c5vettster
Member
- Joined
- Nov 9, 2014
- Threads
- 7
- Messages
- 48
I am trying to find a logical explanation for a low compression on the FC420v motor. It came on an older exmark that sat for years because of no start. I measured the compression and it is barely pushing 5psi e.g. next to nothing.
At first, I suspected the incorrect valve lash or an open valve, but the valve clearance was spot on and valve train move fine with the cycle.
I then suspected blown head gasket or worn out rings. Checked the leakage. The leakage test came out at the low range of the "good", with just a tiny little wind out of the carburetor e.g. slight leak in the intake valve. There was nothing coming out of the oil stick tube. I presume that both rings and head gaskets are therefore in a good condition.
Now I am at a loss to explain the low compression. I've checked and the piston moves up and down with no give on when I put pressure on it with a stick through the spark-plug hole. I am about to recommend the engine replacement because I would darned if I knew what the cause of this mysterious low compression.
At first, I suspected the incorrect valve lash or an open valve, but the valve clearance was spot on and valve train move fine with the cycle.
I then suspected blown head gasket or worn out rings. Checked the leakage. The leakage test came out at the low range of the "good", with just a tiny little wind out of the carburetor e.g. slight leak in the intake valve. There was nothing coming out of the oil stick tube. I presume that both rings and head gaskets are therefore in a good condition.
Now I am at a loss to explain the low compression. I've checked and the piston moves up and down with no give on when I put pressure on it with a stick through the spark-plug hole. I am about to recommend the engine replacement because I would darned if I knew what the cause of this mysterious low compression.