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Old 12 June 2016, 13:01   #1
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Country: UK - England
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Mercury 75 hp problems

Hello all.
I've got a problem with the engine. After a reasonable run at normal cruising speed (20 min @ 3500 - 4000rpm) and a bit of a drift for 5 or 10 min I couldn't restart it. After about 10 min I got life and made it into a nearby bay where it stopped and re started no problem. Or so I thought. After a further half hour or so it wouldn't re start and ended our day early with a tow home.
The following day it has started on the drive no problem.
Any ideas please.
The engine is a 1999 mercury 75 hp 2 stroke. Fuel was new in clean tanks. Filters OK. Battery good. Do they suffer from heat effecting the electronic ignition?
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Old 12 June 2016, 13:53   #2
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I'd start with the jets in the carbs see if they are blocked
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Old 12 June 2016, 14:39   #3
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I'll give them a clean, but why would it re start the next day and stay running for 10 min or so if there was something blocking the jets?
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Old 13 June 2016, 04:28   #4
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Did you try pumping the bulb in the fuel line?
Difficult at sea, but if it does it again checking for a spark at the plugs would be an idea - if you can access one of the leads. Then - unless you have one of the inline checkers - a spare plug is the easiest way. Don't forget that the plug needs to be touched to an earth!

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Old 13 June 2016, 05:36   #5
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I had a problem like this on a yam 40, fuel pump rubbers perished, so would start after pumping the bulb but died when carb ran dry,
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Old 13 June 2016, 09:23   #6
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Sounds to me like it's flooding so the float bowls could be stuck or there is dirt in there. Two jets pilot and main so you may be starting on the pilot with fuel and then getting problems. I'd deffo clean the carbs and jets and check float heights then see where you are
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Old 13 June 2016, 09:47   #7
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was the obm tilted up?
was it tucked under?
does it sneeze when accelerating?
does it reach its full WOT?
If it doesn't get to WOT, does it run better if you squeeze the primer bulb?If it does that could also be the fuel pump diaphragms are stretched.
Is it over oiling from the oil pump?
Have you got the correct NGK spark plugs fitted?
have you tried to start with the throttle wide open, not on the gear lever but on the fast idle leaver?
remove strip and clean carburetors check float heights and needle valve seats for wear, when reassembled set all three idle air screws at one and a half turns out from lightly seated. ignore any w/shop manuals on mixture settings.When you have refitted your carbs, when setting them up have the OBM straight up and down because the design of the carbs will make it run very rich on tickover.when every thing is set up, you should have about a two hundred RPM difference between tickover in neutral and FWD gear. If the engine sneezes on acceleration richen the mixture by the width of the screw slot on the idle air mixture screw.
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Old 13 June 2016, 13:38   #8
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Quote:
Originally Posted by uncle al View Post
was the obm tilted up?
was it tucked under?
does it sneeze when accelerating?
does it reach its full WOT?
If it doesn't get to WOT, does it run better if you squeeze the primer bulb?If it does that could also be the fuel pump diaphragms are stretched.
Is it over oiling from the oil pump?
Have you got the correct NGK spark plugs fitted?
have you tried to start with the throttle wide open, not on the gear lever but on the fast idle leaver?
remove strip and clean carburetors check float heights and needle valve seats for wear, when reassembled set all three idle air screws at one and a half turns out from lightly seated. ignore any w/shop manuals on mixture settings.When you have refitted your carbs, when setting them up have the OBM straight up and down because the design of the carbs will make it run very rich on tickover.when every thing is set up, you should have about a two hundred RPM difference between tickover in neutral and FWD gear. If the engine sneezes on acceleration richen the mixture by the width of the screw slot on the idle air mixture screw.
Thanks for the response.
The engine would have been tilted slightly up.
No sneezing on acceleration.
Yes gets to wot 5100RPM with 21" prop.
Oil consumption doesn't seem excessive and not too smokey.
Yes they are the right plugs but I will probably replace them.
Yes I managed to start it the following day by using max fast idle. Think I tried that while afloat with no luck.
I will follow your settings for the carbs after cleaning at the weekend.
Also my first post made it sound like it cut out on its own in the bay. No that was me checking and finding that it re started several times and thinking were good to keep fishing.
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Old 13 June 2016, 13:50   #9
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Quote:
Originally Posted by paintman View Post
Did you try pumping the bulb in the fuel line?
Difficult at sea, but if it does it again checking for a spark at the plugs would be an idea - if you can access one of the leads. Then - unless you have one of the inline checkers - a spare plug is the easiest way. Don't forget that the plug needs to be touched to an earth!

Engine Ignition Spark Tester AUTO CAR ENGINE IN LINE IGNITION SPARK PLUG TESTER | eBay
Yes the second time it cut out we had the cowel off and checked for fuel leaks and the primer bulb was hard. I did my own test on the plugs by touching the rubber cap and turning the engine over. I only got a belt of the top cylinder whereas I normally get a shock of all three. Not sure if that indicated an electrical fault. My thoughts at the time were it was the kill switch or wires and had the throttle housing apart trying to eliminate it.
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Old 14 June 2016, 09:38   #10
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What you really should use is an adjustable spark tester, with a wide a gap as possible to make the ign system work hard.Bearing that in mind, I think you have the CDM coils fitted, they are the square ones, if so I would check all the earths on the engine disconnect make sure the connector is clean and bright, as the metal its connected to is as well, if you have no spark on the two btm cyls, it could be to cdm at the top cyl.there is a special harness (which makes it easy to test) its a service tool ( expensive to buy) fiddly without though.
you could check all connections back to the stater, is it a black stater, before I retired there was problems with black staters on various sizes of hp engines.
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