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  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
    15,707

    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
    72

    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 08:58 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
    15,707

    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

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