Sticking valves?

Thodge

Thodge

Senior Member
I’m trying to solve a rough running condition on my dad’s 30 coupe. I’ve pretty much exhausted all of the obvious trouble areas and I’m still stumped. The car fires right up when cold, and idles and drives without stalling or backfiring but there is a definite lope to the idle which you can hear in the exhaust and it continues into the driving range. It makes the car shake badly at any rpm and it is clearly down on power because it can’t pull 3rd gear unless the road is fairly flat. What I’ve done so far:
1-set ignition timing with Nu-Rex wrench2-checked points(has Nu-Rex modern points and condenser3-checked plugs(all 4 look perfect, no signs of carbon, fouling, or fuel soaking from misfire)4-checked carb adjustment(runs best at 1.5 turns on idle air screw and 1/4 turn on GAV which seems normal)5-checked compression(cylinders 2&4 were perfect at about 65 psi, cylinders 1&3 were slower to build pressure at first and were about 10 psi lower but came right up with some oil added)6-applied conpressed air to questionable cylinders at TDC(air leakage was through crankcase and not past intake or exhaust valves)
It’s not misfiring completely as the spark plugs look so good and it runs worse when any leads are disconnected but it seems that a couple cylinders are a little off. Seems like it’s mechanical and not fuel or ignition related. Only other symptom is intermittent valve ticking. Seems to run rougher when the ticking is worse so I’m starting to suspect an exhaust valve is just sticky enough to close slowly and cause the slow compression buildup and roughness.
My next step is to pull the valve cover and check the valve clearances and springs.Does anyone have experience with these symptoms and is there anything else that I’m missing?
 
Had the same presentation on my 31 some years ago. Raised the rpm and injected Marvel Mystery Oil directly into the carb intake and the problem was resolved. In my case I think the problem was sluggish valves. As you are thinking, highly recommend checking/adjusting the valve lash. A poor idle is often the result of insufficient clearance on one or more lifters. Let us know what the fix is.
 
Thanks for the replies. I was thinking about trying Marvel Mystery Oil before tearing into it. Nothing to lose. Given the ticking, I should check the valve clearances anyhow. Previous owner had engine out and bottom end was found to be in good shape but valves were lapped so I’m hoping they put in adjustable lifters then. We’ll see.
 
I definitely recommend IMO but also think your rings may be worn, because the compression is slow but fine with oil added. Do the MMO first.
 
How many miles on the engine? It may be time for new rings, lapping the valves, and taking up the bearings. Back in the day, this was usually done at around 25,000 miles. It can be done in the car but I like to take the engine out and put it on an engine stand because I think it makes it easier.
 
Marvel Mystery Oil!!!

Use in gas every time you get fuel.

Give it a chance to work.

I would not worry about your compression readings. Is the car smoking out the exhaust? Going down a hill when you take your foot off the gas, do you get a cloud of blue smoke.

What oil are you using? High detergent? 50/20? Are you adding any STP?

These engines take a licking and keep on ticking. Don’t fix something that’s not broken.

Enjoy.
 
I’m not sure how many miles are on the engine because we only bought it a month ago. We do know that the previous owner had the engine at a shop 15 years ago and they said the bottom end looked new so only valves were done at that time. I’m not overly concerned about the rings yet though because it doesn’t seem to be burning a lot of oil. I will do a hot compression check and see if my numbers are more consistent. I’m also going to do a vacuum check to see if I find any evidence of the valves being sluggish. Then I’ll fog the engine with MMO through the vacuum port and see what happens.
 
I have a 66 ford f 100 and use to if I let it sit for anytime the valves would stick and bend push rods when I tried to start , I poured a couple bottles of Stihl two cycle chainsaw oil in gas tank and have never stuck since.
 
If you look at photos or movies of race cars at the early part of the last century, you will see a cloud of oil smoke behind each car. Oil was mixed with the gasoline. The early cars did not have very good oil systems and the oil in the gasoline kept everything lubricated. For my antique model airplane engines I run 3 to 1 gasoline to 70 weight oil.
 
I seem to have solved the problem today. Vacuum gauge was fluttering terribly when I tested it and when I held a piece of paper in front of the tailpipe, that was fluttering too. Obvious valve problem so I pulled the valve cover. Didn’t find anything out of the ordinary but did find that it did at least have adjustable lifters installed.
I checked the valve lash and found that both of the cylinders with questionable compression readings had exhaust valves that were way too tight. One of them was only .004”. It’s lucky that the previous owner didn’t burn that one. I adjusted all of the exhaust valves to spec and now it runs quite well. Idle is perfect and seems to have quite a bit more pep.
I opted not to adjust any of the intake valves because they were all consistent but about .002” loose. Did I make a good call there? I figure I’d rather have a noisy valve than a burnt valve.
 
I forgot to mention that I did fog the engine with MMO after I did the vacuum test. Made no difference yet. Certainly wouldn’t have fixed the valve clearances anyhow. The car was running poorly enough that you wouldn’t want to drive it long enough to give the oil time to work. Needed more immediate attention.
 
I’m not a member of the local chapter yet. My dad just bought it and I’m trying to get it a bit more mechanically sound before he registers it. It’s a driver in it’s current state but I wouldn’t call it a pleasant driver yet.
 
" I forgot to mention that I did fog the engine with MMO after I did the vacuum test. Made no difference yet."
I don't have that patience either.
 
Thodge said:
I’m not a member of the local chapter yet. My dad just bought it and I’m trying to get it a bit more mechanically sound before he registers it. It’s a driver in it’s current state but I wouldn’t call it a pleasant driver yet.
Click to expand...

The people in your local club don't care if you have a running car, or a basket-case. If your car is broken, typically some of the guys will come over and help you figure out what the problem is for no charge. They are happy to help get another A running again.
 
You must log in or register to reply here.
Back
Top