I decided to do a compression test before pulling my motor to change the timing chain. It is a 1968 350 with #5 heads and has just under 90k miles. It does not smoke or use oil and the vacuum is pretty steady around 19-20. Before I purchase the car, it was only driven several hundred miles a year for the last 15 years or so. All cylinders except #7 were between 150# and 162#. #7 was only 120#. A wet test on #7 did not change the reading. I pulled the valve cover to check the springs and everything looked good there. So now I have to decide whether to just drive it as is or pull the heads while I am doing the timing chain. Also, I noticed the springs are different. The intake springs are raw steel colored with dampers in them and the exhaust springs are tan colored without dampers. Is this how they would have been from the factory?

 

With the rockers off, put air in #7 and see if it leaks and where, from the exhaust or intake are valves, from the oil fill pipe and/or valve covers it rings. Since the wet test didn't change anything it is probably a valve.

Run it or pull it apart? It doesn't matter much as it doesn't involve much overlapping work. If it is rings they MAY free up with use. If you are going to pull it apart start soaking the manifold bolts early and often with Kroil or PB Blaster. If you do pull it apart the compression ratio will drop due to thicker head gaskets.

Good luck!!!

 

Thank you. I will put air in it tomorrow and report back with the findings.

 

Be prepared for the engine to rotate or kick some when applying the air pressure. Unless the piston on that cylinder is right at TDC or at the bottom of its travel it will move and rotate the crank a little.

If it is a valve try rotating the valve a few times in.case your lucky and the seat is dirty. It could also be a weak valve spring, mark the ones on 7 and try switching one if you really want to confirm the source of the problem.

Good luck!!!

 

What is a good air pressure for the test?

 

Pressure for the test 100 psi or better. It will rotate with air. Don't be negative if you get a low reading. Like said before maybe drive it and see if things loosen up. A bit of Marvel mystery oil may help.

 

Well, I put air to it and it is definitely the intake valve. As soon as hooked up the air, it rushed out of the carb. By lightly hitting the valve with a dead blow hammer, I was able to make it stick open a little. Are there any good ways to try and loosen/clean it up?

 

Try to "exercize" it by moving it manually and getting some penetrating oil in the valve guide. If that doesn't work, you may have to dig a little deeper.

 

Agree with exercising it. Pull the spring and seal, clean the stem above the guide the best you can and then work the valve up and down with penetrating oil on the stem until it is totally free and re-test. You could put a drill bit the same size as the valve stem in a drill, connect the bit and stem with a tightly fitting rubber hose and rotate the valve stem in the guide.

Put a tie-wrap in the keeper groove when doing this to prevent the valve from dropping into the cylinder.

Good luck!!!

 

Great idea !!!! I like that idea. Its better than packing rope inside through the spark plug hole.

 

Thanks for the tips. Hopefully I can get to it Friday afternoon.

 

I had the day off due to a delay in materials, so I decided to head out to the garage and work on the valves. Now, I wish I went to work. I am pretty certain that the valve is slightly bent. After removing the spring and seal, I cleaned the stem and sprayed it down with some penetrating oil. I let it set for a while and then went back to move it up and down and spin it. BTW, Sugarbear, the tie wrap tip worked great. It let me disconnect the air to move the valve without letting it drop down too far. I reduced the the air pressure to just enough to hold the valve up and still let me spin it. While spinning it, I noticed I could get the leaking air to almost stop depending on where I positioned the valve. I guess it is possible that it is dirty, but I am leaning towards it being bent. I guess my timing chain project just got a little more involved.

 

If it was bent I'd think changing the position would still create a leak in every spot unless the seat had a matching low spot.

As you know you can pull the head and do a valve job or try putting a tight fitting rubber hose on the valve stem, connect it to a drill and try lightly lapping it in place...not much to lose. Spin the valve while moving it up and down, on and off the seat and re-test. If that doesn't work then all that is left is to drive it or do a valve job.

Good luck whichever way you proceed!!!

 

x 2

 

I ended up pulling the engine and transmission yesterday. I have not pulled the head off yet but I did check the valve again. I am certain that it is bent. I also found the the pushrod on that valve has a small bend also. I picked up a 1970 350 for a reasonable price. It has 14k miles on it since being rebuilt. It will get me back on the road quicker and cheaper than me doing the work to my engine. I also picked up a TH350 that I will be dropping off to be gone through. Hopefully it won't be more than new seals, converter and shift kit. So for now, the plan is to use the new engine and at some point down the road, do a complete rebuild on the original engine.

 

It's good that you have a plan that will keep you motoring. Oil the original engine up well while it is sitting to prevent corrosion.
Good luck!!!