1975 Johnson 6hp not getting full power

[jener8tionx]

I have a 1975 Johnson 6hp (6RL75R converted to standard length). The engine starts pretty easily and will run just fine, but I can never fully plane out in my 12' aluminum boat. The RPMs sound like they are not as high as they should be at WOT. Any ideas?
So far I have:
Cleaned the carburator
Set the points
Checked my fuel mixture (50:1)

Pics:

 

[raczekp1]

Re: 1975 Johnson 6hp not getting full power

hi.
Are you sure that unit is not firning on one cylinder?
I saw you change gasket between powerhad and midsection(maybe this is not fiting good),did you replace cork gasket where drive shaft goes into powerhad?or maybe change front powerhad gasket

 

[BigB9000]

Re: 1975 Johnson 6hp not getting full power

...yeah RTV probably wasn't the best choice for a gasket.
why did you remove the powerhead? on that new of an engine, you should have had a standard shift rod with an extension. I can see why you would pull it for the short convert.

Anyway, if the cork seal/o ring on the crankshaft is bad, this could lead to your problem. as, that is where you get your crankcase pressure from, which powers your fuel pump.

So, if the engine is being starved for fuel, this could be the cause.

While running, you could squirt some premix in via a spray bottle, and see if it livens up. If so, you got a fuel problem. If not, could be a spark problem.
Check all.

check compression first though.
if you have one cylinder with 60psi, and the other with 3, well, you know...

 

[jener8tionx]

Re: 1975 Johnson 6hp not getting full power

Thanks for the replies. I did not do the RTV part. My dad had the unit serviced and shortened at a local shop when he bought it in 2005. Are you saying I should just tear into it and replace this seal? It is fully possible that the shop had no idea what it was doing. I thought it felt like the engine was getting too much fuel and not enough air or spark, but I will try squirting premix into it at WOT today and see if that helps.

Also, is there an online service manual for this engine?

 

[AlTn]

Re: 1975 Johnson 6hp not getting full power

the roller on the throttle linkage should be centered with an embossed mark on the timing cam plate, and the throttle plate just beginning to open, when you have the throttle grip in the start position. Rotate the flywheel cw as you put the motor into gear and assure that the throttle plate becomes horizontal as the roller reaches the highest point on the cam. This advances the throttle opening just ahead of the spark advance....that's about it as far as coordinating the two

 

[jener8tionx]

Re: 1975 Johnson 6hp not getting full power

Altn, I am confused by what I am supposed to be matching with what.

Here is me checking the points. Am I doing it right?

Does the throttle cam need to be adjusted?

 

[jener8tionx]

Re: 1975 Johnson 6hp not getting full power

Just noticed something... The throttle cam does not start to open the butterfly on the carb (Marked A) until the throttle is in the "Fast" position. The part marked A does get to the peak of the throttle cam before the throttle does not open further and the throttle cam hits the intake manifold by the pull starter.

 

[Tim Frank]

Re: 1975 Johnson 6hp not getting full power

Do you know what type of tubing is between the carb and fuel pump?
There is a lot of generic clear tubing that is not really up to this task....make sure it is rated OK for the job. It is not likely related to your current problem, though.

Has this motor ever provided the planing performance that you are after?
I have the E/rude version of this, also on a 12 footer....and with just me (~175 lb.) I need all extra weight forward, and me leaning well forward too, and it just makes it onto the step.
When my daughters were younger, they would zip around, and by themselves the thing would fly ~ 15-18 MPH ....but they weighed ~ 70 lbs

With my 8HP E/rude, the 12 footer planes out fine. You are probably just underpowered slightly.

The 6hp I am afraid will be marginal at best.


On the throttle issue, when the throttle twist grip is turned to what is the "max" , is the throttle butterfly horizontal?
Don't worry about the actual corresponding scale on the twist grip....those tend to lose accuracy over time.

 

[jener8tionx]

Re: 1975 Johnson 6hp not getting full power

I used a mirror to check the throttle butterfly and it is perfectly horizontal when WOT. Maybe I am expecting too much from this motor. I will try it with a lighter pilot in the boat to see if that helps.

 

[Tim Frank]

Re: 1975 Johnson 6hp not getting full power

Has this motor ever provided the planing performance that you are after?

^^^^ you didn't answer that question. If the performance has dropped off, that is a different thing.

 

[AlTn]

Re: 1975 Johnson 6hp not getting full power

GenerX...go to shop2.evinrude.com...and find your year and hp...go to carb and use the magnifying glass feature to enlarge the diagrams..see the throttle to throttle cam linkage?..notice it has a roller on top of the arm?..yours doesn't and it appears that it just slides on the cam plate...regardless, the part that slides should just be about centered with that horizontal embossed mark which appears to be about 1/4" from your letter A...your throttle plate inside your carb throat should just begin to open with these aligned and your twist grip in the start position....as you have the motor in gear and are able to advance the throttle to wide open...<WOT>...your throttle plate should be horizontal and your throttle linkage to the timing cam should be at the highest point on the cam plate....your point setting looks good if that's a .020 feeler...an .022 should be too thick to fit...good pics btw....your coil and coil heels look a smidge close to the inner flywheel as it has rubbed them some...the heels should be exactly even with those machined bosses directly under them..hope this helps

 

[jener8tionx]

Re: 1975 Johnson 6hp not getting full power

^^^^ you didn't answer that question. If the performance has dropped off, that is a different thing.

I have never had good performance, but we have only owned it since 2005 when it was not running at all.

 

[jener8tionx]

Re: 1975 Johnson 6hp not getting full power

Tim Frank was right. I took it out today and rode the boat solo. When I sat in the middle seat (couldn't steer) the boat planed out and the RPMs got where they belong. I guess I was just expecting a little too much from a 6hp outboard. Thanks to all for the replies. At least I have now checked all of those things and the motor runs much better than before.

 

[Tim Frank]

Re: 1975 Johnson 6hp not getting full power

Tim Frank was right. I took it out today and rode the boat solo. When I sat in the middle seat (couldn't steer) the boat planed out and the RPMs got where they belong. I guess I was just expecting a little too much from a 6hp outboard. Thanks to all for the replies. At least I have now checked all of those things and the motor runs much better than before.

Check your trim as well....drop it down one notch and see if that helps....

 

[BigB9000]

Re: 1975 Johnson 6hp not getting full power

replace that not-fuel line too.

Also, the pic is wrong, as my 75 and 76 also have that plastic cam follower.

Check out http://forums.iboats.com/showthread.php?t=408454
for info on that RTV gasket.