QuestionQUESTION: I recently picked up a 1985 Honda V65 Magna (VF1100C) that's been sitting for at least 3 years with no idea how long it was sitting, but it says 5,900 miles.
I got it home and changed the oil, spark plugs, air filter, and battery. It fired right up with some starter fluid but wouldn't idle. I was told to check the compression on each cylinder before cleaning the carbs to make sure the engine was ok, and cylinder 3 read low at around 70 psi while cylinders 1, 2, and 4 all read high around 160 psi.
I added oil to the cylinder through the spark plug to see if it was a ring or valve and the compression level stayed the same, 60-70 psi. I adjusted both the intake and exhaust valves to the specified 0.005" clearance and I still get a low compression reading.
What are some possible reasons I would still get a low compression reading on only that cylinder? And what would cause a bike with that low of miles to have only one cylinder foul up like that?
Thanks
ANSWER: Mike, there's a few things that can cause loss of compression in a cylinder and none of them are easy to fix. The piston could have a hole in the crown, a valve seat could be damaged, the head gasket could be leaking or the piston rings could be broken. You pretty much have to remove the cylinder head to diagnose and repair the problem at this point, unless you have access to an endoscope (the small camera built into a flexible stalk) to have a look through the spark plug hole.
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QUESTION: Your answer was great, and exactly what I've been hearing from a few different people.
I started looking around the backside of the rear cylinder head and I noticed what looks to be a white diagonal line drawn with engine marker across the seam between the cylinder head and the crankcase.
At the slash mark there seems to be some kind of a rubbery substance seeping through the crack. If that's a blown head gasket on the backside of the cylinder head where cylinder 3 is, that would explain the low compression, and all I may need to do is replace the head gasket.
Any other thoughts about this, or am I on the right track with the engine marker pen line I found?
Thanks again!
AnswerIt sounds like the previous owner marked the location of a head gasket leak and then didn't do anything about it. I am not sure what the rubbery substance is, unless someone tried in vain to repair the leak by inserting some silicone sealant into the gap.
When the head gasket blows on a liquid cooled bike, the external indication is usually (but not always, as it depends on the location of the rupture) leaking coolant on the outside of the cylinder head/cylinder barrel junction. If the gasket area around the combustion chamber on any cylinder is blown, you may not get an external indication but you would normally see coolant mixed in with the oil in the oil reservoir.
Either way, I still think you need to remove the cylinder head to see what the problem is for sure.
