1976 Johnson Seahorse 70 HP will not reach full throttle

[gilligan2032]

I just bought my first boat!!! It is a 1976 cobia 14 foot tri hull. It has a 1976 Johnson Seahorse 70HP outboard engine.

The guy I bought it from had the engine serviced back in July. He gave me the receipt for the work, and this is what was done on the boat...

"Checked engine compression- 120 on all cyls.
Checked spark
Checked Gear oil- dirty but no water in housing
R/R starter
Installed new cable ends for starter
Attempted to run engine, engine will not run
Changed gear oil
Carbs were dirty and needed to be cleaned.
removed carbs disassembled and soaked.
rebuilt carbs with new kits.
R/R water pump with kit.
Started engine after rebuilding carbs and found voltage regulator not working properly and engine running hot.
R/R voltage regulator and changed thermostat and gasket.
started engine, Engine did not run hot on shop hose and charging system is working properly. needs lake test
NEW PARTS INSTALLED: starter, carb rebuild kits, water pump kit, voltage regulator, thermostat kit, gear oil, fuel pump kit"


The engine starts, idles and shifts great. It sounds like it is running well. The boat accelerates nicely as I increase the throttle, however when I get to about the 3/4 throttle position, the engine lurches a little and going any further on the throttle does not increase engine power. It's like the boat is stuck at 3/4 power. When the throttle is positioned exactly at 3/4 the engine actually has slightly more power than at 100% throttle.

My max speed calculated by GPS is 25 MPH

I would GREATLY appreciate any help!!!!!

[oldcatamount]

That sounds like a throttle cable adjustment to me but, I would ask the mechanic that did the work to have a look at it before you try to re-adjust it.

[fishfeatures]

what size prop? is it slipping ?

[boobie]

If the carbs were just rebuilt on it you may want to check the sync-link on it to make sure the throttle plates are not going over center at wot.

[gilligan2032]

Prop slipping?

I just bought a new aluminium prop that is in the mail!

The prop that was on there was somewhat badly damaged. It was pretty warped and had very rough edges. I wasn't thinking that it could account for THAT massive of a speed loss though? And I didn't think it explained the 3/4 throttle issue.

When I took the prop off today for the first time, I noticed that the nut that was holding it on was very loose. It unscrewed it with my hands after I took the pin out. There are teeth that keep it from slipping though, right? Does that nut need to be super tight?

I have pictures of the prop, i'll post em!

[gilligan2032]

The prop is 13 3/4 x 15

I ordered the exact same size today from iboats!

It has 13 teeth on it

[gilligan2032]

Full size= http://www2.picturepush.com/photo/a/...mg/7280295.jpg

[gilligan2032]

Thanks for all the input guys!

I took the boat out fishing yesterday (caught some nice striped bass!), and noticed some more strange things about my issue. When I run at full throttle into choppy water, the motor was kicking into full power! The boat was going just over 30mph into the wind and into the chop. The motor only does about 70% power and 20mph in smooth water. Could something be loose and the added vibration is helping it out?

I made a video showing the issue, I'll upload it soon.

[gilligan2032]

I noticed something that might be a BIG PROBLEM, but I would love your input!!

My automatic choke solenoid is STUCK!!!!!!!!!!!!! Does the automatic choke solenoid on my 1976 Johnson 70hp adjust the choke with varying degree? Or is it a binary, full on, or full off mechansim? There is a full choke switch on my control panel next to the steering wheel. I connected a multimeter and read 11 volts going through the solenoid after holding down the switch and noticed that my screwdriver was attracted via the electromagnet. HOWEVER, the stupid metal piston part does not slide easily and is stuck basically, so the solenoid does not work. I was thinking about filing out the inside of the plastic solenoid to try and get it to slide better. Good idea? Any other solutions? It looks like that solenoid is no longer available to purchase online ... http://www.**********/parts/search/B...OLD/parts.html

part number 19.

I don't think I have EVER run full throttle with the choke in the manual off position. The choke solenoid is connected via a spring to the choke flappers in the carbs, and I guess with it being held half closed that only the bouncing of going into the chop allowed them to flap open and thus allow enough air in for full throttle? Could this have been my issue all along?

[kinsy2323]

I have an 81' 35hp Johnson. I don't know much about motors but my throttle does odd things. I can push the throttle almost all the way down before it starts moving and then it speeds up. Not sure if it's the same problem as the poster or a different problem. Other than having a motor wire that obviously corroded and severed which I replaced myself, the motor works awesome! A little louder than I like but hey...it's a 1981!