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Old 12 May 2024, 14:54   #1
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My new 15hp 2 stroke mercury

Right ladies and gentlemen, got myself an outboard, 15hp mercury 2 stroke, its not without issues though runs rough and has low compression accross both cylinders (50 psi), tested it before I brought it and the price was adjusted, it was already being sold as non runner, paid 395, which i thought was OK. Will start the investigations tomorrow.
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Old 12 May 2024, 17:40   #2
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Well the back stop is that the gearbox, cowl, carb plus a couple more bits would probably sell for what you paid so risk is minimal. Hope you get it running OK though as that's the objective. Do you know the year? Any under the cowl pics?

It's rare for a leisure use 2-stroke to be worn out even at 20yrs old so statistics are on your side. Let's just hope there is something weird giving those low compressions.

I'd be getting some clean fuel at the correct mix into it, new plugs (or clean and gap at the very least), clean the fuel filter and give it a run in a tub for a bit until hot then see how the compressions look. Do you know if your gauge is accurate, they can vary loads.

Oh yes and check for splits in any vacuum tubes (particularly the one below/behind the carb). Also check for air leaks.

Next I'd do a carb full clean/overhaul inc fuel pump/accelerator pump and only then if it still runs rough worry further about compressions.
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Old 12 May 2024, 19:01   #3
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If I put some oil in the cylinders the compression raises to about 75 psi. Planning on having a closer look tomorrow which will involve removing the carb. I have downloaded the workshop manual. I'm guessing some one has either run it without oil mixed or run it hot. I will check the carbs first and run it up before I go full on strip down . Thanks for the advice, appreciated.
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Old 12 May 2024, 19:24   #4
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I don't know everything about these but have owned two in 15hp form and a 10hp model same apart from carb.

I can't remember now but is there an exhaust plate you can take off the block easily to see the pistons/bores.. or at least one side?
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Old 13 May 2024, 05:21   #5
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If your compression tester is from eBay or Amazon there is a good chance it is reading wrong. The fact that both compressions are the same suggests it is the meter not the actual engine.

As said I would first clean the carb. Maybe replace the the gasket for the float bowl. Replace the spark plugs. Tighten the HT leads then try her with some fresh fuel. These engines are very simple and parts on eBay are quite cheap
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Old 13 May 2024, 06:35   #6
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Thanks for the advice, I'm looking forward to the challenge. Had a poke around for some parts, gaskets etc and there's a plentiful supply so there's likely to be minimal financial drama.
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Old 13 May 2024, 08:11   #7
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Originally Posted by Easedalenovice View Post
If your compression tester is from eBay or Amazon there is a good chance it is reading wrong. The fact that both compressions are the same suggests it is the meter not the actual engine.
My first thought was 50psi - engine is scrap. But on both cylinders - this is a fair point. Not sure even run on 50 - certainly when pulling it over it'd be particularly very "loose".

For diagnosis purposes, I frequently fall back to using these on an ignition system, for small outboards, the kids go-kart engine, and even the big V8 boat motor : https://www.ebay.co.uk/itm/121981403812

Cheap and easy to use. If you get nice fat, regular sparks - you know ignition is basically OK.
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Old 13 May 2024, 08:53   #8
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>>>My first thought was 50psi - engine is scrap. But on both cylinders - this is a fair point. Not sure even run on 50 - certainly when pulling it over it'd be particularly very "loose".

Yes hence my comment in post 2... "Do you know if your gauge is accurate, they can vary loads"

I would have expected it to have been easy to pull over but hard to get running at 50psi... but I'm assuming it started OK then ran badly from what Tinker said hence advising a logical diagnostic route of a few easier/cheaper things than worrying initially too much about the compression readings.

Here is the issue I had with mine and a split pipe under the carb which is worth being aware of...

https://www.rib.net/forum/f36/marine...arb-67447.html

And here is my carb overhaul thread should you get round to that...

https://www.rib.net/forum/f36/marine...aul-67565.html
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Old 13 May 2024, 09:40   #9
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Great little project. The top ends are easy enough to work on with these, far more-so than the 4-stroke. Fingers crossed it's just worn out rings, but you can still get a full rebuild kit for these anyway.
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Old 13 May 2024, 12:20   #10
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Right then, some scores along the piston from taking the cover off and some signs of debri ( aluminium) in the housing

Dismantled down the whole lot and the conrod needle bearings are shot, one is very lose and the other is notchy to say the least. How the thing run is a complete mystery.

Cylinders aren't scored too badly, think I could probably hone them out. But the bearings......I'm not sure even new ones would rescue the situation. I have found a complete powerhead on ebay but tbh it looks a bit worse for weather to say the least.

https://www.ebay.co.uk/itm/145655263...mis&media=COPY

I have messaged them about compression readings but they have got none, just ran well when it did, spark plugs look rusted in.......

I'm in no rush, happy to wrestle with the 25hp, for now.........
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Old 13 May 2024, 12:34   #11
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Oh that's a shame... I'd hoped you'd found the lucky buy of the year.

I'd be skipping that Ebay powerhead too... you might end up with another rebuild. No engine that's maintained has spark plugs like that unless it's been dunked or left out in the rain with no cowl.
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Old 13 May 2024, 19:27   #12
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Right then, some scores along the piston from taking the cover off and some signs of debri ( aluminium) in the housing

Dismantled down the whole lot and the conrod needle bearings are shot, one is very lose and the other is notchy to say the least. How the thing run is a complete mystery.

Cylinders aren't scored too badly, think I could probably hone them out. But the bearings......I'm not sure even new ones would rescue the situation. I have found a complete powerhead on ebay but tbh it looks a bit worse for weather to say the least.

https://www.ebay.co.uk/itm/145655263...mis&media=COPY

I have messaged them about compression readings but they have got none, just ran well when it did, spark plugs look rusted in.......

I'm in no rush, happy to wrestle with the 25hp, for now.........
Is that notchy feeling on the conrod from just the bearing itself, or is the journal damaged? If the journal is ok then it should still be repairable (you can also repair journals, so long as the damage isn't too severe).

I've certainly worked on much worse, so it's worth a go. It's scrap otherwise, so no loss in trying.
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Old 13 May 2024, 19:59   #13
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It's a mixture of the bearing and the journal, so much play in it, the other ones tighter but the bearing is worn. New bearings are about 90 but the journal isn't repairable without machining and to that's gonna make whole think uneconomical. The only way I can see around it is second hand block with a matched crank and pistons. I could just stick it back together, and I'm.sure it would run for a bit but I can't be risking it, I have a pony motor but it's not good enough to cover any distance and I love to be exposed to the elements.

Not over yet, just take a bit of time.for the right block to come along.
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Old 17 May 2024, 15:42   #14
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Confession time.....i brought the powerhead i wasn't 100% on. Got it today and stripped it down and guess what.......fortune favours the brave (this time!) Anyway it looks good, crank bearings nice and smooth no scoring on the pistons or cylinder walls and the rings look good. Just waiting on a 60mm honing tool to arrive while I clean and paint it.

One question.....where to get parts in the UK, im looking at oil seals that are 40 with delivery! I can measure and order stock of the shelf but theres one, bottom of the crank, thats got and double seal on the ID. Any ideas on a decent UK supplier?
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Old 18 May 2024, 10:44   #15
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I guess you have intensively used Google to search including by part number? Is 40 the dealer price for OE seals? Personally if I was going to the trouble of this rebuild and taking the finished motor to sea I'd be keen on OE even if costly.

I see SSI Marine are listing some on Ebay for 13 but may not be for your specific motor.
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Old 18 May 2024, 12:51   #16
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No it isn't, brought one from them already. I have measured and ordered from a generic oil seal stockist. There's a double lip seal on the shaft, a duo oil seal. I'm replacing a spacer and using two oil seals, that should cover the duo seal.

In other news a 2023 15hp yamaha, 2K

https://www.ebay.co.uk/itm/116186215...mis&media=COPY
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Old 18 May 2024, 13:55   #17
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The Yamaha is interesting. Annoyingly for anyone interested it's plus VAT. Must be a commercial user selling on.
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Old 18 May 2024, 13:55   #18
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There is no need to spend the extra on so called OE Parts. The so called OE parts are usually the cheaper parts in a different bag.

Just see what you can get on ebay to get the engine up and running
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Old 18 May 2024, 13:57   #19
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No it isn't, brought one from them already. I have measured and ordered from a generic oil seal stockist. There's a double lip seal on the shaft, a duo oil seal. I'm replacing a spacer and using two oil seals, that should cover the duo seal.

In other news a 2023 15hp yamaha, 2K

https://www.ebay.co.uk/itm/116186215...mis&media=COPY
In theory that Yamaha even though second hand should only be sold to a commercial user as it was originally bought in 2023
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Old 18 May 2024, 14:07   #20
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>>so called OE parts are usually the cheaper parts in a different bag.

Usually is a misleading claim. You clearly have never been involved with safety or reliability critical marine, automotive or aircraft supply chains where the issue of copies, items rejected from OE factories obtained from"back door" sources and other variable products are a constant issue.

Often the cheapest items are plain bag or white box offerings with the claim they are "the same as OE"... which is frequently a total fabrication.
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