Considering rebuilding 1990 Johnson 70hp - questions

[ShoestringMariner]

I have a running 70 hp with a worn out powerhead. Compression is so bad, I’m surprised it runs. I’ve noticed the crankcase has been opened before which makes me question why. I can only assume it was to be rebuilt once before. Would it be opened for anything else?

in any case, because I’m wondering what sort of internal wear that I’ll be dealing with, it has thinking about rebuild kits.
I noticed rebuild kits have std., .010” up to .040” oversized pistons. But I don’t think I’ve seen undersized bearings for the crank. Are crank journals not usually ground under size like automotive rebuilds typically are?

Do cranks in 2 strokes not see wear like automotive cranks do? Ie; Roller bearings don’t induce the same wear that oil bath/Babbitt bearings do?

 

[PhilSch]

I have a running 70 hp with a worn out powerhead. Compression is so bad, I’m surprised it runs. I’ve noticed the crankcase has been opened before which makes me question why. I can only assume it was to be rebuilt once before. Would it be opened for anything else?

in any case, because I’m wondering what sort of internal wear that I’ll be dealing with, it has thinking about rebuild kits.
I noticed rebuild kits have std., .010” up to .040” oversized pistons. But I don’t think I’ve seen undersized bearings for the crank. Are crank journals not usually ground under size like automotive rebuilds typically are?

Do cranks in 2 strokes not see wear like automotive cranks do? Ie; Roller bearings don’t induce the same wear that oil bath/Babbitt bearings do?

You are correct, the journals on that crank are never ground and there are no undersized bearings. When doing a rebuild mic the journals (there is a spec) and inspect journals for damage. You can put in new bearings but it's possible there is no measurable wear and bearings look so good you won't even need to replace them.

 

[ShoestringMariner]

You are correct, the journals on that crank are never ground and there are no undersized bearings. When doing a rebuild mic the journals (there is a spec) and inspect journals for damage. You can put in new bearings but it's possible there is no measurable wear and bearings look so good you won't even need to replace them.

Ah so that explains kits with and without main bearings!

so that brings a new question; how does one determine wear on main roller bearings?

 

[Faztbullet]

OEM Manual will answer all your question. Tear it down first before buying anything to see if worth repairing.

 

[ShoestringMariner]

OEM Manual will answer all your question. Tear it down first before buying anything to see if worth repairing.

Thanks, defiantly. I won’t buy another Seloc manual.

 

[PhilSch]

Ah so that explains kits with and without main bearings!

so that brings a new question; how does one determine wear on main roller bearings?

If the journals measure well (not close to the minimum) and the bearings show no evidence of excess heat (blue) and feel good I would reuse them. If you are unsure what feels right and what doesn't find an experienced mechanic to look at them. Those bearings generally outlast the rest of the motor. Having said that if you would like to wring out another 20 years or more of use just replace them as they are not that much money in the big picture.

 

[ShoestringMariner]

If the journals measure well (not close to the minimum) and the bearings show no evidence of excess heat (blue) and feel good I would reuse them. If you are unsure what feels right and what doesn't find an experienced mechanic to look at them. Those bearings generally outlast the rest of the motor. Having said that if you would like to wring out another 20 years or more of use just replace them as they are not that much money in the big picture.

Thank you for that advice. If everything looks good inside I will be rebuilding it. I’d like to have a bit of fun with it. (both with the rebuilding project and zipping around with a 70 instead of my 50)

Ultimately though I will probably only run it for a year or two until I can scrape up a bit more cash for a newer four stroke. If it needs lower bearings then so be it. Probably more attractive to a future buyer knowing that it’s been rebuilt top to bottom

 

[flyingscott]

What is bad compression? I believe 1990 was the first yr for decompression holes.

 

[ShoestringMariner]

Thanks. But with decompression, weren’t they supposed to read 120psi?

I’m at 80-82-86psi.
Those numbers seem pretty low to me.

I have a 1995 50 triple (same block?) at 110-115psi.

I bought it running so I might throw it on and see how gutless it is. I’m intentionally leaving a cmc tilt on my boat so I can play with 50-60-70hp OMC’s. This way I can swap motors very quickly without compromising my transom seal.

 

[oldboat1]

Buy a four stroke with a warranty. Work on the current motor as a project while you pay off (and have fun with) the four stroke.

 

[ShoestringMariner]

Buy a four stroke with a warranty. Work on the current motor as a project while you pay off (and have fun with) the four stroke.

I’ve considered that. But uncertain times. I’m considering a career change and don’t want payments or interest. I prefer to buy with cash.