1985 johnson 120 low wot rpms

[bmann]

I have a 1985 Johnson 120(v4 with v6 gearcase) with a 14 1/2 x 20 stainless raker prop. The boat is a 1650 procraft. I have replaced the vro pump(took off), the tachometer and regulator rectifier. At wot it is turning 3200-3400 rpms. It is very slow planing out. The motor checks out good on the dyno. <br /><br />I have been told everything from a cylinder is down to the boat being waterlogged and to heavy for the motor to operate correctly. Procraft has no info on the weight of the boat, but I really don,t think this is the problem. I am going to try different props asap. Any help will be appreciated. Thanks

 

[JB]

Re: 1985 johnson 120 low wot rpms

Howdy, BMann.<br /><br />Can you be more specific about the dyno test? Do you get pressure, torque or hp readings? What do you get at 5500rpm? Can you kill each cylinder and get a drop in rpm under load? If it is a pressure reader, you should be able to get 2500 to 2700psi at 5500rpm.<br /><br />If the engine is in fact okay, you have the wrong prop, but you are only getting to about 60% of correct WOT rpm, which suggests a prop of around 15-16". That just sounds like too short for a 16-17' boat with 120 hp on it. That would be a prop for a 50.<br /><br />I think you need further engine diagnosis.<br /><br />Good luck.

 

[Dhadley]

Re: 1985 johnson 120 low wot rpms

Make sure the tach is set on position 6.<br /><br />Good luck!

 

[wilde1j]

Re: 1985 johnson 120 low wot rpms

I have the same engine, except it's a 140. When I bought it, I got the 2 yr warranty extension. In the second year, the motor gradually got slower and slower (went from 5400 rpm WOT to ~ 4000.) I check all of the obvious stuff and then got a dealer involved. After 6 weeks and a number of factory rep involvments, the fix was to replace the entire induction system including adapter plate and all carbs. It's run fine since and no one was able to provide a rational explanation for why the fix worked. By the way, I bought my motor from my Dad, who owned a boat shop for many years, so I'm pretty good at the simple stuff. My motor complies with a bunch of TSB's now and has been a terrific motor with no major failures in over 600 hours of operation.<br />Just thought I would share the experience.<br />Jim

 

[bmann]

Re: 1985 johnson 120 low wot rpms

I have replaced my tach and tried a 14x19 michigan wheel prop. I am only reaching 4200 rpms at wot. I also noticed melted looking wax or plastic underneath the flywheel. Could the stator be keeping my rpms down a little? Thanks

 

[Dhadley]

Re: 1985 johnson 120 low wot rpms

Is the tach set on position 6?<br /><br />Let us know!

 

[bmann]

Re: 1985 johnson 120 low wot rpms

yes. It is on position 6.

 

[Dhadley]

Re: 1985 johnson 120 low wot rpms

OK, good. We know the prop is most likely OK. If the motor "checks out OK on the dyno" then the motor, including the tach (which should have went to 5500+), is fine. Do as JB suggested and double check your dyno numbers (actual vs target). If the dyno says the motor is OK you can look at the boat and set up for things like trapped water, etc. The dyno has seperated your motor from the boat. You gained a lot of RPM by switching props. Again that suggests too much weight by a lot. That looper makes good torque an should work just fine on that boat. I would even thing your Raker would be close to the right prop. <br /><br />Good luck!

 

[bmann]

Re: 1985 johnson 120 low wot rpms

I am not sure of the dyno numbers. I had a local guy check it out. I am thinking he could have missed something. I know he missed the stator. Could the stator be causing it to not reach the correct rpms?

 

[bmann]

Re: 1985 johnson 120 low wot rpms

could the stator be keeping my boat from reaching the correct rpms?