'78 johnson 85hp rpm

[michael85hp]

First time on forum. Just replaced my 85hp force with '78 johnson 85hp. The boat it cam from did not have a tach. I was wondering if anyone knows what the RPMs should be a wide open throttle.

 

[HighTrim]

Re: '78 johnson 85hp rpm

Depend on the prop, set up, etc... But I believe you should be in the ballpark of 5200-5700, someone correct me if im wrong.

 

[michael85hp]

Re: '78 johnson 85hp rpm

I am only getting about 4600 at 35mph. And the motor does not sound like it is running all out. i had a problem with a coil shorting out right after i bought this motor took awhile to find the problem. now i am running on all 4 cyl. however there is still a little miss. knowing the rpms gives me a number to shot for.thanks for the info.

 

[HighTrim]

Re: '78 johnson 85hp rpm

Not a problem, I would get a second opinion on that though first before aiming too high.

 

[iwombat]

Re: '78 johnson 85hp rpm

That's the right range. What prop do you have on it?

 

[michael85hp]

Re: '78 johnson 85hp rpm

i do not know what prop is on it. it was on it when i bought it and it has no markings anywhere on it. i had a 13 3/4 x 15 from my old boat that would fit on it. i tried it and it would run up to about 1800 rpm then the prop would start to spin in the water like a tire on ice. the boat seems to plane off as good or better than it did with my old force 85hp.

 

[GeneH]

Re: '78 johnson 85hp rpm

What boat is this on? 5500RPM would be a good number and a lighter runabout should take a 19 or 21 pitch prop. Heavier boats- 17 or less.

 

[HighTrim]

Re: '78 johnson 85hp rpm

You "spinning tire on ice" analogy is a spun hub. You will need to replace that prop or get the hub re packed. Fixing it is cheaper, but if the prop is in bad shape, might be better to just buy a new one when theyre on sale this fall. If you want to test it for sure though, draw a line across the hub and prop, or use the cotter pin as a center line, run the motor in the water until you hear it "spin" , pull it out and inspect the lines. If they are no longer lined up, your hub is spun.

 

[iwombat]

Re: '78 johnson 85hp rpm

HighTrim, he was talking about a spare prop he tried out. Not the one he's running now.

 

[michael85hp]

Re: '78 johnson 85hp rpm

the 15 pitch i tried had a new hub that i had just bought to try on this motor. just by comparing the two props i was guessing about a 19. i took the boat out today it ran pretty good no real problems. still has a little miss that you can fill even at wot. the last major problem i found was a cold solder on one of the new plug wires i just changed. i redone all the connections on both coils today hope that takes care of the miss. i will know more sunday after the next fishing trip.

 

[michael85hp]

Re: '78 johnson 85hp rpm

this motor is on a 89 quantum 17ft. it plane off pretty good with the whale tail on the motor. this boat is pretty heavy. the best i think i could hope for is about 38 to 40 mph is i can get the 5500rpm. today was the first fishing trip in a year that i did not have to work on something. so life is getting better.

 

[Boilermaker]

Re: '78 johnson 85hp rpm

Some info that may help you!
I have a 1978 115hp(99.5 cu.in.)...the bigger brother to your 85hp(92.6 cu.in.). The target rpm is 5000 to 5500rpm, but most in the know say the higher the better...try for 6000rpm & you will be happy!
I have my engine on a 1971 180 Westwind (17.5 ft & very heavy). I run a Turbo 15" Stainless Steel prop & get close to 5500rpm around 38mph on the GPS. I also have a 17"pitch Stiletto Stainless prop, but I like the holeshot & all around performance of the 15" Turbo!
I also have the Johnson aluminium prop from about 4 years ago...I didn't like that prop much at all. The original prop that came with the engine back in 1978 was an OMC 19' pitch...but they didn't use any cupping then. The turbo prop has very agressive cupping & I can trim the engine very high & get much better performance.

Does your engine have Tilt-n-Trim??? I noticed you mentioned your rooster tail and was wondering if you have a fixed trim or if you can adjust while underway???
Good luck!
Jim

 

[michael85hp]

Re: '78 johnson 85hp rpm

i think i still have either a fuel or ignition problem. i idles good and it sounds smooth at wot. however when you hammer the throttle it will hesitate a few time then it will smooth out. i have cleaned and rebuilt the carbs, rebuilt the fuel pump, and replaced the power pack and both coils on one side. i have also disconnected the rectifier. this is the best it has run since i have had it, but i am still missing something. i could still be the carbs or the fuel pump. i play a mechanic in real life however i am not a boat nor a car mechanic. yes i have tilt and trim. the boat planes off pretty quick however i could not pull a skier at this point. sometimes it take a little longer to plane off i think due to what i am missing.

 

[michael85hp]

Re: '78 johnson 85hp rpm

is there any reason you can not use a electric fuel pump and do away with the bulb and back flow preventer.

 

[Chinewalker]

Re: '78 johnson 85hp rpm

I would not use an electric fuel pump. If the battery goes dead, you're dead in the water. The pressure pulse pumps are a reliable system. The pumps can be rebuilt and the kits are inexpensive.
- Scott