50hp johnson Low RPM's at WOT

[jsalz]

Hi all, I started this discussion in the general section but I think it should be moved here.
Anyway, we have a 50hp 74 Johnson which I believe has a fuel starvation problem. Initially the motor started and idled and ran at low speed (10mph) very well.
I went ahead and changed the plugs and took it back out. With the new plugs, we were able to reach 25-28 mph gps but only for about 5 minutes. We started to lose rpm's again and went back to about 10mph. After about 10 minutes of putting around, the RPM's came up again for about 5 more minutes. Again, we lost the RPM's and the boat slowed for the rest of the 1 hour trip.
We thought it might be water in the fuel? Fuel Pump? Dirty fuel lines or Carb.
What I don't understand is that at idle, we can use the choke leaver beside the shifter to rev the motor up to high RPM's but when we put it in gear, the RPM's die. The other thing is why did it run well with the new plugs initially? I checked all the hoses for leaks and they all look good.
I have the manual and was going to check the fuel pump but I'm not really keen on turning over the motor with gas flowing out to a container.
The manual also said that if I were to remove the fuel pump and squeeze the bulb that if any fuel is coming out the back vacuum hole, that the pump would need to be replaced. Is the diaphragm the only thing that could be wrong with the pump. Is this a good test? The carbs were rebuilt last year so I don't suspect them but who knows. What about carb cleaner that goes in the gas tank. Does this stuff work?
Would it be ok to run the motor in a 45 gallon drum at high rpm's?
Thanks for your help.

 

[larky]

Re: 50hp johnson Low RPM's at WOT

Ok, the first thing that comes to mind about the rpm's is exhuast fumes under the engine cover. they build up and choke out the motor. so run the engine with the hood off and see if you can cause the same problem. if that cures the problem then you have an exhuast adapter gasket bad or an exhuast cover gasket that is bad.

for the fuel pump, do the test that the manual suggests and check the pulse line from the crankcase for holes or cracks. there is also a filter screen under the screw in the center of the pump.

as far as carb cleaner everyone on here will tell you the only way to do it right is to tear the carb down completely, clean it, and put a carb rebuild kit in it.

hopefully someone with more experience with your motor will be along soon

let us know what ya find

 

[tashasdaddy]

Re: 50hp johnson Low RPM's at WOT

http://forums.iboats.com/showthread.php?t=244013. have you check spark on both cylinders, could be a coil heating up and failing. that is not a choke lever, that is fast idle lever for starting the motor when it is cold. choke is either a button, or you push the key in to activate the choke. you should not be able to shift with the fast idle lever up.

 

[jsalz]

Re: 50hp johnson Low RPM's at WOT

Ok here's my next question. I have identified all the fuel lines except one. It comes from below the carbs and goes to the bottom of the box on the front of the motor where you would adjust the idle mixture. It looks like it is wide open to atmosphere from there but I have not taken the cover off the box yet. Is this some sort of air line? Reason I'm asking is because it was not clamped on at all and there is fuel in it.

 

[larky]

Re: 50hp johnson Low RPM's at WOT

that hose is just a drain for the carbs and airbox. when the outboard is tipped up for trailering a little gas will spill out. that hose routes the gas either into the crankcase or out the exhuast, I can't remember. trust me its the difference between a nice clean power head, and a filthy dirty mess.

 

[jsalz]

Re: 50hp johnson Low RPM's at WOT

Thanks Larky, that makes sense. On the back of that air silencer box, there is a large rectangular opening I assume is for exhaust. It would seem to me that there should be a filter or some sort of cover. Is mine missing or is there one supposed to be there?

 

[larky]

Re: 50hp johnson Low RPM's at WOT

no, that opening is for air intake. and no it does not need a filter. marine engines just like snowblowers are not subjected to environments that contain a lot of dust or debris. that is why neither have air cleaners(filters). I suggest you purchase and outboard repair manual. it will help answer some questions and inform you of maintenance procedures, and the general layout and operation of the basic outboard engine.

It definitely pays to work on these things your self. for me, I'm the only one I can trust not to screw me on a repair job.