Engine Trouble

[A1an]

Ok gents, I'm at my wit's end with this one. Honda 15hp with an intermittent fault. Every electrical component falls well within spec resistance wise. Carb has been swapped for another but its still the same. The previous owner took it to an outboard shop and they said it's an uneconomical repair but no other comments other than that.
It runs as sweet as a nut for an hour then for no reason it starts missing badly, if you can leave it running badly it flicks back to perfect like you hit a switch.
I'm pretty confident when it comes to the oily bits but I'm pegged out with this one.
I'd rather diagnose the issue properly than start throwing parts at it.

A video speaks a thousand words.

I know, it's not pumping water, it's only a few seconds and impeller is due a change.

https://youtu.be/oIBaWVS8_lI

[Fenlander]

Do you think it's random missing or one cylinder dropping out?

[A1an]

Random missing. Tbh it's like it's firing on every eighth stroke. Timing light backs that theory.

[Fenlander]

Do you mean the timing light clipped onto a plug lead shows the loss of spark? If so do you see this on both cyl leads?

If so and the electrical side is indicated I've seen this before with coils and other ign components suffering heat soak then playing up at random... but when tested cold/static their readings check out OK.

[A1an]

The engine will run perfect from cold one morning then play up the next. All ignition component, ignition/pulser/exciter coil all check out whether the engine is hot/cold or running perfect or playing up.
Ive even had a Megger on coil to see if they are going to earth under load but still no further forward.
It looks like a switching issue with the way it just flicks back to running perfect. afaik the only "switched" items in the electrical system are the pulser and CDI.
There is a similar running engine on ebay at the moment, im tempted to buy it and use its parts as a test to see where the fault lies then punt it on after.
I don't think ive ever been this stumped before.

[jeffstevens763@g]

HT leads breaking down ?

[jambo]

Originally Posted by jeffstevens763@g

HT leads breaking down ?

That could be a good call Jeff!

[Allye]

Having spent a few years now testing automotive sensors with resistance checks and other basics checks, I’d say sometimes they may appear ok but are dead. I’m not suggesting fire the parts cannon but don’t rule out a sensor or switch because a basic test suggests it’s ok at that parameter you happen to do the test at.

[A1an]

Originally Posted by jeffstevens763@g

HT leads breaking down ?

Changed coil from a Honda 20 I have in at the moment. Still the same.

[jeffstevens763@g]

just looked at the vid fuel starved then it kicks in had a suzuki like that fuel pump knackered would run ok then die. does it run if you prime the bulb?, spark plugs new?, all fuel lines ok.is there a coarse filter inline [steel mesh] before the plastic one.
you've probably tried all these but just incase.

[A1an]

Not fuel related. When it's running rough I've given it a toot of easy start and it doesn't make a difference.

[Last Tango]

Not familiar with that engine...

Gear shift interlock switch ?......Is the CDI properly grounded. (actually grounded not just to what ever it's mounted on).

[A1an]

Originally Posted by Last Tango

Not familiar with that engine...

Gear shift interlock switch ?......Is the CDI properly grounded. (actually grounded not just to what ever it's mounted on).

Shift interlock is cable operated not electrical.

CDI module is grounded via a common ground. Earth connection has been removed and cleaned and I'm getting continuity to all other ign components.

[Locozodiac]

Is it a 2 or 4 stroke motor ? A barrel test while the motor runs at neutral without load says nothing, powering a boat under load says it all. An impeller change won't fix the problem unless having an intermitent overheating condition which an impeller change should cure.

Besides Ohm resistance testing, have you checked the voltage output of each electrical component when the rope is pulled ? that test will say much more than a simple Ohm resistance test. Will need the Service Manual to check those specs and a DVA tester. A standard one won't read correctly an extremely short duration DVA volts...

If the motor runs OK , misses, cools down, runs great again to repeat the whole failing process seems an intermitent related issue, any of the electrical components could heat up/short/miss, cooles down, works fine. This type of electrical failure is the most complicated to troubleshoot right and fast, unless having spare electrical components to swap and test indviidually till the culprit is found.

Like the Ohm and voltage electrical troubleshooting procedure, otherwise will end being an automat parts changer. Anyway, have a full box of new spare individaul electrical components to swap to test my 3 horses...

Happy Boating

[Allye]

Originally Posted by Locozodiac

Is it a 2 or 4 stroke motor?

One of those classic silver Honda 2 strokes

[A1an]

Originally Posted by Locozodiac

Is it a 2 or 4 stroke motor ? A barrel test while the motor runs at neutral without load says nothing, powering a boat under load says it all. An impeller change won't fix the problem unless having an intermitent overheating condition which an impeller change should cure.

Besides Ohm resistance testing, have you checked the voltage output of each electrical component when the rope is pulled ? that test will say much more than a simple Ohm resistance test. Will need the Service Manual to check those specs and a DVA tester. A standard one won't read correctly an extremely short duration DVA volts...

If the motor runs OK , misses, cools down, runs great again to repeat the whole failing process seems an intermitent related issue, any of the electrical components could heat up/short/miss, cooles down, works fine. This type of electrical failure is the most complicated to troubleshoot right and fast, unless having spare electrical components to swap and test indviidually till the culprit is found.

Like the Ohm and voltage electrical troubleshooting procedure, otherwise will end being an automat parts changer. Anyway, have a full box of new spare individaul electrical components to swap to test my 3 horses...

Happy Boating

Its a four stroke, I don’t know of a Honda two stroke outboard.

I am aware an impeller change won’t sort an electrical issue.

It has been tested on the Orkney Longliner it belongs to.

I haven’t tested output voltage yet, I’m waiting on a DVA adaptor for my multimeter.

Issue appears under hot or cold conditions.

I have purchased an identical engine tonight, I will swap parts from it until I find the culprit then sell it on again. I don’t want to start buying individual parts and be stuck with them when they don’t sort the issue.

[Callum B]

We had an old johnson 2 stroke, which ran a bit intermittently, and as Jeff said, it turned out to be dodgy HT leads.

The other possibility may be the engine management system shutting the engine down to " safe" mode, if it thinks there is a fault..... so possibly an intermittently faulty sensor?? ( sump oil level / or thermostat perhaps)

Just my 2 cents worth.

[neil.mccrirrick]

If all the electrics pan out I'd be looking at a sticking valve, when it starts missing if you are able I'd pull a plug cap off to try and narrow down which cylinder is dropping off. Or stop the engine when missing and see if you can tell from the plugs which one is oiling up.

If you can suss the cylinder you can isolate parts.

[Maximus]

Originally Posted by jeffstevens763@g

HT leads breaking down ?

x3
Can cause all sorts of intermittent and interesting symptoms ...You changed those yet Alan??

[A1an]

Originally Posted by Maximus

x3
Can cause all sorts of intermittent and interesting symptoms ...You changed those yet Alan??

Yep, done that. When I changed the ign coil for a known working one it had leads on it.