1980johnson 85 hp top speed

[321lawncare]

motor runs awesome its on a 1985 bonito 16 ft boat cc
i just rebiult the carbs and all 4 cylenders have good blue spark
but it only runs 35 mph ?? idk why maybe prop ?? but i also have about 450 lbs of peaople on the boat 18 gal tank of fuel full ... and 3 marine batteries fishing trackle etc..

 

[Bob_VT]

Re: 1980johnson 85 hp top speed

Welcome to iboats.

Well you have parts of the equation in your post.

I will move the post to the prop forum

Is the boat new to you?
What prop is on it now? Pitch and diameter? (number's usually cast into prop hub)
1985 boat..... is the flotation dry?
Do you know the boats dry weight?
Is the bottom clean?

MOST Important ........ do you have a working Tach? What RPM's are you at when you are at WOT?

Not trying to shotgun you with questions ....just getting you ready for what will be asked

 

[steelespike]

Re: 1980johnson 85 hp top speed

If you get yourself something like a tinytach about $47 get us some numbers just one in the boat.The antivent plate just above the preop should be a little above the bottom of the boat.It should be visable at speed.In the extreme it could add 2 to 3 mph.Check on the barrel or under the prop nut for prop numbers.Check that throttle plates open all the way.Carefully adjust trim.

 

[WavyD]

Re: 1980johnson 85 hp top speed

no expert here,but with all the weight you describe in that boat,
35 sounds about right for an 85hp.
i have a 1979 85 hp Johnson and it tops out around 39-40 and i also carry 3 batteries along with gear and a 14 gal gas tank........add a few people in the boat and 35 sounds about right......at least on mine it does.
after you get up on plane,try raising your motor with your trim just a little and see if you get any more speed.

 

[321lawncare]

Re: 1980johnson 85 hp top speed

thanks guys ! ive been around boats my whole life and i know what you meen by trimming it out. but this boat if you trim it up at all it starts porposing its got the whale tail also the bottom fin is even with the bottom of boat. bottom is clean, and im pretty sure the boat isnt logged, and ill get the pitch of the prop soon thanks !!!!!!!!!

 

[321lawncare]

Re: 1980johnson 85 hp top speed

prop pitch 13 3/8x17

 

[321lawncare]

Re: 1980johnson 85 hp top speed

Ttt

 

[steelespike]

Re: 1980johnson 85 hp top speed

A 17" prop is in the ball park.In fact fairly typical.As I suggested;the antivent plate even with the bottom is the minimum usually and 1" above is not unusual.Should be visable at speed.
The foil can rob a couple of mph.Something like smart tabs or trim tabs are a much better choice.Tabs set too aggressive can steal some speed as well.

 

[RicMic]

Re: 1980johnson 85 hp top speed

I would say with that load that 35mph is right on the money. I had an 85 Johnson on a 16 Boston Whaler and with 12 gallons of gas, one battery and less than 300# of people it would do about 40 with the same prop. Going to a SST prop, did bump it up a couple mph to about 42 at just about 5500rpm.

 

[321lawncare]

Re: 1980johnson 85 hp top speed

did a compression test to day 135 on left side 140 on right
are these motor that bad on gas ! i get like 1 1/2 miles per gallon !

 

[cougar1985]

Re: 1980johnson 85 hp top speed

did a compression test to day 135 on left side 140 on right
are these motor that bad on gas ! i get like 1 1/2 miles per gallon !

general rule of thumb is 10hp =1 gallon per hour at wide open throttle.but there are a thousand and one things that can influnce the fuel milage usually for the worst.state of tune,load ,prop selection(not reaching proper mfg wot),and the list goes on and on.

 

[321lawncare]

Re: 1980johnson 85 hp top speed

i have a a 18 gal tank and i can barley go 30 miles

 

[RicMic]

Re: 1980johnson 85 hp top speed

Are you running WOT all the time? You can back off on the throttle 10 or 20% and lose very little speed, but get VASTLY better fuel economy and try it without the whale tail.

 

[321lawncare]

Re: 1980johnson 85 hp top speed

i usually just cruise like 30 mph and with out the whale tale it just porpuse

 

[steelespike]

Re: 1980johnson 85 hp top speed

porpoising; have you tried moving some weight forward.
15 gph @ 30 miles = 2 mpg in ideal setup and conditions.Simply reducing speed with out comparing other reduced speeds you could be running at the worst speed to rpm setting.best speed with an outboard can be from just on plane to about 1,000 rpm higher.trial and error is about the only way to test without a flow meter.
And dragging the foil along makes it easier to stay on plane but at what speed loss.I feel a 4 blade prop and/or trim tabs are better.
Where is the gas tank? Are the batteries in the stern?
Part of your problem may be a relatively small boat running close to capacity
where a larger slightly heavier boat might getup better due to the higher capacity rating and more surface area.

 

[321lawncare]

Re: 1980johnson 85 hp top speed

yes the gas tank 18 gal is in the stern along with 3 batteries and a 55qt cooler that is always full and a 20 gallon live well up front, the guy that set up this boat mounted the cooler as a seat behind the cc????

 

[321lawncare]

Re: 1980johnson 85 hp top speed

also i have a budy with a blown up 115 it loos like the carbs will bolt up to my motor will this make it a 115 ?? or is the bore diffrent ive herd there the same !

 

[steelespike]

Re: 1980johnson 85 hp top speed

Displacement depends on the year 115s prior to 73 are 96 cubes.Your motor is a 99.
I would say that a simple carb change isn't likely to net a 30hp increase anyway.
Maybe you could move the batteries to the cc. eliminate 90lbs from the stern
moving it to a more neutral area. Maybe you could move the cooler in front of the cc.I had a little boat with a 10 hp that porpoised at the best trim setting all I had to do was lean way forward to stop it.Obviously yours will require more but the percentage of weight may surprise you. 90lbs out of the stern and added to the forward section is kind of like a 180lb move.

 

[321lawncare]

Re: 1980johnson 85 hp top speed

my motor is a 1980 and the 115 is a 1985 they look exactly the same
and im goin to move the batteries but cant move the cooler