1969 50hp mercury loses rpms after ten minutes

[meyermarine1989]

i have a 1969 50 hp mercury inline 4 that will start right up and then run wide open for about 10 minutes and then lose about 1000 rpms. when this happens i will reduce throttle back to an idle for about 15 seconds and then it will work at full rpms again. it will run again like it should for about another 10 minutes and it will do it again. when i got back to the boat launch and tried to start it again it seemed like my battery was dead. i just rewired everything and i dont know if i wired the rectifier up correctly. would that maked a difference if my battery wasn't charging. the motor has a magneto and it is driven by the flywheel. it is pumping water good and i always mix my oil 50:1 or 45:1. please help. thank you.

 

[emckelvy]

Re: 1969 50hp mercury loses rpms after ten minutes

Sounds like maybe a fuel delivery problem. I'd rebuild the fuel pump and while you're at it, replace the primer bulb. Get a Merc primer bulb if you can find it, the latest ones have been updated with a new check valve that doesn't restrict flow and is less prone to failure.

Your motor has "Lightning Energizer" electronic ignition which doesn't require a battery to operate, so the condition of the charging system will make no difference, as far as the running issue goes.

I'd also check venting on your tank, whether it's portable or built-in. Fuel starvation can be caused by a poorly-vented tank.

Another issue could be with sticking carb float needles/seats or floats. It probably wouldn't be a bad idea to upgrade with the latest-style floats, and change the needle valve/seat assy on both carbs.

BTW a simple way to check fuel starvation is to pump the primer bulb when you start having problems. If the motor picks back up, most likely a fuel pump problem.

Of course, this is just assuming there's a fuel problem. You could also have a cylinder dropping out, caused be something as simple as a bad spark plug (try a new set of Champion L76V plugs), trash in the distributor cap (flush it out with carb cleaner and blow-dry with compressed air), or a 'leaky' spark plug wire (fire it off at night with the cowl removed and look for Christmas Tree Lights!).

HTH & let us know what you find.