low cylinder compression

[jaredts]

Just bought an '83 Silverado a few weeks ago with a 350.  Hoped to just do basic tune-up and get it running well enough.  Problem is after doing everything including new carb. it still wasn't running quite right and occasionally noticed some smoke from the exhaust.  So I did a compression check.  Compression check has all cylinders between 140 and 150 except for #4.  It checked at 105 so I squirted in some oil and get 120.  Is this certainly a falied ring???  I don't have a garage so am a little concerned about trying to pull the motor.  I've done that a few times before but never outside.  Is a cylinder leakdown test the next best step?  I don't have one but gather that compressed air and listening for hissing from the carb, valve covers, and exhaust as well as looking for bubbling in the radiator can pinpoint the problem.  If I verify bad rings in this cylinder is removing the engine the only way to replace a ring?  I've heard of people pulling the pistons and crank and all from the bottom with the engine installed, but not sure about this particular vehicle.

[jaredts]

No help yet?  How can I verify for sure if I am losing compression from rings or top end?  Oil in the cylinder will raise the compression of even a good cylinder, won't it?  Only thing is that some smoke and low compression in one cylinder seems to point to the worst.  What do you guys think?

[maddog]

what did the spark plug look like in that cyclinder ??
Unless you want to fix it right I would just drive it.

[jaredts]

All plugs have some oil on them, but there was nothing remarkable about that one.  I still have to narrow down whether its top end or not.  I would have no hesitation about yanking the heads off for a valve job, but don't want to then find out its rings.  On the just drive it idea--believe me thats appealing.  I am a little worried about further damaging the cylinder.  Anyone know how likely a bad ring is to break and destroy a cylinder beyond a standard overbore?
I'll try and confirm that its the rings this weekend using compressed air in the cylinder.

[VileZambonie]

yes do a leakage test. A Dual leak down gauge is the best method. Harbor Freight has them. I would avoid trying to re-ring one in the truck. If you're going to go that far just yank it and rebuild it. If the engine was rebuilt and someone used moly rings on a bad hone job it could definitely be rings but how old is the rebuild on this engine?

[jaredts]

The truck is an '83 but the motor's not original.  Casting numbers say its an L48 350 made in '79.  Its hard to say when or if it has been rebuilt.  I don't think the previous owner took very good care of it, though.  I was hoping to just take that cylinder to top dead center and put compressed air into it and listen for air leaking noises from the normal spots.  Do you really need a leakdown gage if you already know there is a problem and just want to pin it down?

[VileZambonie]

Do you at least have a regulator? Try it and see what you find.