75HP Mariner no power under load

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    Posted: 09 Sep 2018 at 9:08pm
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I have had intermittent power issues over past 12 months in my 1994 2 stroke. A similar issue a couple years back ended up being a corroded HT lead. Three trips ago I could not go more than 3300 revs. Enough to Go about 20mph. Changed other two leads and all new plugs. Checked all sparks OK then would not go more than 3000revs not enough to get on plane at 12mph. Next I have stripped all three carbs cleaned out jets re assembled test run today no change. Same on both underfloor and tote tank. Starts well. Idles well. Any other ideas?
Why choose either diving or fishing when you can do both. Besides crayfish tail is very good bait!!
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Post Options Post Options   Likes (0) Likes(0)   Quote Grasshoppa Quote  Post ReplyReply Direct Link To This Post Posted: 09 Sep 2018 at 9:35pm
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Have you checked compression and this is healthy?
Fuel lines/connectors in good condition with no leaks?

Sounds like your dropping a cylinder, given your previous issue and Mariner/Merc engines being fairly consistent with ignition issues i'd check this out

I assume your testing sparking while cranking engine?
If so you need to test spark with engine under load, this can be accomplished at the boat ramp, ideally you'd use a test prop that loads motor without generating drive but if your careful you can back boat in with std prop so that water intake is just under the surface with engine lowered and some of prop out of water, the idea is to have enough prop in water to create some load so motor not free-rev but enough prop out of water that it ventilates and dosent load transom excessively
You'll need a timing light on respective cylinder verifying you have spark while a capable/trusted colleague operates throttle with motor in gear, go thru all cylinders until hopefully one of them stops sparking

While your there you could check engines high speed timing althou this dosent go "out of tune" unless tampered with, or you shear a flywheel key but i think that motor actually has a splined crank/flywheel making this unlikely

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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 09 Sep 2018 at 9:36pm
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Is it firing on all cylinders? You may need to do a compression check. What is the motor?
Best gurnard fisherman in my street
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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 09 Sep 2018 at 9:44pm
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Ooops 75 Mariner hehe
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Post Options Post Options   Likes (0) Likes(0)   Quote CrayZfish Quote  Post ReplyReply Direct Link To This Post Posted: 10 Sep 2018 at 10:11am
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Originally posted by Grasshoppa Grasshoppa wrote:

Have you checked compression and this is healthy?
Fuel lines/connectors in good condition with no leaks?

Sounds like your dropping a cylinder, given your previous issue and Mariner/Merc engines being fairly consistent with ignition issues i'd check this out

I assume your testing sparking while cranking engine?
If so you need to test spark with engine under load, this can be accomplished at the boat ramp, ideally you'd use a test prop that loads motor without generating drive but if your careful you can back boat in with std prop so that water intake is just under the surface with engine lowered and some of prop out of water, the idea is to have enough prop in water to create some load so motor not free-rev but enough prop out of water that it ventilates and dosent load transom excessively
You'll need a timing light on respective cylinder verifying you have spark while a capable/trusted colleague operates throttle with motor in gear, go thru all cylinders until hopefully one of them stops sparking
 
 
Yes it feels like its electrical again, i had thought testing at home was enough, did not think about spark changing under load.
I havn't actually checked compression, so will get that done.
Assuming now one of the cylinders is not firing under load, what is the likely cause/fix??
 
Thanks for the help?
Why choose either diving or fishing when you can do both. Besides crayfish tail is very good bait!!
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Post Options Post Options   Likes (0) Likes(0)   Quote Tagit Quote  Post ReplyReply Direct Link To This Post Posted: 10 Sep 2018 at 10:12am
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Prop is in good condition? Compression test is a basic one to prove that you aren't wasting time chasing something that can't be fixed without a teardown. If compressions are good then you need to determine fuel or ignition. 
If one carb is bad you could be running on only 2.5 cylinders under load and hence not making the power. Carbs can be hard to clean without untrasound and/or compressed air etc. A bad carb could lean out and wreck a cylinder. If you could run the engine under load on flat water with the cowling off, you could spray a little fuel or ether etc into each carb throat to see if it will pick up noticeably with extra fuel into one particular carb. Just do that enough to prove it. Don't do more than you need to as there is no oil in that fuel. 
The other possibilities could be the fuel supply due to a bad pump or pinholed fuel line letting air in or a leak around the filter or a blocked tank pickup or a blocked tank vent. Generally though I would expect any of those to cause a miss rather than just low rpms. A small air leak might just cause low rpms though. 
Ignition problems I would expect to generally cause a miss rather than just low rpms, but might be hard to detect on just one cylinder. You could do the drop test at a high idle by removing each plug lead in turn to see if one cylinder drops the rpms less than the others. This can also be a very rough guide to compressions, or at least a pointer where to check first. The other thing that might need checking is your ignition advance to make sure it is working properly. Also check that your battery is getting the right charging voltages as a basic check of the stator etc.
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Post Options Post Options   Likes (0) Likes(0)   Quote MacSkipper Quote  Post ReplyReply Direct Link To This Post Posted: 10 Sep 2018 at 5:57pm
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My 1998 Merc 115 was dropping a pot when cold ok warm - One suggestion re spark is to run engine on ear muffs at night with cowling off - you may see arcing around an HT lead power pack?  My mechanic suggested this when no problems seen he came around with a "spark testor" which is basically plugged in between plug and lead and has a gap for spark to jump that can be observed - should be nice fat blue spark - one power pack was weak - replaced and problem solved.
Good fishing trip nothing breaks, great trip catch fish.
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Post Options Post Options   Likes (1) Likes(1)   Quote CrayZfish Quote  Post ReplyReply Direct Link To This Post Posted: 11 Sep 2018 at 12:38pm
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I've given in an its off to an outboard mechanic she goes. He'll probably figure it out easily. Shame I could not figure it out but I have learnt a lot about the motor and myself trying. Thanks for the help I'll post back on here what he finds.
Why choose either diving or fishing when you can do both. Besides crayfish tail is very good bait!!
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Post Options Post Options   Likes (0) Likes(0)   Quote CrayZfish Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 2018 at 12:36pm
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Originally posted by CrayZfish CrayZfish wrote:

I've given in an its off to an outboard mechanic she goes. He'll probably figure it out easily. Shame I could not figure it out but I have learnt a lot about the motor and myself trying. Thanks for the help I'll post back on here what he finds.
 
 
I dropped the boat in on the weekend to my motor mechanic, and get a call mid morning, she's all good to go. A seized spark advance lever. The washer under the lever was steel and had rusted up, restricting the advance under throttle.
 
Not sure if you are on here Willy but top man.
 
Looking forward to getting on the water, heck i'll be happy just to get on the plane.
 
 
 
Why choose either diving or fishing when you can do both. Besides crayfish tail is very good bait!!
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Post Options Post Options   Likes (0) Likes(0)   Quote CrayZfish Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 2018 at 12:42pm
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Originally posted by Tagit Tagit wrote:

The other thing that might need checking is your ignition advance to make sure it is working properly.
 
On the money Tagit.
 
Hopefully that is the end of that. Test run next on the way to the Scallop beds, then gear up for the next decent spell of NE.
Will be good to have some confidence in the Ol girl again.
 
Why choose either diving or fishing when you can do both. Besides crayfish tail is very good bait!!
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 17 Sep 2018 at 4:00pm
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All those little things should get a clean and lube with a good brand name mariner grease.. at least once a yr.. ideally 2x per yr.
Its in the service manual and should be done at each service.
 If sticks.. that part of the service not done.
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