johnson 75hp 1979 wont go past 1400rpm under load

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,047
????---You will not get spark on this motor by turning the flywheel by hand !!
 
Joined
Nov 6, 2014
Messages
50
Hey Blueberry where is it :bump2:dont make me come a knocking for it:love-struck:

LOL wish I had some good news for you guys, but it turned out to be a disaster :facepalm:
I took the boat out on the sunday and although at first the engine wasnt running too good but then it kicked right into life all of a sudden and I was getting up on plane and hitting about 28mph :D compare this to the 8mph i used to get :facepalm: Every now and again though it would start acting up and almost dying on me when I advance the throttle but then all of a sudden as if another cylinder started firing it would take off. I have a feeling this is due to me having to use the old powerpack as the problems is almost definatly electrical.
According to my tach (which is just a cheap digital one from ebay) I was only getting 2400rpm at wot at 28mph, Although the tach might not be really accurate Im sure I am still missing 1-2 k rpm. Hopefully the new powerpack sorts this out.

However the next day I decided to go out for a couple of hours for another test run as the day before went pretty well.
I was just at the slip about to launch the boat and when I jumped out the boat my phone jumped out my pocket and fell to its death :grumpy: I use my phone for gps and I had all my routes save on this phone too. Not to mention the videos from the day before were also lost im afraid.
Soooo I went out anyway and I used my partners phone for gps along with my navigation guide, but this phones gps was not accurate enough (out by 20 - 30 metres) so long story short I ended up in waters I wasn't experienced with and all of a sudden - right in the middle of the loch - I must have hit a massive rock and my gearcase took the hit and smashed, it threw my outboard right up too and damaged the tilt swivel bracket as well :blue: I managed to get home with my little auxillary 2hp outboard.

All in all a really expensive and unlucky day. My outboard is now in bits waiting for parts, a gearcase and swivel/tilt bracket. I was gutted at the time but I suppose it could have been alot worse, so I am now off the water for a few weeks till i repair all of this :mad-new:
 
Last edited:

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
Yep that read like a disaster story all right:smow:
Ive never had an outboard fly up but id imagine thatd be pretty freaky
Look forward to the next update:clap2:
 
Joined
Nov 6, 2014
Messages
50
another bit of advice needed here :)

I have ordered a second hand gearcase from ebay, it looks identical and the part number listed is 385560 wich matches the part no. listed on the parts catalouge on boats dot net and crowleymarine. The problem however is the pictures on ebay of the case I have bought looks like a different bearing shifter assy, I am thinking it is the hydro shift model case I have purchased and mine is just plain mechanical.
Does anyone know if the case will be the same and will accept all the internals from my old case.
There has been a supercession as the 385560 is now 393752.
does anyone know if there was a major change when the part superceded.
Also the skeg on mine slopes at the front and the one I am buying just drops down verticaly.

Again thank you for everyones help and advice
 
Joined
Nov 6, 2014
Messages
50
Well a bit of an update here. New tilt bracket and gearcase eventually arrived and its all put together now. I took it out for a water trial and it was much the same as my last outing.
I am suspicious about a couple of things here and hopefully you guys can help.
1st problem is i recently purchased another new power pack as the last brand new one failed to spark cyl 1 and sparking on other cylinders was eratic. So i re fitted the old powerpack untill i purchased a replacement. The wierd thing is that my 2nd new powerpack (a different brand) also sparks eraticly so much so that the outboard wont start. It fires and backfires and basically all over the place. I fitted the old one again and it started and ran fine again. Whats the chances of 2 brand new failed powerpacks? Its a mystery to me :-(

Next thing that is bothering me is my rpm vs speed. According to my tacho i am at 2400rpm at 25mph (gps) i used some online calculators and this just doesnt add up. My rpm should be much higher at that speed. Can anyone clarify this?
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
have you a number on that first powerpack?
maybe your motor is a frankenstein with odd parts all over it
 
Joined
Nov 6, 2014
Messages
50
have you a number on that first powerpack?
maybe your motor is a frankenstein with odd parts all over it

Yip I will check that next time I am the boat but so far all parts seem to be correct as per parts diagram, I have done a full rebuild on this motor so many new parts have been ordered and fitted with no problems. Maybe just bad luck with 2 new failed power packs, My fault really for trying to save a few $. I am going to order a CDI one this time along with CDI coils, that way my full ignition system will consist of CDI components :joyous:
 
Last edited:
Joined
Nov 6, 2014
Messages
50
Well a positive update this time :laugh:
I purchased a second hand 15p prop to try out and the difference in performance is amazing :happy:
Pops up on plane in seconds and gets to 28mph in no time at all.
According to the online prop calculators my engine is running right in middle of the max rpm range.
28mph with aprox 10% slip with a gear ratio of 2.44 equates to aprox 5200rpm
So obviously my tachometer was WAY out as at this speed the tacho was showing 2700.
It is quite suspisious that this is almost half of what the true rpm is so leads me to believe the tacho is not set up for a 2 stroke, I have went through the settings on the tacho and made sure it was set to 2 stroke so it must be an internal fault.

Lessons learned : -

Dont buy cheap universal tachometers

Realize that experimenting with different props can save me a load of head scratching

and most importantly keep away from shallow water where big rocks are hiding :facepalm:


I think after all this I can put this thread to a happy ending
Although I replaced quite a few parts I can safely say from my findings is that the cause of bogging down under load was a combination of a bad stator and the prop having too high a pitch. And obviously the faulty tach made these problems less evident at the time.

A big thanks to all of you that helped me so much in tackling this issue and Im sure this thread is going to help others in the future.

Happy boating everyone :laugh:
 
Top