1979 Johnson 70hp will not rev up

[Hoochie]

Hi,

1979 70hp 3cycle Johnson

Had it out on the water, going great guns, full speed for 20kms or so, when it idled down and refused rev up when throttle was applied, could only tick along at a bit over idle and couldn't rev up past that. For whatever reason it can't generate power under load.

Squeezed bulb to see if pump was stuffed, no difference.
Timing, link and sync is all good.
Compression is good.
Spark is good.

Now I'll have a go at carb rebuild cos all I can think of is that something has managed to clag up the carbies.

Can anyone throw a few ideas at me regarding what would cause this?

Cheers,
Chris.

 

[mrcj001]

Re: 1979 Johnson 70hp will not rev up

Double check the quality of the spark. It sounds like its dropping a cylinder. Use a cheap spark tester so you can see the spark. Make sure its consistant and can jump 1/2 inch or so.

Some ignition componets will start to fail once the engine gets hot. Test spark on cold engine then on a hot engine.

Also, if the foam insulation inside the cowl cover is deteriorating, get rid of it. The carbs will suck in small bits and cause carb issues. I found some in my carbs last time I rebult them and found some of that stuff in the bowl.

 

[soundchaser]

Re: 1979 Johnson 70hp will not rev up

Yeah' I put a fine brass screen over the holes in my air box. Take a vacuum cleaner to the screens once in awhile.

 

[jjacobs007]

Re: 1979 Johnson 70hp will not rev up

I have the same mtr and it does the same thing once in a while.runs good at open throttle then when i slow down to wake less speed i have to slowly take off and wait for engine to change noise,which lets me know when i can go.rebuilt carb and all other stuff except changing the stator and timing base.i believe theres a problem in my timer base but i havent seen one on ebay for cheap in over a year.hopefully yours is just the carbs!

 

[Hoochie]

Re: 1979 Johnson 70hp will not rev up

Ta for that.