50hp bogs after 30-40 minutes of running (runs ok with cowl off)

[ashorvath]

I have been hitting my head against the wall trying to figure this out. It is an 87 merc 50hp 2 stroke and it will bog down after running for about 30-40 minutes. It happened a lot faster in the summer time (about 15-20 minutes).

I have replaced fuel pump, good fuel, ensure good tank breather.

I figured it was worth a shot to take it for a good run this past weekend but this time the cowl was taken off as I am pretty sure it has to do with temperature. The motor ran great without issue...

so what to do next?

 

[roscoe]

Ignition coil may be failing when heating up.

Or, you have an exhaust leak and the exhaust is actually smothering the engine, starving it of sufficient oxygen.

When it starts to bog, and you remove the top cowl, does it immediately run better?

 

[racerone]

Or is one of the plug wires shorting to cowling latches.----Up to you to look at that possibility, I can not look at that for you.

 

[Texas Prowler]

How about a vacuum leak at the top or bottom crank seals and or carb gaskets. That would give the same symptoms.

 

[QBhoy]

Is it cooling alright ? Strong tell tale stream of water ?

 

[ashorvath]

cooling well - strong pee stream. I will try and take it out next week and give it a run with the cowl on and have it bog then pop it off to see if it corrects immediately,

thanks so far guys!

 

[Texasmark]

Nobody mentioned it but if your "to be done test" proves positive, check the air inlets at the top rear of the cowl. Around here dirt dobbers just love to get in your engine and pack it with mud, air inlets on the cowl are no exception, air inlet box (inlets) over the carbs is another hot spot.

Also for a data point, if you pop the cowl and no immediate response, how long does it take to get one and what kind of ambient temp.

One thing that can get neglected over the years is heat sinking of the electronics to metallic parts of the engine proper. Sometimes white thermal compound is applied between the sink and the item to be "sunk". Over time this dries out and not only fails to provide the necessary conduction path, but impedes it due to a smaller contact area from it shrinking as it dries out. The "how long" question above would help in identifying this as a root cause.

 

[Learningboating359]

My problem is similar! I bought an older alum boat with a 84’ cylinder 50 hp Mariner and I got it on the water after replacing the fuel lines and put new diaphragm gaskets on carbs! Brought it out for the 1st time on the water and open up the throttle and it ran well for 30 minutes until I slow down and then began bog down as though it wan’t getting fuel! Finally died and would not start! Had to get a 40 minute tow back to Landing! But started when I got to the landing! Vapor lock?

 

[roscoe]

not vapor lock.
Probably dirt plugging the fuel pick up in the tank.
Or overheat from a weak water pump.