Low Compression 70 hp Evinrude

70's (unknown year) 70 hp Evinrude has 120 on two cylinders and 70 on third cyl. (cold) (bottom cyl) will start but won't take gas bogs and dies. on occasion I see a puff of smoke and the motor picks up rpm momentarily. but will not take gas . I used to be able to hit the choke quickly and the motor would pick up and go ...but it has gotten worse and worse over time and now is useless... Looked at plugs and found the low compression cyl. plug to look silver and clean , other two were black and somewhat oily. I run 50:1 oil mix... Have rebuilt carbs, new gas lines, hoses , fuel pump, gas, cleaned screens , etc. thats not my problem .. I sure hate to have to replace rings etc. Any input please !

you really have two choices...fix that bottom cylinder or scrap the engine....

Sounds like a blown head gasket to me. Clean plug, low compression.
Relative easy fix.
Only way to know for sure is to pull the head.

Re-reading....is that silver on the plug aluminum or steel deposits? If so....uuurrrrfffff.

I vote for worn out motor that has given good service and deserves a good rest.

Won't take much to remove the head and take a look.