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Results 1 to 18 of 18
  1. #1

    Unhappy Need help with motor not starting

    Just picked up a 7.5 Mercury (was told it's a 78'). Anyway it will not turn over. I've checked spark which is very good and strong, compression is good on both cylinders at #120, re-built the carburetor and fuel pump all new gaskets, new gas tank and fuel line with new primer bulb. Doesn't leak any gas from the carb.

    I'm not getting any fuel to the spark plugs, they should be getting wet with fuel on a 2 stroke. I removed the carburetor and air is coming back twords me and not twords the plugs, does that mean the Reed valves are bad?

    The guy I got it from says it hasn't ran in a while, came on a boat he bought and didn't need the motor so I got it for $120. The carburetor doesn't have any issue as far as I know and I cleaned all the passages and adjusted the float to be level (it was missing the spring that goes on top of the needle).

    I can see the Reed valves and they "seem" to be closing properly but not 100% sure. Any information and imput would be greatly appreciated.

    Serial number is 5146276

  2. #2
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,790

    Default Re: Need help with motor not starting

    I've rebuilt dozens of these motors and I can assure you that your Most likely problem is the dumb throttle stop Merc installed on these motors; it does not allow you to give it enough gas (throttle) to get her going. It may have worked fine when they were new, but that was many decades ago.

    Look at the bottom left side of the carb and you will find a bolted-on tab that is limiting the throttle opening. Remove the bolt and toss the tab out. With it gone you can advance the throttle far more and it should start. Be prepared to idle it right down as it does in case you 'goosed it' too much. (With a little practice you'll know how much.)

    Good luck!

    Jeff

  3. #3

    Default Re: Need help with motor not starting

    Nice! Thank you very much, I will definitely try that and report back.

  4. #4
    Join Date
    May 2015
    Location
    Crosslake, Mn
    Posts
    81

    Default Re: Need help with motor not starting

    The last time I worked on one of these motors, it was a carb cleaning for my neighbor. After running it through my ultrasonic cleaner it was spotless. All small passages probed... Rinse with hot water... Blow out with compressed air... Re-assembled with new gasket... Still wouldn't start... Remove the plugs, added a couple drops of mixed fuel, replaced the plugs, pulled the rope, and it popped right over and ran for a few second.... Removed the carb again... pulled the float bowl off... dry as a bone.. the float needle was sticking in guide... cleaned the needle and guide with two or three Q-Tips and spray on carb cleaner.... needle now moving freely... reconnected fuel line... pumped the tank ball... gas flowed freely from the float inlet valve... re-installed float bowl and crab to motor...

    Motor now starts on first pull...neighbor happy...learn something new to check....

    Hope this helps...

  5. #5

    Default Re: Need help with motor not starting

    Haven't had a chance to try the advice I got yet but hopefully will soon. I'm still confused though as to why there would be air coming back out the carburetor (choke opening)?? All the research I have been looking at says the reed valves are bad?? Even if I do remove the throttle limit tab, the air from the carburetor.....????

  6. #6

    Default Re: Need help with motor not starting

    Spitting back out of the carb sounds like bad reed valves to me. Pull the carb and have a peak, you should be able to shine a flashlight in and see them. If one is missing or has a chunk out of it, that will be your problem

  7. #7

    Default Re: Need help with motor not starting

    The reed valves are on the crank on that motor not an easy task to change. Do what chris said the only way to know if a reed valve is bad is to get it running. Are you turning the flywheel by hand or using the recoil ? You may be feeling air out of the carb just because there is not enough crankcase pressure to close them.
    Last edited by flyingscott; 10-15-2018 at 07:58 AM.

  8. #8

    Default Re: Need help with motor not starting

    I have been using the recoil rope to pull it. The air that is felt is just a quick burst as the Piston moves up and down in the cylinder. I looked at the Reed valves and they look fine and are appearing to close all the way. I can stick a small pick in the passage way and there is a lot of tension on the Reed valves and they spring right back up and close, wether or not they are actually sealing, I don't know. After I get my other parts in I'll try to mess with it some more and if no luck, looks like reeds are only thing left

  9. #9

    Default Re: Need help with motor not starting

    ##Update##. I installed a new baffle plate and gaskets, new spark plugs. When I pull on the starter rope after about 5-6 times it backfires and shoot a very small flame out of the carburetor choke opening then the carburetor bowl starts to leak and the fuel pump gasket area. I have replaced/rebuilt both the carb and fuel pump and I can not make the screws any tighter orther wise the screws will strip out.

    When I initially try I pump the primer bulb till it's hard and no leaks anywhere then I pull out the choke knob to try to start it and after the 5-6 pulls it starts leaking and backfires??

    What else can I do? Any help would be greatly appreciated

  10. #10
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,790

    Default Re: Need help with motor not starting

    Are you sure the plug wires (or cool wiring) is not reversed? Also, look for a sheared flywheel key.

    Jeff

  11. #11

    Default Re: Need help with motor not starting

    Quote Originally Posted by fastjeff View Post
    Are you sure the plug wires (or cool wiring) is not reversed? Also, look for a sheared flywheel key.

    Jeff
    Plug wires are not reversed and flywheel key is ok. I took it into a shop and they looked at it and verified what I told them. Prime with gas and after 5-6 pulls starts leaking gas out the front of the carb and side of the fuel pump followed by a 3-4" flame and backfires out the carburetor. They suggested I take it to a shop that specializes in Mercury as they do Johnson mainly.

    I'm lost on this one...... Spark and compression are great, fuel is the issue.......

  12. #12
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    16,256

    Default Re: Need help with motor not starting

    If you can not do the repairs yourself it is time to look for another motor.

  13. #13

    Default Re: Need help with motor not starting

    How do I check the timing on this motor? Or can you? I would assume if the flywheel key is sheared then that would be a easy fix BUT it isn't and it's fine. Plug wires are specific and each one is a certain length so they can not be reversed. Fuel pumps three the carb just fine and holds UNTIL 5-6 pulls then starts leaking followed by the backfire....

    What to do???????

  14. #14
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    16,256

    Default Re: Need help with motor not starting

    Does spark jump a gap of 3/8" , yes or no.----The term " strong spark " means nothing to me.

  15. #15

    Default Re: Need help with motor not starting

    Quote Originally Posted by racerone View Post
    Does spark jump a gap of 3/8" , yes or no.----The term " strong spark " means nothing to me.
    Yes the Gap is easily a 3/8" gap. I have a spark tester and it jumps the gap with no problems

  16. #16
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,790

    Default Re: Need help with motor not starting

    Any decent shop should be able to square away that carb. It should run fine after it is.

    Did you remove the neutral stop, as I told you? If you can't get it to throttle up enough it won't start.

    Jeff

  17. #17

    Default Re: Need help with motor not starting

    Quote Originally Posted by fastjeff View Post
    Any decent shop should be able to square away that carb. It should run fine after it is.

    Did you remove the neutral stop, as I told you? If you can't get it to throttle up enough it won't start.

    Jeff
    Yes I removed the neutral stop as well. There is a shop that charges $50 to diagnose, probably going to have to take it there, they specialize in Mercury as well.

  18. #18

    Default Re: Need help with motor not starting

    Took it to a shop to have it looked at. They verified everything I told them then they switched the wires coming off the coils and it started up on first pull. Of all the things I checked that wasn't one of them...... So yeah that was dumb of me. Now they say it leaks water bad from the baffle plate exhaust cover. Can I just put some silicone on the gaskets and seal it up that way? This is what they told me to do. I will try to drill out the rest of the broken off bolts if I can but they aren't very easy.

    Thanks for any help you can give

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