MTFCA Forum

What are the symptoms of sticking valves? Unsuccessfully trying to diagnose rough idling and once smoke emanating from the throttle arm hole in the valve cover on a newly rebuilt engine. I've gone thru the coils, carburetor, timer, plugs, all the easy stuff to no avail. Car runs strong at higher rpm.
I get different compression readings by hand cranking. Don't know if its my technique.
I do recall sticking valves on a 1910 on a new engine I rebuilt years ago that was cured when I cleaned the valves and guides and opened up the guides a few thou.
As always, any help appreciated.

It's easy to take the valve cover off and watch them as the engine is turned over. I have seen new valves stick if the engine has sat for a while.
Rich

Richard - for a proper compression test, remove all 4 spark plugs with the throttle wide open & crank away

A sticking valve may not show up at cranking speed as it has lots of time to get closed. Having said that, if the engine runs strong at higher RPM, valve sticking is probably not the problem. A rough idle (especially on a fresh rebuild) is more likely fuel mixture, or one or two valves set a bit too tight. I'd check the clearances again (0.012 int. and 0.015 exhaust) and then look for a vacuum leak at the intake/block fitting. I'm assuming you're using the brass ring and steel sleeve gaskets to achieve a proper seal and prevent exhaust manifold sag. As I rebuild a lot of carbs, I use an intake with a vacuum fitting installed so I can connect my vacuum gauge which can expose any mixture or valve issues instantly . Good luck : Bruce

Steve, I did best I could. Removed all the plugs, opened the throttle and hand cranked. I don't have a starter on the car.
I went thru each cylinder 6 times to make sure I got the highest reading before moving to the next cylinder. I got readings varying between 42 and 60 psi. That may be indicative of a newly rebuilt engine, I don't know.
Reluctant as I am to do this, I will remove the head and manifolds/valve cover and check things out. I have questionable threads in the head at the water outlet so I'll use the opportunity to install either a helicoil or solid insert. In addition to any valve work, I may decide to change rings. If so I dread getting under the car trying to remove the caps and keep everything in place.
The problems I am experiencing certainly suggest a carburetor problem, but I've substituted carbs known to be good with the same result. Really puzzling.

How "new" of a rebuild is it ? I'd attempt to squirt a small amount of motor oil in the low reading cylinder and check it again to verify the reading.

I vote for vacuum leak at the intake manifold. I would not pull the pistons to change the rings at this point. Also, you said there was some smoke coming out of the hole in the valve cover. That would indicate to me loose valve guides, which would be less likely to make a sticky valve. Before you tear anything apart, spray some carb cleaner around the intake ports while it is running and see if the idle changes.

All ideas/suggestions good food for thought. I'll check for a vacuum leak before I do anything.

I rebuilt the engine last year and had the car running in October. I had to pull the engine because the .080 os pistons were hitting the head and I wanted to recheck all my work.

Since 2 of the rods were bent from the pistons hitting the head I pulled the pistons and aligned all 4. One mistake I may have made was I reused the rings thinking the car had run so few minutes, it would not be a problem. Now I wonder.

Like I said earlier I tried running 3 other carbs on it with the same result, two of which were known to be good running carbs. I'm running an nos roller timer and new Champion X plugs. I've checked the wiring for continuity and the rebuilt coils checked out perfectly on the Strobo meter. Im running out of ideas.

How much smoke is too much smoke! coming out of the throttle linkage hole ? I have a little after the engine warms up at idle.

A little is fine - it's blowby from your engine and oil vapor off the drain holes in the bottom of the spring chamber. If it's more than a little, then I'd be concerned as you have issues starting to develop - worn valve guides, rings, etc...

Richard: I'm getting mixed signals. You say it's a "rebuilt" engine while suggesting changing the rings, talking about bent rods and as well as pistons hitting the head so I'm confused. If you have correctly checked the plugs, timer, timer wires, coils, coil box, carburetor and possible leaks at the intake manifold the only things left are valves/compression. I really doubt that you could get 60 psi in a stock T and cranking by hand, and I also question the huge variance from 60 to 40 even with 0.080over pistons so something's screwy. To verify any piston/ring issues, squirt some oil in each cylinder and repeat the compression test ( I'm assuming you have help with someone holding the gauge while the other guy does the cranking). If your piston/rings are ok the readings should be the same as the earlier test, and if so , the problem in with the valves, either not seating properly ,sticking, or set way too tight .

Bruce the engine was finished initially in October. A total rebuild. When I ran it, I discovered the os pistons were hitting the head. I decided to remove the engine and go thru it again fearing the piston contact had bent the rods. I was right . Two of the four were bent so I realigned them. I beveled the tops of the pistons on a lathe to achieve clearance and put things back together after rechecking the valves just to be sure all was OK. Since the rings were practically new, I reused them.
When I started the engine back up in December I was confronted with the problems I describe above. I put things on hold due to medical issues and have been trying to remedy the problem for the last couple weeks.
Just today I performed an intake vacuum leak and found a significant leak on the 1-2 intake port. I think I found the problem. If not THE problem, a significant part. Thanks to all those who offered suggestions and advice.