Johnson 7.5

[paul-j]

I have a johnson 7.5 on the duck boat.... I'm pretty much a new guy with motors, and I was running it not too long back and it just started making a knocking noise and a friend told me I threw a rod... I was hoping this wouldnt happen considering i just bought the motor but could someone provide me with some assistance please? thanks paul

 

[crb478]

Re: Johnson 7.5

What model number do you have? Have you done a compression test? Once you provide the year and model number it will be easier to help you.

 

[paul-j]

Re: Johnson 7.5

Ill go look right now...
And no I havent done a compression test.
Thx.. hold on one sec and Ill get it

 

[wbeaton]

Re: Johnson 7.5

Do you hear any knocking, binding or anything out of the ordinary when you pull the rope out? Was there a loss of power when it started knocking?

What fuel ratio are you using? 24:1, 50:1, 100:1 etc.

 

[paul-j]

Re: Johnson 7.5

model number is J8RCIA SERIAL NUMBER... F5740469

these should be right.. the I in the model number might be a T but i dont think so

 

[paul-j]

Re: Johnson 7.5

wayne... there was no knocking when i pulled the rope it was only after i took off.. i use about 40ish to 1. and it lost a good bit of power after the knocking

 

[wbeaton]

Re: Johnson 7.5

Its a 1981 unless the "I" is a T then its a 1983. Takes 50:1, but that's not your problem. A broken connecting rod is.

 

[paul-j]

Re: Johnson 7.5

Ah crap! Which one is that? lol Im pretty illiterate with these outboard motors and I have no manual.. and how much should I expect to pay?

 

[wbeaton]

Re: Johnson 7.5

It would be cheaper to buy another motor than to have it done at a shop. In fact, I'd suspect most shops wouldn't do the work because of it. You could consider finding a good used powerhead or rebuilding it yourself, but the block may be scrap depending on the damage. You'd really be farther ahead buying another one and using the one you have for parts.

 

[wbeaton]

Re: Johnson 7.5

However, what you should do before buying another motor is to take out the plugs and check the spark plugs. If there is a damaged plug that is the bad cylinder. You can also remove the head or bypass cover and see what the damage is.

I'd do that before I did anything else.

 

[paul-j]

Re: Johnson 7.5

okay thank you so much for your help..
it shouldnt have tooo much damage done to it.
I ran it a long time the time before it happened and it ran great.
It ran perfect the whole way to the fishing hole that day and then when i went to take off it started making the noise so i shut it off immediately.
I just went and looked at it againa and there is oil leaking from the foot.

 

[paul-j]

Re: Johnson 7.5

oh yeah, and the main reason for fixing the motor was so I could sell it..
I have a new boat now and this one has became obsolete and is burning a hole in my pocket. Any suggestions?

 

[wbeaton]

Re: Johnson 7.5

The sound is coming from the powerhead, right? Where is the oil coming from exactly and what does the oil look like?

 

[paul-j]

Re: Johnson 7.5

im not quite sure where the sound is coming from. As soon as I heard it I shut the motor off.. and the oil is coming from the spot where the foot connects to the the rest of the motor...

 

[wbeaton]

Re: Johnson 7.5

Ok, that's just unspent gas and oil from the exhaust. Check the cylinders like I suggested.

 

[paul-j]

Re: Johnson 7.5

oh okay... thank you.
I will do that tomorrow and then give you some feedback!

 

[paul-j]

Re: Johnson 7.5

OKay so I have began day 2 of the rebuild process..
I pulled the foot off and drained the oil
no broken connecting rod...
any other ideas?

 

[HighTrim]

Re: Johnson 7.5

You have to remove the head or bypass cover like Wayne said. Try removing the plugs first though and see if any have any damage as stated above.

"http://www.crowleymarine.com/brp_parts/979.cfm"

 

[wbeaton]

Re: Johnson 7.5

I'm surprised. You looked in the cylinder and found that everything was fine? When you pull the rope slowly do you hear or feel anything out of the ordinary?

 

[jbjennings]

Re: Johnson 7.5

A compression check ought to immediately identify a thrown piston rod, shouldn't it guys? Or, take the plugs out and but a pencil or screwdriver and turn the flywheel where the piston is at top dead center, go a hair further, and push in with the screwdriver gently to see if there's any play. What do ya'll think? Those are 2 easy techniques you guys have taught me in the past that were very helpful.
just a thought,
JBJ