75 Johnson Problem

[highwayman]

I have a 75 Johnson on a 16ft mckee craft. it runs great but: When i fire it up it runs great I will take off and its runing like a champ 25 to 30 miles an hour but youll go for a while then youll stop and when you try to go again it wont go. it will only get up to about 4 or 5 miles an hour and thats with the throtle all the way down. Some times this wont happen for long time like usally on the way back. Dose this sound famillar to anyone?

 

[jvaccarino]

Re: 75 Johnson Problem

Are the RPMS only going to a certain limit when it slows down? I had a similar problem where I could only rev to 3500 rpm.. due to a faulty temp sensor..

 

[highwayman]

Re: 75 Johnson Problem

Yeah i beleive the rpms are staying low, do you think a new sensor will fix it,and can I change it?

 

[jvaccarino]

Re: 75 Johnson Problem

I believe it's a simple fix.. Only 2 or 3 wires to disco and reconnect. Local marine should carry, probably would come with a manual also.. I would investigate more tho before you take my word for it.. But if the RPMs are not increasing, thats probably the cause..

 

[highwayman]

Re: 75 Johnson Problem

If that isnt the problem what else could it be?

 

[highwayman]

Re: 75 Johnson Problem

It only goes to 3000 rpms, but if i put in nutral and rev it it will go higher. Is that consistant with a bad sensor.

 

[highwayman]

Re: 75 Johnson Problem

just a bump so more folks might be able to help:'(

 

[F_R]

Re: 75 Johnson Problem

Let's start by finding out what year that 75 is. Maks a big difference in trying to figure out what's wrong. Model number?

 

[tashasdaddy]

Re: 75 Johnson Problem

i.m with FR, start at the beginning, there are a lot of 75's what have you checked?. last time it was serviced?

 

[highwayman]

Re: 75 Johnson Problem

Its a 1981, I just had it serviced and had new motor mounts installed.

 

[F_R]

Re: 75 Johnson Problem

Suspect a bad coil breaking down when it gets warm, causing it to not run on that one's associated cylinder. Are you running QL77JC4 spark plugs? You should be.

 

[highwayman]

Re: 75 Johnson Problem

I will go ahead and get some new spark plugs. How hard is it to change the coil,and is there a way to check if the coil is bad?

 

[tashasdaddy]

Re: 75 Johnson Problem

if you have an induction timing light available, i'd take it for a run until it starts acting up, the connect the light to the battery, and put the induction pick up, on each plug wire. it will show whats, not firing. coils are simple to replace. just watch where the wires come from, and replace in the right spot. do one at a time. make sure the connections are clean and tight.