Wonder if it is getting hot due to less water flow at low RPM, and vaporising fuel in the line? Has the impeller/thermostat been replaced lately?

Drain the water separator/filter onto a patch of concrete and you will see if there is water in the gas. Check the plugs but you will need to know what you're looking for. Besides the plugs on a modern 4 stroke should be well good after those few hours.

Hopefully you have a good local dealer who can figure it out if it's more that that

If that's is the case it is likely no fault code, as the engine doesn't have sensors on the leads, so can't detect anything wrong.

I would buy a spare lead and next time it does it (I would try and make it happen some where safe, like 2 minutes from the ramp or even better drum in backyard), then you can pull one lead if the problem gets worse then you know it's not that lead. If it doesn't then it's that lead and you replace it with spare and bingo engine fixed. Worse case scenario that doesn't work and you have a spare lead.

The possible issue is a leakage with a broken insulation. The conductivity/resistance of the leads will all test the same with a voltmeter - unless there is a permanently faulty connection - which would misfire all the time.

Alan

Um no. I didn't say that. Your original advise was to use a voltmeter. A multimeter will read ohms and volts plus a heap more stuff. An ohm meter will show discrepancies with the conducting part of the lead but won't account for insulation leakage voltage. Trust me, I'm a rocket doctor. Well ok I'm just a retired sparkie. While this may seem like pedantics to some, it would be a very bad thing to test a HV lead with a voltmeter for two reasons: you will get a nasty shock & your voltmeter will be buggered.

There are two parts in determining the effectiveness of an HV lead. The resistance and the insulation resistance.

Your approach/advice is incorrect.

Alan

  And whoever said about the crack on the lead opening up when its hot ...........lemmie tell ya , as a retired EST  , if there is a crack or pin hole in the HT lead temperature wont make a 5hit of difference.

How rude of her! It reminds me of the time(s) I showed my wife how to diagnose and replace a faulty coil on her Honda Accord. Apparently all the answers on facebook said it was 'something else'. 

Great question Kandrew

   Once you have a crack or pin hole that goes from the inner core to the outer surface  of the insulation ( wether its pinched closed or pulled open )  the insulation is completely perforated  and there is no dielectric strength, all you have left is air gap  , regardless of temp . We are talking 10's of thousands of volts   and if you provide an alternate path to earth via said perforation ......like put your finger on it  ..... it aint going to matter if its hot or cold . 

Its like a hose  with a pin hole thats closed over . If you put 10psi into that hose it may well not leak at all  but if you put 40,000 psi ( like 40,000v )  it will .

 I dont doubt for a second that this happened as you said , all Im sayin is that temp is pretty much irrelevant in this instance . 

Your engine is a couple of years younger than mine, and I didn't have the issue with it in idle. so may not be connected. You're just going to have to visit a dealer I'm afraid.

Joys of technology. Big call for younger people in industry as trhey have caught up on new technology.A lot of the older people still stuck in theirs ways. Just dont understand how new stuff works. Hence why boy is off to kiwirail to work on hybrids,old farts dont understands cambus! Even outboards have itr.

This applies to any circuit board  be it outboard , TV  or whiteware.

 Now days we are expected to cough up hundreds of dollars to replace a whole circuit board  when there may only be a 50c  part thats actually failing . By spraying  instant freeze  on the whole board the thing will often start working  and if so  wait till it heats up and fails  and then give each individual component  a wee squirt of instant freeze  and you can often identify either a faulty component or a dry joint , and that component can be replaced  ( using appropriate heat sink and static protection  )

kimber7wsm , sounds like your inviting me to call you a liar  , but I dont think you are ( and I have no wish to offend you either ). I didnt come here to win arguments   or be smarter than everyone else.........  so I dont mind if you think Im wrong here . Lets just get on with helping each other  where we can .

Hamish

My Banax 1000 elec reel failed last yr. It has done a lot of work, could not really complain. But I figured there had to be a reason. It would light up etc but not respond to the wind instruction.

Finally decided to rip it apart, nothing to loose. It had a sealed circuit board which I finally got to and tested various components. Bingo. Failed Mosfet. Once located, was a $10 part. Works great again. But it took some time to get there. So this stuff gets dumped for lack of ability to repair and cost of someones service time - prob in the order of $100+/hr? If I charged my hrs out to myself, would have been cheaper to buy another Banax.

Working on a mates 911 right now. High rpm miss. Fuel or spark. Not sure - going thru various tests. The answer will be there to be found. But it is the time it takes. Mine is free, for my mate.  I may, or may not, be slower than a certified garage (I know these things inside out), but you can clock some big bills up pretty quick at over $100 hr to find a fault in something..

Alan

My trusty VN holden (350+km) developed a misfire acouple of weeks ago. Ran pefect when cold, when up to temp had a miss, especially under load.

Turned out to be an HT problem. They are heat sensitve systems. Never threw a multi meter at the HT leads - waste of time - didn't fit the symptoms and likely to give a misleading result.

I am still racing cars. Not sure when that will end. Hopefully not with a big crash :-).

Regards

Alan 

 Like your Banax reel Alan  , if you have the nouse  there is so much that can be fixed  for whoever you want to help out . Case in point my 18hp Tohatsu .... got it for free coz it was rooted  according to the owner  . Im going to wind up with a better than new  ( every bush , bearing , bolt, seal , short block electrix and a proper paint job ) outboard for well under a grand . My stupid rich lawyer brother  paid way more that that just to get his boat serviced .

Done a few cam belts  and oil changes  for friends  , box of beer dozen eggs sort of thing   

But you are left with just a few basics to check. An HT lead breaking down under heat would be a pig to diagnose, and some hrs (ss clocked up).

I just ordered today a Yam 70 4S to replace one of my trusty reliable Merc 90 2 S. Reluctant to say goodbye to it - but at 2500 hrs, I need to do something. Never missed a days fishing with it. The 4S with a black box does worry me. I guess I will cope.

The 911 is sorted - coil.

Alan

Agree it is a punt - but I really don't use my 90 HP. I never really run over 3500. And I am doing as much knts as I would want the hull to run, and most times I can't run at that speed.

I think it will work out OK. Fingers crossed. It is the same (slightly less) weight as my 90. The next step up is another 40 kg.

But I think the point is, I would be putting 40 kg extra weight on for HP I wouldn' t be using.

I have to conceed - at this point I don't know, but at some point a few months ahead I will know if it is the right call. Being a smaller motor I am quite prepared to invest in a 2nd prop and change it when the load rating changes - eg a couple of mates coming over to fish for a few weeks. otherwise, it is 90% solo.

Alan

I bought a 90hp 2/ yam that I was assured by the dealer had less than 10 hrs on it , and I can remember thinking that was bugger all . I didnt have an hour meter  but I figured that at  20l/hr ( it was surprisingly consistent ) I would get 2 1/2 hrs from a litre of oil  and I reckon I put 12l of oil thru that motor . 12 times 2 1/2  is 30 ......30 hrs . That surprised me coz it felt like I was superglued to that boat ...... and I only managed 30 hrs  . Then I get you with your 2500 hrs   make me feel like a friggin amateur , but I now get a better handle .

 What sort of oil do you use ?  I always went for the TCW3 Valvoline outboard oil , and I love the smell of 2/ smoke  so I  was in nirvana there . 

But to round it off, I used to use the Merc oil. Then I came across Lubrimax (Aus I think). 1/2 the price. (The boat is in Vanuatu) Used it for the last 4-5 yrs. No issue. I have 2 boats like this - use one in the summer and one in the winter. So around 3-400 hrs per yr.

How much did I use? Dunno exactly.I have a 40 l fuel tank - would use 30-35 l most trips and went thru a 4 l bottle about every 4 tank refilss - eg about 120l. Sound right? Don't have any exact figures. Fishing costs money . Rather spend it on boat fuel etc than pee it up a wall in the pub. So I don't really count that stuff, and I don't go to the pub.

Alan

Put the engine on muffs and run it at idle for the critical period. Check the fuel bulb.

If you can, check the cyl temps. Prob need an IR gun. For cars I just feel the headers after a few seconds running. But you get the idea. Also suggested has been stray HT. You could chance your arm as suggested and run your hand over them. Also I find tracking stray HT works good in the dark. You can see it.

Then engage a gear and run the engine hoping it will misbehave - and check all again.

If it does start misfiring, pull the plugs..

If nothing shows up, then it is likely to need a diagnostic plug in.

Good luck

Alan

 Yeah , sorry about that 

Sent you a PM - hadn't answered all your Qs,

Alan