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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10897
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Mariner 40hp 2cyl 2stroke. Vibrates badly at tick over.

Customer Question

Mariner 40hp 2cyl 2stroke. Vibrates badly at tick over. Passes lots of water and engine does not get hot. Runs fine from 1/4 to WOT..I have rebuilt carbs and checked timing with strobe. It's the bad tick over and cold running that bothers me. Plugs look good after fast run but wet when it has been ticking over. This was originally an oil injection engine. Now running 100:1.
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.

Hi Ernie, my name is Jason.

All Mercurys are cataloged by serial number, can I get that number please.

Customer: replied 1 year ago.
Mariner 40 ELTO 6E9.L.502230
Customer: replied 1 year ago.
Telltale and holes in back of leg pass copious water. Much more than I would expect. Engine never warms up. Thermostat opens at 60*C in hot water. I replaced thermostat with disc with 3mm hole. There is a spring loaded plastic valve beside the thermostat. Looks like it is a pressure relief valve. I've never seen this arrangement before. Spark plugs are a good colour afterwards and a fast run but wet at tickover. Very rough tick over and engine still cold. Is there too much cooling water passing through engine preventing engine from warming up?
I have set ignition timing to be almost fully advanced just before throttle begins to open as in previous thread.
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Jason replied 1 year ago.

Hi. Phone service is available for customers in the US, are you located in the USA?

Customer: replied 1 year ago.
it's twin cylinder 2stroke twin carburettor. Your link is the closest to this engine I can find. My friend took off the oil injection years ago. It appears to have an electric motor and a PP3 battery, maybe to sound a low oil alarm. He says it was linked into the fuel supply. I've never seen an injection system like this. The engine has never run well in his ownership. I now have it running well on all throttle openings except tickover up to 1/4 throttle.
Expert:  Jason replied 1 year ago.

Have you compression tested the engine yet, and if yes, what did you get for actual numbers?

Also, 100 to 1 is not enough oil for this engine, and you can be damaging it. You want to run it at 50 to 1

Customer: replied 1 year ago.
Ok I'll check compression on Monday and post the result. The engine is incredibly smokey on 50:1. That's why I tried 100:1. Although sold as a Mariner, this engine was built by Yamaha. Many Yamaha engines of that era are labeled 100:1. I run my other engines on 50:1 even if they are labeled 100:1. I agree 100:1 is very marginal and does not leave enough residual oil to protect the engine during storage, especially since we are now using 10% alcohol in petrol and causes corrosion.
I recently rebuilt a Mercury 2.5hp for a club member. It was a good example of a 100:1 engine that had rusted in storage. I've never seen an old 2stroke rust like this. They may have been run on 25:1 and petrol didn't have 10% alcohol in the good old days.
Until Monday
Customer: replied 1 year ago.
Sorry I didn't tell you that I am retired in Spain. I guess that the phone service is not available to me.
Customer: replied 1 year ago.
Sorry I left my compression tester at home. I'll do it on Wednesday.
Customer: replied 1 year ago.
Hi Jason
I'm back again with the results of the compression tests.
Top cylinder 110 psi
Bottom cyl 100 psi
So there is a discrepancy there. This engine lay around for several years because it was giving trouble. The rings might be stuck in the pistons. Do you have a favourite ring free spray that you use?
We're back to 50:1 as you suggested. Thats pretty smokey but better than a damaged engine. We're using synthetic oil. Do you have any preferences on oil?
Thanks for your advice so far.
Expert:  Tan replied 1 year ago.

Hi Ernie. My name is Tan.

I like the Quicksilver Power tuner for freeing up rings.

Have you tried setting the idle jets a little bit richer to see if that helps with the throttle issue?

I use only Mercury Marine Premium Plus synthetic 2 stroke oil.

Post back with questions and findings with the idle adjustment.

Customer: replied 1 year ago.
Hi Tan
Thanks for the links.
I've already used the Quicksilver power tune. I'm glad you advised that.
We're using synthetic 2stroke oil. I'll buy the Quicksilver synthetic you advise.
I've trimmed the tickover mixture screws in and out. The engine runs best about 2 turns out on both carbs.
I still don't understand why this engine pumps SO much water and never heats up. I've never seen what looks like a pressure relief valve (or bypass) beside a thermostat in my life before. I've ordered a water pump impellor so I'll be examining the water pump when that arrives. I wonder if someone has fitted the impellor the wrong way round with the blades leading rather than trailing. I've never seen it done that way and I wonder if that would cause the water pressure and flow to be far to high and cause over cooling. There is no feeling warmth in the water exiting the two ports half way down the leg. There is very little heat in the engine even after a run at WOT. And the tickover is still rougher than normal for a two stroke twin.
Expert:  Tan replied 1 year ago.

Hi Ernie. What can cause the water flow to be continuous and not get warm is a open thermostat. Pull the thermostat cover and inspect the thermostat it is most likely stuck open. There is also a popit valve that could be stuck open allowing full flow of water. This is also in the thermostat cover. Both should be closed till the engine gets up to temp. Then the popit valve should only open when engine is running at higher rpms 2500 and up. In this diagram the t-stat # ***** and the popit #26 & 27.

The pump will not cause excessive pressure. Lots of water isn't a problem but the thermostat being stuck open is as it will cause poor idle and the engine will never get up to temp.

If the engine still idles rough at idle after the t-stat has been verified then we need to do a spark and compression test.

Post back with questions and findings.

Post back with questions.

Customer: replied 1 year ago.
Hi Tan
Are you able to go back to the start of this question and read the whole thread?
I've had the thermostat out. It was closed and opens at 60 * C. I replaced the thermostat with a disc with a 3mm hole drilled in it. The poppet valve is closed and I've stretched the spring a bit to increase the pressure. Still lots of water.
Compression on top cyl 110, bottom 100. My gauge is very old and may be low.
Single coil with twin HT leads. Sparks both cyls at same time with lost spark at bottom.

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