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Old 28 June 2015, 02:47   #1
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Engine problem

Hello
Iam using my boat which is a sib with a two stroke Yamaha twin on it .
It has started missing and running lumpy on the way back in yesterday . I would like any help on finding out what is wrong with it .
Regards chris
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Old 28 June 2015, 03:02   #2
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Well there are three areas I'd think of first... Fuel... Spark... Mechanical.

If you had a compression tester you could quickly eliminate any (very unlikely) issue there.

Very quick job to remove the spark plugs and see if they look OK... cheap to replace if there is any uncertainty over them.

But for me the most likely is a fuel issue of some kind so first I'd be checking the tank pickup, bulb and lines are fine then looking at the fuel filter and after that the carb/s for muck in the float bowl and/or jets.

After that probably consider a fuel pump issue.

And then if all the above have checked out I'd return to looking at the ign system considering the possibility of lead and/or coil pack/ign unit problems... rare in my experience though.
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Old 28 June 2015, 04:26   #3
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Spark plugs

Thanks for reply
I've took the plugs out cleaned them as well as I can their is a build of carbon on the one .
Will put them in and see if their is a good spark on them .
Does it hurt this engines running them on an hose and baffles ?
Regards Chris
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Old 28 June 2015, 04:36   #4
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If one had far more carbon than the other seems that cyl isn't firing properly.I'd fit new plugs as cleaning is not regarded as a good thing like it was 30yrs ago.

Loads of folks flush/run engines on a hose. I always use a tank for testing/tuning as... for example... idle speed and mixture should be set when in gear in water.

Is your engine single or twin carb?
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Old 28 June 2015, 05:31   #5
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Engine

Thanks fenland
I am going to replace plugs was trying to see if the problem would go away as I want to go out at 4 to do a bit of fishing .
The problem is still their . I've checked the carb out and cleaned it their was nothing init and it's a single carb model .
If a plug was breaking down that would leave carbon wouldn't it ?
Regards Chris
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Old 28 June 2015, 05:34   #6
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Yes... any thing that causes the cylinder not to fire will cause the carbon buildup... plug itself, its cap or lead and the coil pack for the cyl (if they are separate).
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Old 28 June 2015, 05:52   #7
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Misfire

Thanks fenlander
Are they all the issues with the cylinder or could it still be mechanical I.e barrel / rings perhaps ?
Regards Chris
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Old 28 June 2015, 05:57   #8
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Plugs

Hello ,
Can spark breakdown and misfire or do they just stop sparking all together
?
Cheers
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Old 28 June 2015, 06:38   #9
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Yes they can have intermittent breakdown... for example due to faults in plugs or leads that causes tracking... where the spark voltage "leaks" away to earth rather than sparking at the plug gap. Tracking can come and go according to how dry/moist conditions are.

Also ign coil packs can fail in a gradual or intermittent way.
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Old 28 June 2015, 06:41   #10
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>>> could it still be mechanical I.e barrel / rings perhaps ?

Personally I've never had any noticeable cylinder wear or any mechanical failure within an outboard engine which is why I'd be looking at fuel first then spark.

I mentioned cylinder compression as if you had a gauge it would be quick and easy to check.
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Old 28 June 2015, 07:24   #11
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Engine

Hi fenlander
Iam looking at the same sort of area as you think . I can't do a lot more everything seems fine . I will do a compression test when I get back do you know what it should be (psi) and I will change the plugs .
I like a workshop manual for this engine it's a 30mshu 1996 I have had a look and managed to find on .
Regards Chris
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Old 28 June 2015, 07:38   #12
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Thanks do you know what the compression should be and have you any idea were I could get a manual for it ?
Thanks
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Old 28 June 2015, 08:14   #13
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Just search online or Ebay for manual... someone else may have more idea than me where to look.

Similarly I do not know the figures for your outboard but I'd be guessing compressions around 110psi. If you have a firing problem on one cylinder then the comparison between the two readings will give away any issue with one cylinder.
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Old 28 June 2015, 08:54   #14
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Cheers

Hello fenlander
That's great I will do that . Have good day and what left of the weekend
Regards chris
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Old 28 June 2015, 10:14   #15
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No problem... I hope someone else will post later as I might not have thought of everything.

As a matter of interest I'm checking over and servicing the Mercury 10hp 2-stroke I bought recently... its compression test made an identical 9bar (130.5psi) on each cylinder which was pleasing.

The gauge's reading hold valve lets by a little but it was exactly 9bar when the engine was pulled over.
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Old 28 June 2015, 11:08   #16
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When you say lumpy, that sounds as if the engine was at times missing. You say you found carbon build-up on one plug. Clean both plugs, re-gap and try again. After a few hours if running well, take the plugs out and inspect. If there is a build-up of black..do a compression check -- three types "Dry, Wet, Hot"..you can look up the differences. In total, each test type should be no-more than 10-14 lbs different from the other. If one is and it rises during the wet test..Rings, Reeds or other is the problem. Before spending money on a pump kit do the above first; also look for burn-marking on the plug high-tension wires as that means there's arching to ground before current gets to the plug. Good Luck
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