Outboard engine ignition problem diagnosis

[JerryHawkins]

My totally-reliable-to-date Tohatsu 8HP 2-stroke (about 2 years old) suddenly stopped over the weekend and I couldn't re-start it. There seems to be plenty of fuel getting through but no (or at least very weak) spark. Can anyone give me any ideas on how to diagnose if the problem is with the magneto or the CDI pack? If not then I'll replace the megneto first (@ about £50) before the CDI pack (@ about £120)!

Cheers,

Jerry

 

[William_H]

Yes it is a real pain that a dealer can try a component and if it doesn't fix the problem he puts it back in stock while us poor DIY have to buy the replacement just to try it.

If you have a Digital multimeter you should be able to measure several thousand ohm from the plug lead to the engine block. This test will check the plug lead and the high voltage coil. Leads do give trouble but this test is not really applicable if it is a twin cylinder engine as you would hope only one lead or coil would fail so giving a miss on one cylinder.
My experience has been with Johnson 6 of 20 plus years old where the CDI pack does fail from time to time. It is only diagnosed or fixed by replacement.

In the case of the Johnson and I imagine the Tohatsu is somewhat similar there are 2 coils under the flywheel which are energised as the magnet passes them. The first generates a voltage of 100 volts plus which is rectified and applied to a capacitor (for storage) the second coil generates less voltage which fires a Silicon controlled rectifier (SCR) a kind of trnasistor when it reaches enough voltage with the passage of the magnet. The SCR switches the charge of the storage capacitor out to the coil which transforms this rapid rise of voltage to a very high spark voltae of some kilovolts. In a twin cylinder there are 2 SCRs in the CDI pack and a coil for each plug.

This explanation may or may not help you decide what to buy to fix the problem. But I have never seen a coil fail on Johnson.
good luck olewill

 

[JerryHawkins]

Hi Olewill,

Thanks for your message. Hopefully it isn't the CDI pack; as you say since it is a 2-cylinder unit, and I assume that the CDI pack is pretty much two separate coil units (one for each plug) that it would be very unlikely that both would fail at the same time - no miss-firing or one-cylinder firing, just total stop ( as if I'd pulled the kill cord). I did check the kill switch by the way!

I've yet to start taking the thing apart, but it looks like I'll go for the magneto, unless something more obvious turns up as I unbolt things!

Cheers,

Jerry

 

[lexi]

Just be careful with your diagnosis as some engines can lose the spark on both cylinders when one fails.

Alex

 

[VicS]

[ QUOTE ]
and I assume that the CDI pack is pretty much two separate coil units

[/ QUOTE ] I'm not familiar with your engine but what i would expect to find is the CDI unit or "powerpack" and two coil units. The coil units may even be combined into one. They/it will easily be identified by the fact that the plug leads go to them/it.

You are unlikely to find anything wrong under the flywheel, the most likely thing to have failed is the powerpack/cdi unit. They do just pack up in the way you describe (got the teeshirt!) and the only way you can check them is by substitution.

Do double check all the wiring and connectors though, including the stop switch.

 

[captainslarty]

I would check the wiring harness first, have a good look at the bullet connectors and check for any chafe in a wire to the engine block. I have a workshop manual for the toh 8 , will dig it out tomorrow if you like...

 

[JerryHawkins]

That would be great - where did you get the workshop manual from. I'd like to get hold of a copy myself!

I'm hoping that when I start taking the thing apart, a lose connection or chafed wire comes to light. If not, then its is a case of test-by-replacement; hence the magneto first since it is about 1/3 price of the CDI pack.

Cheers,

Jerry