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FishonFabrications

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PostPosted: 02 Oct 2016, 22:18 

Joined: 13 Jun 2012, 14:13
Posts: 42
My 86 25HP two stroke Yamaha seems low on power, I pulled plugs when I got back to the dock, One plug was back(top plug, no bad little dark) the bottom plug was white whit a touch of bronzing. I'll post pics tomorrow as it is dark now. Its like the top cylinder is not firing all the time and when it does it is fouled. Is there any good way to check this out with out going to a shop?


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PostPosted: 03 Oct 2016, 08:20 

Joined: 13 Nov 2014, 08:01
Posts: 737
You can buy a inline spark tester for under $30 at auto parts store.. That should confirm the failure.


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PostPosted: 03 Oct 2016, 13:30 

Joined: 13 Jun 2012, 14:13
Posts: 42
I'll do that, I did not know if anything other than a the ignition could do that? I do not know much about the ignition of boat motors, is their separate mags or coils?


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PostPosted: 03 Oct 2016, 19:06 

Joined: 13 Nov 2014, 08:01
Posts: 737
Newer engines have a CDI system, have no idea if your is points or CDI.


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PostPosted: 04 Oct 2016, 11:56 

Joined: 13 Jun 2012, 14:13
Posts: 42
It is CDI Ignition.


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PostPosted: 04 Oct 2016, 15:40 

Joined: 13 Jun 2012, 14:13
Posts: 42
This is what the plugs look like, the black one if from the top cylinder. How do you check the coil and see if it is good?


20161004_123017.jpg
20161004_123003.jpg
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PostPosted: 04 Oct 2016, 19:20 

Joined: 24 Dec 2011, 22:06
Posts: 977
Location: 72032
Does your motor have 2 carburetors or one?


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PostPosted: 05 Oct 2016, 11:38 

Joined: 13 Jun 2012, 14:13
Posts: 42
It has just one carb.


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PostPosted: 05 Oct 2016, 19:21 

Joined: 24 Dec 2011, 22:06
Posts: 977
Location: 72032
If it's got spark on both cylinders, the coil is ok-usually. Failed coils on Yamahas are pretty rare in my experience.
Single carb, so that rules out one carb being plugged.
Could be a reed valve issue. Or air leak. Could be air leak at the lower crank seal. I ain't been into one of those in a while but some had a plate that is bolted to the block which retains the seal-and if an object was struck hard enough, the plate could break, then causing air leak-showing up as a lean lower cylinder. Check the engine compression.


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PostPosted: 06 Oct 2016, 01:03 

Joined: 13 Jun 2012, 14:13
Posts: 42
I'll take a look at the compression tomorrow. The motor pulls like it has good compression. Seems to run good for a bit then missses, or stumbles, like a ignition problem. Visually the whole thing looks good. The engine has not struck anything since I have had it. Only has about e or four hours on a tune up. I beieve a compression Check was done then, and it was good. Was told the motor was in good shape by the mechanic


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PostPosted: 06 Oct 2016, 18:32 

Joined: 13 Jun 2012, 14:13
Posts: 42
Both cylinders were about 140 PSI. I did the test dry and with the other plug out


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PostPosted: 07 Oct 2016, 00:43 

Joined: 20 Jul 2014, 22:11
Posts: 88
Location: Oregon
Any way of using a timing light to check the spark while it's running? Does it run with one cylinder pulled?


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PostPosted: 07 Oct 2016, 01:37 

Joined: 13 Jun 2012, 14:13
Posts: 42
I sure can, where do I chack the timing. I see there are marks on the flyheel, nextra to where the rod is that adusts the timing. I have not tried pulling a wire, was going to do that. We are in a good rain storm so I have not messed with it too much.


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