My dad bought this nice little 1967 Johnson in 1980 or so for our 18' aluminum sport canoe.
All the water passages had clogged up sometime between when my dad bought it used in 1980 or so to around 2010 when I stopped using it much. My son overheated it a couple times before we figured out what was going on. The pistons were pretty scored up. I pulled it all the way apart, had a machine shop acid bathe the block to clean it out, re-assembled. So it's got new pistons, rings, all new gaskets everywhere, haven't been broken in. (I've done fair amount of work on cars, but little on boat engines.). I've rebuilt the carb, including soaking overnight in carb cleaner and a new float. I've checked that the spark jumps a 10mm (>1/4") gap on both cylinders. New plugs when I rebuilt it. New fuel pump.
Since the rebuild, it starts very hard and has low power at WOT. In another thread (asking for help with the link & sync procedure), it was suggested that it might be running on one cylinder. Indeed, by starting and pulling one plug wire then the other, I've verified that it appears to be running on the bottom cylinder only.
Compression seems a bit low to me - 80 bottom, 70 top. My manual says normal rule-of-thumb is 100+. Does that mean that the old pistons ground out the cylinder walls too much and I need to re-bore and get oversize cylinders/rings? Or something else? It's within the 80% spec given in the manual (80*0.80=64).
Any ideas how to narrow it down further?
All the water passages had clogged up sometime between when my dad bought it used in 1980 or so to around 2010 when I stopped using it much. My son overheated it a couple times before we figured out what was going on. The pistons were pretty scored up. I pulled it all the way apart, had a machine shop acid bathe the block to clean it out, re-assembled. So it's got new pistons, rings, all new gaskets everywhere, haven't been broken in. (I've done fair amount of work on cars, but little on boat engines.). I've rebuilt the carb, including soaking overnight in carb cleaner and a new float. I've checked that the spark jumps a 10mm (>1/4") gap on both cylinders. New plugs when I rebuilt it. New fuel pump.
Since the rebuild, it starts very hard and has low power at WOT. In another thread (asking for help with the link & sync procedure), it was suggested that it might be running on one cylinder. Indeed, by starting and pulling one plug wire then the other, I've verified that it appears to be running on the bottom cylinder only.
Compression seems a bit low to me - 80 bottom, 70 top. My manual says normal rule-of-thumb is 100+. Does that mean that the old pistons ground out the cylinder walls too much and I need to re-bore and get oversize cylinders/rings? Or something else? It's within the 80% spec given in the manual (80*0.80=64).
Any ideas how to narrow it down further?
. Finding a machine shop to bore out the head would be actually harder - there are fewer and fewer of those around. 