75hp johnson bog issue

[75-JOHNSON]

engine is a 1976 75 hp johnson, on a 15 foot trihaul boat. the carbs are clean, and prop is correct. The issue is, about 25% of the time when you go to take off it boggs out and dies. this is especially noticeable if you slowly increase the throttle. if you give it full throttle right off the bat, MOST of the time it will quickly pick up rpm and off you go, but sometimes not. It seems like if the fuel is mixed slightly rich, the condition becomes worse. if i mix the fuel 50:1 or slightly leaner, it performs better.

my question is, is this a carb issue, if so what does a person set. Also, would burning premium fuel better the situation?

What are opinions on running 100:1 amsoil premix?
thanks for your time.

 

[Joe Reeves]

Re: 75hp johnson bog issue

Do not attempt to use a 100/1 mixture, stick to the 50/1 mix.

Pump the fuel primer bulb, acting as a manual fuel pump. If this cures the problem, either the fuel pump is faulty or a fuel/air leak exists at some point between the fuel pump and the fuel supply. If the above does not cure the problem, the usual problem is that the throttle butterflies are opening slightly too soon or something has been overlooked within the carburetors.

With the carburetor roller NOT touching the throttle cam roller, all of the carburetor throttle butterflies must open and close at the same time.

Butterflies should open when the scribe mark on the cam is dead center with the throttle cam roller.

Did you make sure that the high speed jets were perfectly clean? If not, do so.

 

[jtexas]

Re: 75hp johnson bog issue

take a look at the idle set screw...is it butted up against its stop?

premium gasoline won't help.

this pic shows a later model than yours, but the all the parts are there:

 

[boobie]

Re: 75hp johnson bog issue

Try hitting the choke when it does this and see what happens.

 

[papaterk]

Re: 75hp johnson bog issue

I had the same problem, tried pumping the bulb, no help. Tried pushing in the key trick and it would surge and run up to speed. Found # 3 carb high speed jet with just a tiny bit of trash in it (after rebuild) That fixed it. I got all my troubleshooting advice right here. These folks are great!

Thanks Guys,

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