Yamaha 90 2 Stroke No Spark


Lieutenant Commander
Aug 22, 2013
Thanks. I only did the CDI readings because I thought I might as well, wasn't sure about it anyway. The main point was the other readings, all those coils.

I had a guy rebuild the carbs for me a few years ago, he's been a Yamaha approved repair agent for 30+ years so he knows his stuff. I assumed it could be faulty engine stop switch or key switch. I have confirmed 100% it's not the engine stop switch. I mentioned that I wondered if a faulty key switch could be to blame but he said no, if the engine cranks (which it does) then that switch shouldn't be causing no spark. I wonder if he's wrong about that? I hope not, he seems to know his stuff.

He said I would need to take it to him for proper diagnosis, but he's a long way from me and booked up for weeks/months :(

I was hoping someone might be able to confirm some of my coil readings were good or bad, and lead me in some direction towards something to try replacing!
A swith can fail is one of several different ways. The starter portion may work while the kill switch portion may have failed.

If the kill switch circuit (white wire) has chafed to ground anywhere along its path from the kill switch/key switch to the CDI, the CDI will think that the kill switch circuit has been activated. Disconnect the white wire to the CDI. See what happens.


Fleet Admiral
Mar 10, 2016
A 90 HP Yamaha two stroke does not have a TPS. Throttle Position Sensor.
Good info. Only mentioned it because I’m aware some larger Yamaha (models older than this 90 even) with a cdi, have a TPS on them.


Aug 13, 2023
I took readings a few days ago using Boscoe's chart. I will show them below.

Today I took readings again. And again the charge coil resistance tests showed odd results. 22,000 ohms. Yet the engine did spark.

I connected fuel, ran engine for 10 minutes, no failures. Totally stumped, i checked resistance again on charge coil and got 74 ohms which is almost bang on what the Yam service manual says it should be.

I have no clue where to go from here. I have it booked in with a Yam agent but really can't afford to pay someone if they will only repeat what I have already done and come to the same conclusion, i.e. none!!
I daren't take the boat out but if i can't replicate the fault I guess one day I will have to pluck up the courage. No more boat for a while, sorry kids!

I'd love any more ideas but I don't think there are any. My most recent readings (before today's) are below just because I may as well! Thanks to everyone who chimed in to help.