Thread: Loss of power 50 hp merc

Might be the stator


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If your engine is experiencing a loss of power, even though it has good compression, new plug wires and carbs seem to be fine, there could be a few other reasons for the issue:

1. Fuel problems.
   
2. Ignition system issues.
   
3. Exhaust restrictions.
   
4. Propeller problems.
   
5. Overheating.

Without seeing a tach, or hearing the motor that's going to be a tough one to "internet diagnose". Could be as simple as a loose spark plug wire, or clogged high speed jets in carb(s).

I would also suspect fuel pump.

Could it possibly have water on the boat and you can't see it?

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My 88 50 classic had a loss of power that would come and go. It ended up being the switch box. No problems since having it replaced.

You already checked compression. My next step would be to do a static spark test with a true gap tester, not plug resting on cylinder head. Make sure each plug lead will jump a 3/8" gap to ground and give a good, thick snappy spark. That doesn't mean you will have good spark at all rpm ranges but it does tell you that its all properly grounded and has good contact everywhere. Most ignition systems have a high speed and low speed windings so you can have healthy spark at lower rpms but it can fail once engine speeds increases. Also ignition components, like the coils and CDI boxes can fail once they get hot after the motor has warmed up. I have had that happen to a Mercury switchbox on a 70hp 3 cyl where it would drop spark to a cylinder intermittently but only after the motor warmed up. Also had an Evinrude that acted like you described and it had a bad power pack that was failing after there was any rpm load up on the motor. If you have a timing light you may want to check that now to make sure its not off and the flywheel key has sheared. I would also check that the timing advance linkage is functioning because if it's not advancing then it's never going to rev like it should.

Also at this point, I would inspect the plugs. If you haven't done a ton of idling after having the wide open problems, then your plugs can help at least give you an idea of whats going on. If you are slow running at wide open throttle and spark is still working as is, then you would be running lean and the plugs would start to turn white from running dry. If you are getting fuel, but spark is becoming weak or dropping off then they will look rich and/or look uneven in color if only one cylinder is seeing ignition issues.

If that passes, I'd move to the fuel delivery system starting at the tank and working up to the carbs. Make sure tank vent is open and the pickup screen isn't clogged. Unhook the fuel line at the motor side quick connect (or at the next connection past the primer bulb if you have it plumbed straight to the fuel pump or a filter) and pump the bulb to pump fuel into a jar to make sure it flows freely. Those gray fuel lines have a liner in them that will deteriorate and clog them up. Has happened to me once before and that was with running 100% gas virtually all of the time too. Continuing to work down the fuel line, I would skip the messing with the fuel pump for now. Reason being is that if you suspect a faulty pump, the best way to diagnose it is on the water with the throttled fully opened. When you are wide open and barely chugging along simply pump the primer bulb. If it is not totally soft and pumping it does nothing then it means your carb bowl are likely full and its doing its job. If it is soft and pumping it gives you immediate power back then you know its not pumping right and needs to be addressed. If you get this far and you think everything is good from tank to carbs, including the pump, then you are probably looking at carb issue. In my experience, if the motor idles well, usually the high speed fuel circuit isn't clogged I guess because the idle circuit would be as well if that were the case. I most often see the floats sticking (which usually causes flooding at idle though) or the inlet needle sticking. If I got to this point, personally I would pull carbs and rebuild.

If you want to avoid that, I would revisit the ignition system. I would find some specs on the ignition system and do all of the resistance checks to see if anything is out of spec. If it passes, I'd do another water test where I pull the plug for a cylinder and take it to WOT to see how it responds and repeat for all plugs. This will maybe help you figure out if maybe one plug is loosing spark, they all have weak spark, or maybe something else. You will have to use this data with what you saw when you inspected the plugs to make an educated guess on if you now think its a fuel or spark issue.

My main recommendation is don't throw parts at it unless they are cheap. Probably not a bad idea to go ahead and replace the fuel lines and fuel filter, and if you rebuild the fuel pump yourself, the kit is probably pretty cheap. Same for a carb kit if you can do it yourself. But definitely avoid experimenting with ignition parts. They are so dang expensive and they are typically nonreturnable. How many cylinders is your motor? Is it a single carb or multi carb motor?

Originally Posted by scout-j-m

You already checked compression. My next step would be to do a static spark test with a true gap tester, not plug resting on cylinder head. Make sure each plug lead will jump a 3/8" gap to ground and give a good, thick snappy spark. That doesn't mean you will have good spark at all rpm ranges but it does tell you that its all properly grounded and has good contact everywhere. Most ignition systems have a high speed and low speed windings so you can have healthy spark at lower rpms but it can fail once engine speeds increases. Also ignition components, like the coils and CDI boxes can fail once they get hot after the motor has warmed up. I have had that happen to a Mercury switchbox on a 70hp 3 cyl where it would drop spark to a cylinder intermittently but only after the motor warmed up. Also had an Evinrude that acted like you described and it had a bad power pack that was failing after there was any rpm load up on the motor. If you have a timing light you may want to check that now to make sure its not off and the flywheel key has sheared. I would also check that the timing advance linkage is functioning because if it's not advancing then it's never going to rev like it should.

Also at this point, I would inspect the plugs. If you haven't done a ton of idling after having the wide open problems, then your plugs can help at least give you an idea of whats going on. If you are slow running at wide open throttle and spark is still working as is, then you would be running lean and the plugs would start to turn white from running dry. If you are getting fuel, but spark is becoming weak or dropping off then they will look rich and/or look uneven in color if only one cylinder is seeing ignition issues.

If that passes, I'd move to the fuel delivery system starting at the tank and working up to the carbs. Make sure tank vent is open and the pickup screen isn't clogged. Unhook the fuel line at the motor side quick connect (or at the next connection past the primer bulb if you have it plumbed straight to the fuel pump or a filter) and pump the bulb to pump fuel into a jar to make sure it flows freely. Those gray fuel lines have a liner in them that will deteriorate and clog them up. Has happened to me once before and that was with running 100% gas virtually all of the time too. Continuing to work down the fuel line, I would skip the messing with the fuel pump for now. Reason being is that if you suspect a faulty pump, the best way to diagnose it is on the water with the throttled fully opened. When you are wide open and barely chugging along simply pump the primer bulb. If it is not totally soft and pumping it does nothing then it means your carb bowl are likely full and its doing its job. If it is soft and pumping it gives you immediate power back then you know its not pumping right and needs to be addressed. If you get this far and you think everything is good from tank to carbs, including the pump, then you are probably looking at carb issue. In my experience, if the motor idles well, usually the high speed fuel circuit isn't clogged I guess because the idle circuit would be as well if that were the case. I most often see the floats sticking (which usually causes flooding at idle though) or the inlet needle sticking. If I got to this point, personally I would pull carbs and rebuild.

If you want to avoid that, I would revisit the ignition system. I would find some specs on the ignition system and do all of the resistance checks to see if anything is out of spec. If it passes, I'd do another water test where I pull the plug for a cylinder and take it to WOT to see how it responds and repeat for all plugs. This will maybe help you figure out if maybe one plug is loosing spark, they all have weak spark, or maybe something else. You will have to use this data with what you saw when you inspected the plugs to make an educated guess on if you now think its a fuel or spark issue.

My main recommendation is don't throw parts at it unless they are cheap. Probably not a bad idea to go ahead and replace the fuel lines and fuel filter, and if you rebuild the fuel pump yourself, the kit is probably pretty cheap. Same for a carb kit if you can do it yourself. But definitely avoid experimenting with ignition parts. They are so dang expensive and they are typically nonreturnable. How many cylinders is your motor? Is it a single carb or multi carb motor?

Thanks for your help that’s where I’ll start


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