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Author: Subject: No spark on an old Honda outboard engine
computid

posted on 3/6/24 at 04:44 PM Reply With Quote
No spark on an old Honda outboard engine

I thought I'd ask the LCB font of knowledge before I start pulling my hair out!

I've got a Honda BF75 7.5hp outboard powerhead in my boat attached to a Volvo Saildrive (if you don't know what that is it isn't really relevant).

The Honda engine is a little two cylinder four stroke from the mid-80's. It uses points ignition. Now, I haven't touched a set of points in 16ish years and TBH I could just do with some advice so I'm not stabbing in the dark and wildly parts swapping.

Last weekend on Sunday the engine started and ran great - no issues. Even started twice. All fine and dandy. On Monday it wouldn't start at all. Didn't even try. Ended up figuring out that it has no spark.

Tracing the wiring, I'm a little confused by how its setup. There is an inline fuse in the ignition circuit that goes somewhere which I found to be blown. I replaced this but it made no difference and hasn't blown since.


- I've checked the resistance values of the coil and all seem within spec.
- I've replaced the plugs.
- I've checked the continuity of the wiring as best I can.
- I've cleaned (with some emery paper) and checked the points and found the gap to be far too small (though there is weirdly no specified points gap for this engine, I googled and found a forum post mentioning 0.3-0.4mm) so I adjusted it within spec.
- I've taken off the flywheel, cleaned any rust off of the magnets, and made sure the wiring to the exciter coil & pickup coil (I think that's what they are) is in place.
- The engine stop switch works by shorting the coil to ground. I've made sure this switch works, disconnected this switch, and ensured the coil isn't shorted to ground.

After this, I'm a bit stumped. I'm slightly confused by what should be happening, which probably isn't helping. Am I right in thinking that there should be no continuity between the two sides of the points until near the top of the ignition stroke? The points are located under, and driven off of, the cam shaft sprocket, which means there is no separate distributor to adjust the position of. It also means they can only be accessed at that point in the engines stroke.

Basically, any advice for diagnosing no spark issues with points ignition systems would be appreciated. I haven't yet measured the resistance values of the coils under the flywheel which I probably should do. I've ordered new points & condenser just in case, but these look like a real pain to fit. Very open to ideas!

Thanks in advance!

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ReMan

posted on 3/6/24 at 09:24 PM Reply With Quote
In brief the points shout be closed, short circuit if you like, until the cam, reaches the firing point just before the piston TDC, when the points open and it causes the magic to make the spark.
The fact that you had it working so recently at least means that the magnets and coils in the flywheel should still be good and the issue is more likely to be points and possibly condenser else something else odd......





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JAG

posted on 4/6/24 at 10:35 AM Reply With Quote
So a points ignition system is complicated to look at but the genral principle is simple enough.

On a boat engine I think it will be a 'total-loss' system. No battery to power the coil basically. The IGNITION coil is driven by the magnets and a second coil that sit inside/behind the flywheel. This is where the power is generated.

This power is fed to the IGNITION coil, via the points. When the points are closed the IGNITION coil is fed with the power and creates a large magnetic field.

When the points OPEN the coil is dis-connected and the collapsing magnetic field induces a high voltage spark in the coil's second set of windings and that flows from the coil to the spark plug and set's fire to the fuel.

The Condenser is there to reduce the damage that happens to the points when they open and close whilst 'live' - whether the Condenser is good or bad the engine will run.

Timing is managed via a cam (the centre post of the distributor) but your boat engine won't have that. I guess it runs a 'wasted spark' system where both plugs fire at the same time - but only one cylinder will be on the compression stroke at once.

[Edited on 4/6/24 by JAG]





Justin


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Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!

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JAG

posted on 7/6/24 at 08:24 AM Reply With Quote
As the engine was running and has now ceased - I'd be checking that everything is clean, dry and connected.

Once you're sure that it's clean and dry I'd check the points gap and the plug gap (although it's difficult to imagine these changed overnight and stopped the engine).

Then I'd follow all the wires and make certain you have connectivity and that all the joints are 'good'





Justin


Who is this super hero? Sarge? ...No.
Rosemary, the telephone operator? ...No.
Penry, the mild-mannered janitor? ...Could be!

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ReMan

posted on 7/6/24 at 03:32 PM Reply With Quote
thinking out loud, if it as stood for years then its possible that the ignition cam is dry and rusty and in running it, it has worn away the fibre heel of the points, making them be permanently closed and hence no spark now





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