Yamaha and Johnson motor swap

[BassAssassin2009]

This is my first time swapping motor from a 1977 Johnson 85hp 115 to a 1990 Yamaha 70hp on a 1986 Procraft 1650 bassboat. There are 4 bolts holding the johnson...are these bolt patterns universal for all motor? Do I have to drill new ones? The Yamaha is oil injected while the Johnson is oil and fuel mix. Do I need to change the fuel tank? Hope I dont have to change the fuel tank because its pretty tight getting it out.

 

[rodbolt]

Re: Yamaha and Johnson motor swap

bolt pattern is universal.
dont sweat the premix as long as its not old or soured. if fuel is good just run it until its gone. wont hurt anything.
make sure the 70 is propped as close to 5500 as you can at WOT and dont even attempt to hook up a water pressure gauge.

 

[BassAssassin2009]

Re: Yamaha and Johnson motor swap

So how do I know if the fuel is old or still good? The Johnson overheated last year in August and that was the last time I put in gas and took out the boat. What about the fuel filter? The Yamaha is coming off an older Crestliner that has a fuel filter.

Thanks for the quick reply...I'm making the swap this weekend and will post pictures and updates.

 

[rodbolt]

Re: Yamaha and Johnson motor swap

ditch the fuel.
when was the last time the yamaha was known to run CORRECTLY ?
if unkwon clean the carbs,fuel pump and lines then replace the water pump and tstat.I would do the cooling system at a minimum.
as you found out, lack of maint can cost way more than doing a bit sometimes the engine only has to overheat once.

 

[BassAssassin2009]

Re: Yamaha and Johnson motor swap

The yamaha impeller and bottom unit oil was replaced a couple weeks ago. The motor ran fine and it was last ran a week ago. I'm going to replace the thermostat and sparks and wires when i do the swap this weekend.

 

[rodbolt]

Re: Yamaha and Johnson motor swap

dont sweat the plug wires.
they simply dont fail.
that and they are epoxied into the ign coil.
do check the little spring in the plug cap for rust,the cap and spring can be replaced seperatly from the coil assy.