wot rpm 4200 78 johnson 85hp

[tyl]

I have a 17.5 sea nymph aluminum runabout with a 1978 Johnson 85hp manual tilt. I am running a 17p stainless prop it has had carbs rebuilt ,new tanks, new lines new coils, new powerpacks,new fuel pump . compression is 120 on all cylinders.It cruises at 3400 24 mph and top speed is 33 at 4200. with 2 people and 2 batteries 2 full 6 gallon tanks.Does anyone have a similar setup does this sound normal and tach not right or do you think I have another problem. I tilted engine either way and the tilt pin hole is third from bottom and boat runs the best.It runs good and starts good. I worry the rpms are too low. One other thing the idle screw is almost maxed out I don't know if that makes a difference.

 

[jakedaawg]

Do yiu have soft spots in the floor and waterlogged floatation?

Have you done a compression test?

Have you preformed a cylinder drop test?

 

[tyl]

compression 120 on all 4 cylinders all drop about 200 rpm when disconnected.floor is
good

 

[jakedaawg]

Have you ever been able to get it to full rpm with the tach you are useing?

 

[tyl]

No the tach is an hour meter/tach it seems to be accurate but may try another tach. I was hoping someone with a 85hp johnson or evinrude could tell me what they are getting or am i expecting too much from an old motor.

 

[boobie]

Try another tach and see what happens.

 

[Baylinerchuck]

I have the same motor, 85ML78C. Mine has a 13-3/4 x 15 prop. I used to have it on a 16' bayliner capri and also set the pin in the middle hole. I put a HydroStabilizer on it because it had a hard time getting on plane with 4 adults in the boat. The fin also improved performance pulling towables. 5000 rpm is max WOT and mine would run around 4800 WOT with the stabilizer. Max speed with 2 adults, 2 kids was about 33mph. That is a very good 2 stroke motor. I upgraded a little and put an 87 90hp Johnson with trim tilt on the same boat. Max speed now with trim on that 90hp motor is 42mph. What a difference trim makes!!!

 

[Baylinerchuck]

Also the idle screw on my 85 is also about maxed out. Nature of the beast on that old hog I suppose.

 

[jimmbo]

If your engine is in tune and the carbs are opening fully, you are over propped. Your engine WOT RPM range is 4500 - 5500. Improper engine trim will lower rpm. Is it running an all cylinders?
Your rpms don't appear out of line for the speeds you are reporting with a 17 in prop.
When the carbs were rebuilt was the timing checked, and a carb-ignition sync done?

 

[tyl]

I have manual tilt when i moved it out 1 hole it took longer to plane out rpm rose about 400. (4th hole out) no speed increase. Timing at idle 4 degrees carb sync is good as far as i can tell the boat runs perfect it is just slow