bobgritz
Petty Officer 2nd Class
- Joined
- Sep 22, 2009
- Messages
- 185
The top cylinder was not firing (at idle) on my 1983 15hp Johnson although at high rpm's both cylinders would kick in. I had 1/2" of spark from both and plenty of fuel. The compressioin tested at 115 lbs each. I rebuilt the carb, replaced coils and powerpack, rebuilt fuel pump then replaced spark plugs and wires. There was still no effect when the top plug wire was pulled at idle whereas the engine would die immediately when lower wire was pulled.
Next I pulled the powerhead and checked reed valves. One pair didn't seat perfectly but it didn't look bad. I finally called a Johnson mechanic for his opinion and when I told him what was wrong (and what I repaired) without hesitation he asked if I had inspected the pistons. I hadn't pulled the side plates and told him, "no". He said, "you've got a damaged piston"! I pointed out that there was 115 lbs of compression and he said that didn't matter and that the 'skirt' was likely damaged. When I pulled the side plates I could see the pistons and just like he said, the upper piston has some serious scoring. Koodos to the mechanic.
Could someone explain why a scored skirt of a piston affects the idling of a motor?
If the cylinder walls haven't been damaged should I simply replace the piston and rings with a standard set or should I hone the cylinder first?
If cylinder is scored, can I just have the damaged cylinder bored or do I need to oversize both cylinders? In other words, do you need both pistons to have matching sizes or can they vary slightly?
The first two pictures show the upper (scored) piston and the third pic is the piston rod (notice the strange discoloration at the midpoint).
Next I pulled the powerhead and checked reed valves. One pair didn't seat perfectly but it didn't look bad. I finally called a Johnson mechanic for his opinion and when I told him what was wrong (and what I repaired) without hesitation he asked if I had inspected the pistons. I hadn't pulled the side plates and told him, "no". He said, "you've got a damaged piston"! I pointed out that there was 115 lbs of compression and he said that didn't matter and that the 'skirt' was likely damaged. When I pulled the side plates I could see the pistons and just like he said, the upper piston has some serious scoring. Koodos to the mechanic.
Could someone explain why a scored skirt of a piston affects the idling of a motor?
If the cylinder walls haven't been damaged should I simply replace the piston and rings with a standard set or should I hone the cylinder first?
If cylinder is scored, can I just have the damaged cylinder bored or do I need to oversize both cylinders? In other words, do you need both pistons to have matching sizes or can they vary slightly?
The first two pictures show the upper (scored) piston and the third pic is the piston rod (notice the strange discoloration at the midpoint).