Evinrude 50 (Lark) full throttle problem

[jasonk1747]

Motor idles fine and will run on full throttle for 20 mins then seems to starve. All fuel hoses, the powerpack and impellor have been replaced. Has been in the workshop several times over the past 3 months. Mechanic can't seem to find the problem. Fuel replaced twice. New plugs. 165 psi on both cylinders. The only thing I have left is to check if there is any rubbish is in the tank. A mate suggested that it could be an exhaust issue. Any suggestions? Fishing is becoming unenjoyable and the minister for home affairs reckons I should give up.

 

[tashasdaddy]

Re: Evinrude 50 (Lark) full throttle problem

ok Jason, we know you have a 50 hp evinrude lark, what year or model # ??? you say you've replaced power pack, but nothing about coils. the reason i ask as this sounds like a classic case of a coil getting warm and failing. have you checked spark, on each plug when the motor acts up. i had a 73 50 that did that, and it was a bad coil, that failed when warm. what do the plugs look like when it does this, one dry? one wet? i think if it were fuel starvation, it would happen sooner. just for kicks. when it starts to act up, have some one pump the bulb on the fuel line, if the motor picks up with this, i would suspect a fuel problem. if not, electrical. throughly confused???

 

[larky]

Re: Evinrude 50 (Lark) full throttle problem

The first thing that comes to mind is the gas tank vent. but if you have had it worked on by marine Mechanic, then hopefully they checked that. about the exhaust, try and take the hood off and running it full throttle and see if that helps. if it does, then it is most likely an exhuast leak. my 50hp had an power head base gasket leak, which let exhuast gases feed into the carb thus choking it. I could clearly see the misshaped gasket on the port side of the power head.

 

[jasonk1747]

Re: Evinrude 50 (Lark) full throttle problem

Not sure of the year. I suspect early 70's. I haven't checked the plugs when warmed up but I will. Though I would've thought the mechaninc would do this as part of a troubleshooting routine. I will also check for visible exhaust leak. I have tried pumping the bulb whilst in motion and run it at full throttle without the hood with no change. I am planning to take it back to the mechanic but am gathering intelligence on what it could be before I do.

 

[tashasdaddy]

Re: Evinrude 50 (Lark) full throttle problem

mechanics are not going to take you boat out for a 1 hour cruise. unless you want to pay him $80-100 an hour, to go boating. the tank vent is also a good idea, but if pumping the bulb made no difference, i doubt that is it. take the boat out, when it starts acting up. with insulated pliers pull one spark plug wire. listen for a change in the rpms. if the engine dies, or the rpms drop, that cylinder is working, if no change it is not working. then replace the wire, and do the other plug.

is your mech a certified Johnson/evinrude mech? look on the transom bracket for a 1"x2" plate with numbers on it. what are the numbers. if no plate look on the engine block for a plug like in this picture, what are the #'s?

 

[jasonk1747]

Re: Evinrude 50 (Lark) full throttle problem

There is a plaque on the mechanics wall that suggests he is a qualified Johnson mechanic. The serial is 50273C which I believe makes it a 72.

 

[larky]

Re: Evinrude 50 (Lark) full throttle problem

1972 is correct