Tapping Then Vibration In #4 Intake Valve Area F Head

Went on a run this weekend. All highway 45-50 mph top speeds, maybe up to 55 occasionally. Longest I have driven this since putting it back together. All was well until 140 mile into 160 mile round trip. Started to get a vibration in the engine that was rpm related. When I got home there was a fairly loud tapping/knocking sound above the #4 cylinder. Vibration was still present as it was still hot and the outside temperature was near 100 degrees. Put the jeep away and pondered, then searched the forums. Today I pulled the rocker arm cover and did some investigating. Nothing apparent, but adjusted the tappet clearances just to be sure. Everything was correct, .018" intake, .016 exhaust. Put it back togehter and started it up. Much quieter and no vibration. As it warmed up, the tapping, knocking grew louder. Checked various places with a screw driver to the ear and isolated it to the rocker arm cover above #4. Louder on the passenger side than drivers side. Could this be a weak or broken valve spring? Any other suggestions appreciated. Engine has 29,000 miles, and was dissassembled and inspected closely during restoration. Tried to make sure engine was to FSM specs, and it was. Just reassembled with all new gaskets, altough that was 10 years ago. Has about 300 miles since I started driving it again in May. Was a fun drive!

 

Can you take a video of it running? Might help.

 

Did you listen with the screwdiver under the engine and crankcase, for lower end noises?
Is it better or worse under load, or on decel?

100ºF on a highway for hours…? I'd be fearful of a bearing going out. DAMHIK.

 

No lower end noises that I could hear. All the noise is from the top. It was only a little over 90 minutes at that temperature. Coolant temp never went over normal.

 

Start it up with the valve cover off so the valves can be seen. Maybe the valve is sticking in the guide.

 

Sticky valve guide? I'd think that would fit the noise pattern.
But "vibration?"

 

If the valve is open, that cylinder isn't firing. If it's not firing it will cause the engine to shake.

 

Excellent point. That didn't occur to me.

 

X2 on post #5. I expect your valve guides are too tight, at least one of then. I have had this same thing happen on a sbc marine engine with a fresh rebuild.
Cold, everything was fine. Run easy up to mid-throttle, still good. Start running at moderate load, after about 5 minutes, the engine would start missing and running rough. A couple bronze valve guides were too tight and caused the valve to stick as things got to full temperature with a load. A light ream of all the guides fixed the issue. The shop pressed in new guides, and the valves seemed fine on assembly. They needed the required reaming to size after pressing in. All will be fine until full temperature is reached.
Did this engine receive new guides or valves as part of the rebuild?
-Donny

 

In recent years I've seen stuck valves in several small engines.
That makes me wonder about the newer formulas for fuel and oil, especially in an older design that was built for traditional additives.

 

I've had similar problems and had success with Marvels M O in the gas and oil,used to get gas additive by CENPECO,don't know if they are around any more.

 

Engine was not rebuilt, just dissassembled inspected and reassembled.

 

Broken or weak valve spring cause this?

 

I would think a weak spring could. A broken spring would have the symptoms cold or hot.

 

Replaced the valve spring for #4 intake. Issue is gone. Old spring was not to height spec in FSM so assume it was weak. Old spring measured 1.920" and new spring was 1.990". Test drove this morning and had more power. Will replace the rest as a precaution. 60 year old springs.

 

Your local shop will have a spring tester. There should be a spec.

 

Glad you found what may be the issue, though it could return with another long trip. I still suspect your valve guides are on the tight side, or could have carbon buildup on the valve stems.
Free height of the valve spring is one measure, but a more accurate measure is spring pressure at the height of the valve installed on the head, or seat height. It is common for a machine shop to add shims under the springs to achieve the specified seat pressures, while making sure there is sufficient clearance as to not approach coil bind.
It would be interesting to measure the spring you changed out to see if it was near the factory spec. I'll offer to measure that spring if you want to send it to me, as I have the fixture to properly determine the tension.
-Donny

 

Good job analyzing and looking for the fix.

 

All the specs for the valve springs are in my FSM, I'm going to take them in to a local shop and have them measured. It is possible there is some carbon build-up. Don't want to pull the head at this point.