What does this compression test mean?

-
Shunyun

Shunyun

New Convert
Joined
May 27, 2013
Messages
66
Reaction score
0
Location
Escondido, CA
Hi folks, newbie here... for that matter, to MOPAR and especially to /6.
Just bought a 65 Dart GT w/ 225. Doing first maintenance and have this compression reading. It idles rough until warm, dry soot from tail, a very little black smoke when first starts. Sat for quite a while, Los Angeles car. Plugs were all dry but black, not pitted. I know I'll need to do a valve job but does this reading point out anything more? I only ask because I don't know /6 and wouldn't know if it points to something everyone here knows about.

1 - 96
2 - 117
3 - 115
4 - 96
5 - 92
6 - 113

Thanks!
 
There's an old rule-of-thumb that says the lowest cylinder should be at least 90% of the highest cylinder. Based on the data provided, this engine is right at the point where it needs a rebuild. Just barely. I'm used to seeing much greater disparity in the numbers.

If you're going to have a machine shop do the cylinder head, suggest they fit hardened valve seats to make the engine more friendly with no-lead fuels.

Hope your experiences with the Dart and FABO are good ones.
 
Try adjusting the valves.
 
how long did it sit before it was driven on a regular basis?im with rusty on this one try a valve adjustment and a tune up/oil change.drive it for a while then re-test.
 
2 Darts said:
...
If you're going to have a machine shop do the cylinder head, suggest they fit hardened valve seats to make the engine more friendly with no-lead fuels.

Hope your experiences with the Dart and FABO are good ones.
Click to expand...

Thanks 2D, for the advice and the welcome! I expect that I'll be doing a valve job first, then determine if I'm at the point of new rings; if so, I'll take advantage of having the head off to do as you advised. OD says 6k, so I'm thinking at least 206k. My son found it in a garage on a jobsite... owners husband had passed away 10 years before, that was 3 years ago, so I'm thinking he drove it conservative considering the condition. Rust in the usual spots, interior in great condition, headliner perfect. Plus, given that its only exibiting rough idle at cold, and smooths out at warm, I'm thinking the bottom end is fair... so that's how I conclude 2 rollovers.
 
340 8BBL said:
how long did it sit before it was driven on a regular basis?im with rusty on this one try a valve adjustment and a tune up/oil change.drive it for a while then re-test.
Click to expand...

Thanks Rusty, TBeast and 3408BBL, that confirms my plan. I wouldn't try anything else at this point till I've looked at the valves. Knowing what you know about this engine, do these patterns indicate any typical weakness with the head gasket? When I see two low cylinders between high, I'm suspicious. I know the only way to know for sure is to do the valve job but just thought it was worth asking.
 
Try simply adjusting the valves first. It may not need a valve job....or much else.
 
Does the motor tick while it is running?
Give it a good tune up, points, condenser, clean / rebuild the carb, plugs wires, etc.
Adjust the valves and run it for a couple of weeks, it may just need run some to loosen back up the rings, and knock the carbon off the valves.
Sitting is very hard on an engine.
 
could of had some rust in the cyl walls from sitting also. I would adjust the valves as stated, and run it a few hundred miles if possible to let the rings re-seat and then check the compression again.


Shunyun said:
Thanks 2D, for the advice and the welcome! I expect that I'll be doing a valve job first, then determine if I'm at the point of new rings; if so, I'll take advantage of having the head off to do as you advised. OD says 6k, so I'm thinking at least 206k. My son found it in a garage on a jobsite... owners husband had passed away 10 years before, that was 3 years ago, so I'm thinking he drove it conservative considering the condition. Rust in the usual spots, interior in great condition, headliner perfect. Plus, given that its only exibiting rough idle at cold, and smooths out at warm, I'm thinking the bottom end is fair... so that's how I conclude 2 rollovers.
Click to expand...
 
sireland67 said:
Does the motor tick while it is running?
Give it a good tune up, points, condenser, clean / rebuild the carb, plugs wires, etc.
Adjust the valves and run it for a couple of weeks, it may just need run some to loosen back up the rings, and knock the carbon off the valves.
Sitting is very hard on an engine.
Click to expand...

Good call, no it doesn't tick. Yeah, I think first step is simple adjustment; thanks, all. I'll let's y'all know how it turns out.
 
Where this car has not been run for several years chances are good that there the rings need to be run in, and most likely the valve lash hasn’t been adjusted for the last 50K miles or more.

Where that car is a 1965, and probably has not been fitted with hardened exhaust seats once unleaded fuels came on line in mid-seventies there most likely some exhaust valve recession has taken place… This is all normal for an old car, and often all that is needed on a slant is to adjust valve lash, run engine gently for a few hundred miles varying the rpms as if you were braking in a new engine’s rings. You may find it burns some oil, this could be due to dried out valve seals, and or stuck rings, this is normal. Excepting valve seals, often just running the car for several thousand miles will free the rings, and the cylinder compression will improve.

I would run the car for a while before pulling the head, and see how well it comes back to life. Be warned if you try to burn old gas, chances are good that one or more of the valves will become stuck, so clean out fuel system, and use fresh fuel.
Also plan on installing a carburetor rebuild kit to refresh seals, needle valves, set float level, and clean out any crud. Also properly adjust choke to eliminate black soot, it is running rich when cold.
 
You must log in or register to reply here.
-

Back
Top