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I have a 2003 Mercury 75hp 3 cylinder that idles a little…

I have a 2003...

I have a 2003 Mercury 75hp 3 cylinder that idles a little rough and has problem planing off (misfires) and once get planed off can get up to about 5000 rpms just about wide open and then push throttle open and the tach will drop to about 4500 rpms, also when trimming the motor ( pressing the trim button affects the tach) I have cleaned carbs three times, pumped tank and put fresh gas, new filter, new fuel pump kit, brand new trigger, new plugs, swapped coils from 115 Mercury, when pulling the plug wires with motor running the motor does drop with each cylinder, but the top cylinder seems to have weaker spark.

Mechanic's Assistant: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else important you think the Marine Mechanic should know?

Thanks, tommy

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Customer reply replied 7 months ago
Has 130 psi compression on all three cylinders
Customer reply replied 7 months ago
Load or strain seems to cause problem and when do get on plane and trimmed out seems to surge
Answered in 8 hours by:
12/12/2017
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 17,818
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Hi Tommy, Good morning my name is***** read what you wrote. Is there a story or history to this outboard, have you ever personally seen it run correctly? Can I get the serial number on the outboard.

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Customer reply replied 7 months ago
Jason, The ser# ***** ***** I don't know history of the motor, I just bought the boat a few weeks ago to fix up and sell, so I have never seen it run correctly, I forgot to mention that the oil injection has been removed and fuel is now pre-mix, the flywheel magnets were toast and had rubbed the stator, I put a new flywheel on it and had a red stator from a 115 that I put on it, but did try the original stator and it ran the same way, but I am not sure the stator from the 115 is good, as it was just loose in a box from a previous boat I had purchased, so I've never had it on a motor until this one.
Tommy
Customer reply replied 7 months ago
I'll add that it looked like the guy had been messing with it to try and get it to run right, as there were pierce marks in the wires from stator and trigger from a tester, and at idle it is rough, but as first accelerate it seems to clear up until about 2500 to 3000 rpm then fall off, they may have messed with timing, don't know if high speed timing would cause the problem, but I can baby the throttle and lean forward in the boat and eventually get it to plane and once planed can get up to 5,000 rpm, still hear a miss, but apply last littl bit of throttle and the tach will act up,
Customer reply replied 7 months ago
Also when purchased it I did replace the water pump impeller and it would never get warm, the block, cylinder head cool to touch, so I put new thermostat and poppit valve in and still runs very cool, I did a high speed shut down and the plugs have a little oil on them and no sign of water, so I think I may have ruled out cracked block and when I had carbs off the reeds looked good
Customer reply replied 7 months ago
I've probably had at least thirty of these Motors in last 4 years and this is first one to stump me, it seems like fuel, but the tach acting up is suspicious, oh and also I replaced the tach, as old one the needle had fallen off, could this be a ground or stator issue, or maybe timing? I don't have timing light, probably wouldn't know how to use one, but thinking about buying one, as I am pretty good at learning and fixing "stuff". We don't have a reliable Mercury mechanic around here and I have always resolved problems myself

Hi Tommy. Okay got it. Did you look up the part numbers for the stator and flywheel by serial number, and then use those specific part numbers for your engine?

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Customer reply replied 7 months ago
No, got the part number off the old flywheel and purchased one with same number and the stator is one I had from a previous motor which was the red 4 wire stator and there is no part number on either stator, the one I removed or the one I put on, my suspicion has been the stator, but didn't want to buy one until I was reasonably sure that's the problem since I've already thrown parts at it

G​ot it. Everything is testable if you have the tools to do it. Do you have a good quality shielded Volt meter as well as a DVA?

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Customer reply replied 7 months ago
Have a volt meter but not DVA, I am not at home right now but can buy a DVA while out if you want to give me directions on how and what to test and get back with you

A​ DVA is something you would get from Snap On. Click this link, this is it. https://store.snapon.com/Multimeter-Accessories-Adaptor-Direct-Voltage-P649400.aspx On your stator. Does your stator have a white with a green stripe and a green with a white stripe wires that come off it?

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Customer reply replied 7 months ago
It actually has a solid green wire and a white wire with green stripe, the solid green wire plugs into butt connector with two green with white stripe wires, the stator I removed does have the green with white stripe, could this be my problem, as the stator on the motor now came off a 4 cylinder 115, but a friend of miy son that's in the marine salvage business said they are the same, both the stators have two yellow wires

Okay got it. I'm kind of guessing at this because it should have a green/white and white/green. That's what it should be. So what you would do is hook your meter/DVA up to the green and white/green wires. Set the meter to DC volts, auto ranging, crank the engine over, and see what you get for a reading. Let me know how you make out.

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Customer reply replied 7 months ago
Sure will, it will be later today before I can get back and test

N​o problem, let me know what you find.

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Customer reply replied 7 months ago
Do I unplug the two wires at the butt connectors to test?

Yes, unplug them then hook your meter leads directly up to the 2 stator wires.

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Customer reply replied 7 months ago
Ok, I'm going by an Auto Zone to see if they have DVA, if not I'll try a couple of other places

A​uto Zone and any other auto parts stores will not have a DVA as DVA's are not used on cars. DVA's are used on outboards, motorcycles, some types of power equipment. Where you would start is by finding out if you have a snap on guy in town, and call him first. If he doesn't have one then call marine dealers/marina's, or try motorcycle shops, and see if any of them have a DVA. These are specialty tools so chance are you are simply going to have to order one.

Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 17,818
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Customer reply replied 7 months ago
Ordered DVA meter and timing light, waiting for meter to arrive, did leave with marine mechanic last Wednesday, he put in water and has him stumped too, can't figure out why it is running so cold (80 degrees) with new thermostat and poppet valve, was running cold before replaced those, did not change, also will not plane off trimmed down, but will plane off eventually if trim up while accelerating really has him scratching his head

Good morning. How are you taking the engines temperature? Are you scanning the tops of the cylinder, where the plug wires are, or are you scanning a different part of the block, such as the exhaust cover?

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Customer reply replied 7 months ago
I don't know how he took engine temperature,
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