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  1. #1

    Default Newbie needs some help with an old Mercury 500

    Hi -

    I am new at this posting stuff and not so good with computers. So, hopefully I won't mess this up. Apologies in advance.

    A few weeks ago my 2 young grandsons combined their savings and bought an 1968 Starcraft with a 1977 Merc 500 (50 hp). The motor ran well in the test drive. When we got it home we changed the oil in the lower unit and added fresh regular gas using a 50:1 mix. to the mounted aluminum tank.

    Boat ran great. They used the boat over a period of a few weeks and probably put about 3-4 hours on the motor during that time. All was well.

    Yesterday my younger grandson put the boat in the water and started it up. It was idling at a much lower RPM than in the past. When he gave it gas it seemed to plow. He was not able to get the boat on plane and came back in.

    We pulled the plugs. Based on a quick look on the internet they are the correct NGK center electrode plugs. They seemed a little wet to me, so we cleaned them with carb cleaner and put them back in (should the plugs be 100% dry like in a 4 stroke?). Back in the water it went. But, same problem. We pulled the boat and I ran it with the muffs for a while. It seems to have an inconsistent miss at a variety of RPMs (did not put in gear, just used the neutral throttle).

    I'm pretty good with a wrench but haven't has a 2 stroke Merc in 35 years. Generally, before I dig in too deep, I like to try, and rule out the easy stuff first. So, my next steps in diagnosis are to change the plugs and put in new high test gas mixed 50:1. Let me know if you think this is a good place to start or if you have any other simple diagnosis steps. For example, I wanted to try to see if one of the plugs/cylinders was the culprit but really did not know how to test for that.

    Thanks in advance for your suggestions. I'll let you know how it goes.

  2. #2

    Default Re: Newbie needs some help with an old Mercury 500

    I agree that looking at the simple stuff first is a good idea: Do you have an owners manual for the motor; have you checked compression; have you checked timing; does adjusting the fuel mixture screws make a difference; is 50:1 the right oil mixture ratio - my old 1956 Johnson and a friend's 1958 Evinrude engines required 24:1; have you tried new plugs - my 1992 40 Merc began missing and using new plugs solved that; don't know if premium gas is a good idea but some fellas on the forum use non-alcohol regular; maybe the tank, fuel line, fuel filter may need attention/cleaned.

  3. #3
    Join Date
    May 2007
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    Ireland,the only country in the world you can go North to go South
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    Default Re: Newbie needs some help with an old Mercury 500

    50:1 is the correct mix.And I like to see wet plugs as you know there is lubrication on the cyls.
    First is check spark jumping 7/16" Best to use a spark tester though I made my own up before I bought one. Compression and fuel lines.Were the carbs cleaned? I know you ran it for a few hours but it could have picked up some crud from the tank or lines.But with intermittent miss,I would suspect electrical.
    While it's running pull one lead at a time to see which one makes no difference to the running.It could be a weak coil,this is the one with four coils?

  4. #4
    Join Date
    Sep 2010
    Posts
    17,431

    Default Re: Newbie needs some help with an old Mercury 500

    Was a new impeller installed , yes or no ?----Compression test is next.----Then the maximum spark output test at 3/8" + on a test device.

  5. #5

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by haventaclue View Post
    50:1 is the correct mix.And I like to see wet plugs as you know there is lubrication on the cyls.
    First is check spark jumping 7/16" Best to use a spark tester though I made my own up before I bought one. Compression and fuel lines.Were the carbs cleaned? I know you ran it for a few hours but it could have picked up some crud from the tank or lines.But with intermittent miss,I would suspect electrical.
    While it's running pull one lead at a time to see which one makes no difference to the running.It could be a weak coil,this is the one with four coils?
    Thanks. Will check basics tomorrow. Need to get a spark tester

  6. #6

    Default Re: Newbie needs some help with an old Mercury 500

    We did not install a new impeller but water was coming from the telltale. I’ll check the spark next

  7. #7

    Default Re: Newbie needs some help with an old Mercury 500

    Will check spark and will try to determine if there is one cylinder/coil which is the culprit. There is a filter installed so I was thinking that crud wasn’t likely.

    Thanks

  8. #8
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
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    Default Re: Newbie needs some help with an old Mercury 500

    Get yourself an infrared temp gun and shoot the metal base of the plugs while she's idling. That will tell you if they're all running.

    Jeff

  9. #9

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by fastjeff View Post
    Get yourself an infrared temp gun and shoot the metal base of the plugs while she's idling. That will tell you if they're all running.

    Jeff
    Great suggestion. just ordered one. Thanks!

  10. #10

    Default Re: Newbie needs some help with an old Mercury 500

    Maybe after you have the problem solved, a new fuel filter would be a good thing or before.

  11. #11

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by RickM46 View Post
    Maybe after you have the problem solved, a new fuel filter would be a good thing or before.
    Yes thanks. That is part of the plan.

  12. #12

    Default Re: Newbie needs some help with an old Mercury 500

    Got the spark tester. Hoping to test it later today.

  13. #13

    Default Re: Newbie needs some help with an old Mercury 500

    Multiple ignition system problems with possible solutions on you tube. I have a 73 t/bolt 650 and there are a couple of videos that may help.

  14. #14

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by masterknowitall View Post
    Multiple ignition system problems with possible solutions on you tube. I have a 73 t/bolt 650 and there are a couple of videos that may help.

    Thanks!

  15. #15

    Default Re: Newbie needs some help with an old Mercury 500

    Sorry for not posting for a while. I was called away on business and just got back to this today. Thanks for your patience.

    While I was away the diagnostic gear showed up. Today I used the spark tester. Good spark on the top two plugs. No spark on the bottom two.

    I did my best to try to test the two bottom coils. The #3 tested NG. The #4 seems to be okay. So, I guess this is progress. But, it's a bit confusing since neither coil was producing a spark.

    I've ordered two coils off eBay. They should be here this week some time.

    If anyone can point me to a really easy to understand video on how to test these coils I'd appreciate it. I think I did it right, but maybe not. I'd like to be sure I test the new-to-me coils properly when they arrive.

    Thanks

  16. #16
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
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    16,216

    Default Re: Newbie needs some help with an old Mercury 500

    Ohm meter is the only way. Rare that two coils go out. I suggest you swap one of them with one that works and see if the problem follows or goes away.

    Jeff

  17. #17
    Join Date
    Sep 2010
    Posts
    17,431

    Default Re: Newbie needs some help with an old Mercury 500

    What does the wiring look like on this motor ?----Insulation crumbling ?

  18. #18

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by racerone View Post
    What does the wiring look like on this motor ?----Insulation crumbling ?
    The wiring looks great. A little dirty but no crumbling insulation

  19. #19

    Default Re: Newbie needs some help with an old Mercury 500

    I agree that’s very strange to have two fail at once. I was wondering if there could be anything “up stream” that might cause two coils to not energize. Two work fine generating strong spark. Odd. Will recheck all coils when the new ones arrive and use the best four
    Last edited by Seefra; 07-28-2019 at 10:25 PM.

  20. #20

    Default Re: Newbie needs some help with an old Mercury 500

    Latest update....

    I decided to start at the beginning, rigorously following the diagnostic test sequence recommended in the SELOC manual I purchased.

    Spark plugs
    1- All properly connected. Good continuity on all the plug to coil wires
    2- Plugs installed correctly. Correct plugs installed. Inspection found no unusual conditions
    3- Spark plug test - No spark on Plugs to Cyls 3 and 4. According to SELOC manual this is most likely an issue in the Capacitor Discharge (CD) system (made up of lots of elements). Began the CD element test sequence.

    CD element tests:
    a) Stator Coil test - NOT in spec!
    b) Trigger Assy tests - ok
    c) Ignition coil tests - ok
    d) Control harness connector continuity - ok (but I will replace if I get the motor running well. Control harness wire insulation is cracking in some locations. Since it's an expensive Assy I will temporarily insulate cracked wires with electrical tape)

    Since the Stator tested out of spec I have decided its replacement is the next step. Once accomplished I will insulate cracked harness wires with electrical tape (temporary measure) and then start the diagnostic test sequence all over from the beginning again.

    Will update thread after new Stator is installed

  21. #21

    Default Re: Newbie needs some help with an old Mercury 500

    Seefra; great documentation; can you show disassembly pics of how you got to the stator and removed it? I may be in the same boat with my Merc 40.

  22. #22

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by RickM46 View Post
    Seefra; great documentation; can you show disassembly pics of how you got to the stator and removed it? I may be in the same boat with my Merc 40.
    Sure thing.

    Awaiting stator and flywheel puller tool deliveries.

  23. #23

    Default Re: Newbie needs some help with an old Mercury 500

    Replaced the stator with a CDI aftermarket one for $160 and ran diagnostic tests from the beginning. No joy. But, not unexpected. When testing the old stator there was continuity to ground. So, if I understand it right, the old stator had a short to ground. I've read that when stators "fry" they can cause damage down stream. Since the only part down stream we had not tested (because it can not be tested effectively with an ohm meter) is the switch box. We replaced that and then re-ran the diagnostic sequence from the start. SUCCESS! Spark at all four Cyls.

    So, it appears the stator fried and took the switch box with it.

    Ran motor. Sounded better. Took it out. Not entirely happy with how it's running but will do a full tune up, fresh gas, new plugs etc and I suspect all will be well.


    Rickm46 - will post pics on stator removal if I can figure out how. But, it was easy. Here are the steps:

    Disconnected battery
    Removed engine covers.
    Used an impact wrench to remove flywheel retaining nut
    Used special tool purchased on Amazon to pull flywheel ($18 tool)
    Removed 4 Allen screws which retain stator
    Stator pulls up easily
    Removed stator wired from switch box
    Inspected new stator. Wire colors matched the old one so installation on a 4 cylinder is straight forward.
    Installed new CDI brand stator reversing the disassembly sequence above.
    Last edited by Seefra; 08-09-2019 at 09:38 AM.

  24. #24

    Default Re: Newbie needs some help with an old Mercury 500

    Thanks Seefra - will save this thread.

  25. #25

    Default Re: Newbie needs some help with an old Mercury 500

    Seefra, where did you get the new switch box?

  26. #26

    Default Re: Newbie needs some help with an old Mercury 500

    I purchased a used one off eBay from a seller with a good eBay reputation. It was a mercury unit. CDI makes them too. They are for sale on Amazon. Just be sure to get he correct one.

    I tried to upload the pics of the stator replacement but could not make it work. Sorry.
    Last edited by Seefra; 08-09-2019 at 08:02 PM.

  27. #27

    Default Re: Newbie needs some help with an old Mercury 500

    No need for pics but thanks a bunch anyway; none of my 4 cylinders are getting spark on my '92 40hp Merc 2 stroke; your success has encouraged me to take on further diagnostics; I decided to RTFM - I have the factory shop manual produced by Merc which contains step by step diagnostics. I may tap you on the shoulder via the forum when I go after the components with a multi-meter if that is OK with you.

  28. #28

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by RickM46 View Post
    No need for pics but thanks a bunch anyway; none of my 4 cylinders are getting spark on my '92 40hp Merc 2 stroke; your success has encouraged me to take on further diagnostics; I decided to RTFM - I have the factory shop manual produced by Merc which contains step by step diagnostics. I may tap you on the shoulder via the forum when I go after the components with a multi-meter if that is OK with you.
    Sure thing.

    The good old news is there I are a finite number of components. All can be tested with an ohm meter except the switch box. And if youíre willing to purchase used parts off eBay (and test them to make sure they are in spec) it can be done affordably.

    A friend of mine has the mercury shop manual vs the Seloc. He said itís really good.

  29. #29

    Default Re: Newbie needs some help with an old Mercury 500

    Hi Seefra,
    I am prepping up for replacing my stator on my '92 Merc 40 4 cylinder 2 stroke; my thread for that is http://www.marineengine.com/boat-for...-With-No-Spark
    I noticed you bought a CDI one and have some questions:
    Is the CDI still working for you?
    Where did you get your flywheel holder?
    Where did you get your flywheel puller?
    Many thanks!

  30. #30

    Default Re: Newbie needs some help with an old Mercury 500

    Quote Originally Posted by RickM46 View Post
    Hi Seefra,
    I am prepping up for replacing my stator on my '92 Merc 40 4 cylinder 2 stroke; my thread for that is http://www.marineengine.com/boat-for...-With-No-Spark
    I noticed you bought a CDI one and have some questions:
    Is the CDI still working for you?
    Where did you get your flywheel holder?
    Where did you get your flywheel puller?
    Many thanks!
    The CDI brad unit seems to have worked out (Iíve only had it a week)

    I did not need a flywheel holder. I used an impact wrench (think air tool for taking lugs off wheels). It spun the nut right off without any need to hold the flywheel from spinning.

    This is should be a like to the flywheel puller tool I used. Be sure to get on for your specific motor.
    Zinger Mercury Mariner Force Yamaha 1 1/2" -16 Flywheel Puller Removal Tool 91-849... https://www.amazon.com/dp/B075R414NZ..._EHWuDbFGQTXNM

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