TR6 Compression help in one cylinder

[Gliderman8]

Compression help one cylinder is low

As some of you may remember, in May I bought a '76 TR6 to restore. Over the last few weeks I've rebuilt the carbs, and drained and cleaned the gas tank.
I finally started it up a few days ago and have been trying to tune it as well as get the carbs set correctly. This morning, I had the car running while making some adjustments and heard a noise coming from the engine that you know when you hear it, its not good. It didn't make the banging sound continuously, but it did it every once in a while. I pulled the plugs and did a compression test and I don't like what I found so I'm looking for advice as to which way to proceed.
Here's the compression results (dry):

1=110 2=100 3=45 4=100 5=105 6=110

Obviously #3 has a problem. I added some oil to #3 cylinder and redid the test and this time it only came up to 60 lbs.

What is the best way to determine if its the valves or rings? If it is broken rings in #3 then whats to best way to proceed?
Thanks for any help.

 

[Deleted member 8987]

First, do a wet test. One squirt of oil in each hole, crank a couple of times, and re-do the test. If it's worn rings, you'll see a substantial jump in readings.

Poor man's leakdown is a fitting on an air hose that matches your plugs.
Remove rocker assembly. Air up each hole, listen for air out carbs, exhaust, adjacent holes.

I would also be looking closely at valve springs. Push down with the palm of your hand on each valve. Does one (especially the low hole) go down MUCH easier than the rest?

Could also be your source of noise.

Once you have done that, and it all checks out, it's broken rings or piston or both.

Dave

 

[Geo Hahn]

Poor man's leakdown is a fitting on an air hose that matches your plugs.
Remove rocker assembly. Air up each hole, listen for air out carbs, exhaust, adjacent holes...

Just to expand on that:

Many compression gauges have a quick-connect on the gauge -- remove the gauge and what you're left with is a hose with a quick-connect on one end and spark plug threads on the other.

To do this test you want the cylinder to be at TDC on the compression stroke. I also have it in gear and handbrake set though in theory the piston won't move it you are exactly at TDC.

 

[Gliderman8]

Thank you gentlemen... I will need to get a hose with the proper fittings.
George: I do have a compression gauge, but I'll have to look and see if it has the quick connect on one end.

 

[Gliderman8]

To make matters worse, I just went to reinstall the plugs and I notice #3 spark plug has had the tip pushed in; ie, the gap is closed. Could a bad valve have done that?

 

[Deleted member 8987]

All stop. Plug smashed is something inside the bore. Added to that low compression, just pull the head, pan, and piston/rod.
Plan on a full rebuild.

Sorry.

Dave

 

[TR3driver]

That was my comment: you know there is something bad in #3, might as well pull the head and find out what. Since you are "restoring" the car, I can't see any reason not to restore the engine as well.

 

[svtmikey]

Yes, all stop.....no way to have the plug smashed if all is ok...very bad
Just as a comment, better way to check things is a leak test.....we don't do compression tests very much anymore.
A Leak tester is relatively cheap, you position the cylinder you are checking so it is TDC with all the valves closed and then pressurize it with compressed air then see how much air is leaking out of the cylinder.
A certain amount of leak is allowable, most gauges show what is acceptable.
The advantage is it tells you where the problem is.
You simply then "listen for where the air leak sound is.
If you hear it out of the exhaust pipe.....exhaust valves
Radiator.....most likely means head gasket
Intake.....intake valves
Oil filler/ dipstick....looks like rings.
Pretty easy and a lot less guessing

Good luck on the problem though



Not saying you never use a compression test, but there are more precise ways of trouble shooting.