Export thread

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.

pour about a tablespoon of oil into each plug hole the rotate the engine several times by hand.
If you have some old plugs handy . fit them loose and crank the engine with the starter ( this will oil up the plugs ).
Now crank the engine plugs out to expell the excess oil.
Try to start the engine.
The seal between the rings & the bore is actually made by the oil and when the bore dries right out you will not get much compression.
If you have ever boiled your car engine you will have noticed that when cooled down it spins very fast, for the same reason .

bertsmobile1 said:

pour about a tablespoon of oil into each plug hole the rotate the engine several times by hand.
If you have some old plugs handy . fit them loose and crank the engine with the starter ( this will oil up the plugs ).
Now crank the engine plugs out to expell the excess oil.
Try to start the engine.
The seal between the rings & the bore is actually made by the oil and when the bore dries right out you will not get much compression.
If you have ever boiled your car engine you will have noticed that when cooled down it spins very fast, for the same reason .

Click to expand...

Hmm, let me try it! I am at the end of my wits so willing to trying anything.

Ok, the oil trick worked. The compression is now about 60psi. I believe it is too low given that 71PSI is the minimal compression required for this engine. You reckon the original recommendation of engine replacement is still a go?

Don't give up on it yet.
Try some starting fluid.
even if you need to give the engine continious short bursts down the carb throat.
It does not take much in the way of rust on valves & cylinder walls to knock the compression for a 6.
Ten minutes running will oft burn/ wear off the rust & have it back in running order.
This is fairly common with antique motorcycle engines that only get run once every blue moon.
oil down the plug hole really only restores the top ring oil coating it does not do much for the second compression or oil rings.

On really old engines we toss some thin oil down the plug hole and leave it a couple of days to penetrate all of the ring grooves.
Even then it still takes a fair bit of running to restore things to normal.

bertsmobile1 said:

Don't give up on it yet.
Try some starting fluid.
even if you need to give the engine continious short bursts down the carb throat.
It does not take much in the way of rust on valves & cylinder walls to knock the compression for a 6.
Ten minutes running will oft burn/ wear off the rust & have it back in running order.
This is fairly common with antique motorcycle engines that only get run once every blue moon.
oil down the plug hole really only restores the top ring oil coating it does not do much for the second compression or oil rings.

On really old engines we toss some thin oil down the plug hole and leave it a couple of days to penetrate all of the ring grooves.
Even then it still takes a fair bit of running to restore things to normal.

Click to expand...

Thanks for your advice! The coil and/or igniter are faulty, so I will have to order a new set to try to get it running.