blue/yellow at the ignition module. Both of these tests are with the wires disconnected at the ignition module and match what the manual says I should find.

The next test in the manual is to check the voltage at the primary of the coil with the wires connected. Here I only get a fraction of a volt, when I should get over 100V. If I disconnect the wires at the coil, I get 100+V. What I can't figure out is do I have a bad coil that's loading down the ignition module, or a bad ignition module that can't drive the coil? I don't have anyone near by that I can borrow one from to swap and check and both parts are expensive enough that I don't want to guess. Plus I don't what to have to wait for parts twice.

The primary side of the ignition coil measures around 0.2 ohms with the meter properly zeroed.

So, any thoughts on how to figure out which is bad?


I would just replace the cdi and try again they go bad all the time and you can get them for 5 bucks just about anywhere

Try unplugging the ignition switch. I had mine go haywire and cause similar symptoms.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now