Does a tightly shaking vacuum gauge REALLY mean an ignition problem? Chasing my tail.

"With engine off, key on the voltage reads 12volt on one side and about 9.5 on the dist side" Meant to say coil side of the ballast.
 
Hogie said:
100 psi sounds a little on the low side to me for a rebuilt motor… the erratic vac reading sounds suspiciously like an intake valve that is leaking slightly, you may not not notice it under light load conditions.. under normal conditions if the valves are seating properly you really shouldn’t see much movement of the vac gauge. I would highly recommend doing a cylinder leak down test before you chase your tail around too much. Just a suggestion.
Click to expand...
start with a leakdown test like Hogie suggested
 
Rusty Muffler said:
So you think this random misfire and cyclical vibration is caused by 1 sticking valve? Solution?
Click to expand...

Well, the diagnosis for a sticking valve is a messy pain. And the cure can be even more so.
So, where are you at. The timing light says the ignition is good when you check each bank plug. But a fouled spark plug can do that. Not likely because you have checked the condition of the plugs. A new plug can look good but be bad. Not likely. And I don't think it would present as you are describing.
The distributer to coil wire can show very slight erratic behavior with the timing light and the car can run fine, I don't know why.
Do you still have points ignition? A bad condenser or coil can run fine under load and still miss fire in the driveway. Not likely.
Most fuel problems roll in and roll out. But you are describing a nervous motor. Not a rich one down on power. It could be going lean. But under a very specific load and rpm? Not likely. But possible. Vacuum leak? But how could it idle so good and stable?
Most of the very clever at hiding sticking valves I've diagnosed was done by removing the valve cover and aiming the timing light at each and all the rocker arms, one plug wire at a time. Check all the rockers with one wire, then all the rockers with the next wire, until you have been through all of them. It's easy to catch the rocker arm out of position. It should be in the same position each time the light hits it. When you freeze it with the light in another position, it's sticking open. You are looking for a sticking intake valve.

But, I would change the condenser and coil and put a dwell gauge on the points before I messed with the valves.

There's lots of speculation with everything I just said. Need more options and opinions here from anyone that has any suggestion/clue.
And your compression gauge is suspect at 100 psi.
 
Just Carbs said:
Well, the diagnosis for a sticking valve is a messy pain. And the cure can be even more so.
So, where are you at. The timing light says the ignition is good when you check each bank plug. But a fouled spark plug can do that. Not likely because you have checked the condition of the plugs. A new plug can look good but be bad. Not likely. And I don't think it would present as you are describing.
The distributer to coil wire can show very slight erratic behavior with the timing light and the car can run fine, I don't know why.
Do you still have points ignition? A bad condenser or coil can run fine under load and still miss fire in the driveway. Not likely.
Most fuel problems roll in and roll out. But you are describing a nervous motor. Not a rich one down on power. It could be going lean. But under a very specific load and rpm? Not likely. But possible. Vacuum leak? But how could it idle so good and stable?
Most of the very clever at hiding sticking valves I've diagnosed was done by removing the valve cover and aiming the timing light at each and all the rocker arms, one plug wire at a time. Check all the rockers with one wire, then all the rockers with the next wire, until you have been through all of them. It's easy to catch the rocker arm out of position. It should be in the same position each time the light hits it. When you freeze it with the light in another position, it's sticking open. You are looking for a sticking intake valve.

But, I would change the condenser and coil and put a dwell gauge on the points before I messed with the valves.

There's lots of speculation with everything I just said. Need more options and opinions here from anyone that has any suggestion/clue.
And your compression gauge is suspect at 100 psi.
Click to expand...
Thank Carbs,
I have points and have metered the condenser and the coil. I've seen in my life ONE condenser go bad and NO coils. I do have a spare coil I could try for laughs. Dwell is fine. Still suspicious of the carb I bought used and rebuild but sat in storage for years. I opened her up and found silt covering the jets in the bottom of the bowl. I cleaned all that up and basically rebuilt it again. There's pictures of it on another post here. Ever since then the plugs look as in the pics.
If vales were sticking wouldn't I be getting a back fire in the carb or exhaust?
 
Rusty Muffler said:
Thank Carbs,
I have points and have metered the condenser and the coil. I've seen in my life ONE condenser go bad and NO coils. I do have a spare coil I could try for laughs. Dwell is fine. Still suspicious of the carb I bought used and rebuild but sat in storage for years. I opened her up and found silt covering the jets in the bottom of the bowl. I cleaned all that up and basically rebuilt it again. There's pictures of it on another post here. Ever since then the plugs look as in the pics.
If vales were sticking wouldn't I be getting a back fire in the carb or exhaust?
Click to expand...

Yes, if an intake sticks while the car is being started. Or the throttle is goosed.
It can present a variety of symptoms. A lot of it depends on where in the valve travel the valve sticks and how long it sticks.
Sticking intake disrupts other cylinder air charge loading. So, the nervous engine syndrome.
Sticking exhaust give a puff at the tail pipe when idling. Like a severely burned exhaust valve.
Early stages are almost always erratic. It usually first shows up as a slight rough idle on the first startup of the day when the motor is cold, when the head and valve guide have closed in on the valve stem and the oil goo is still sticky. In a few seconds after the head chamber heats up and expands, the valve works as normal. That would be mistaken for dirty spark plug burn off from a rich startup or bad valve stem seals and worn valve guilds that load the plug overnight.
You have a new motor, so oil goo would be almost nonexistent. Unless you have a gas tank contaminated with varnish. That diluted liquid stuff can make it past the filter and carb and stick to the valve stem. Does your endoscope show any carbon or oil goo on the valve stems? Oil goo on the stem is easy fix, carbon is a big different job. Carbon on a 600-mile motor? Nah.
So, you're thinking one half of one of your fuel feed circuits is misbehaving?
I'm still thinking ignition or sticking.
Or a broken 12-volt wire hiding inside the insulation. Or something.
 
No smoke ever, starts right up cold, idles perfect and the dual exhaust at that time sounds even and crisp. After warm up the sound isn't as distinctive. Before the very first start after installing the engine, I replaced the gas tank with a big M dry junk yard tank that leaked so I replaced the tank with a new one. I doubt a dry tank would have absorbed residual bad gas from a dry tank? I know what bad gas can do, glue the valves. I also know carb cleaner will loosen them up. I just adjusted the valves hot a second time, no major differences and the valves look perfectly clean. I do see blow by coming through the valve cover cap though, thought that was normal for the engine only having 700 miles on it.
 
I once witnessed a sticking valve free up on a 400 cuin. Ford by putting 1 quart of transmission fluid in the engine oil.

It may not have been sticking real bad as I was surprised at how fast it cleared up.
 
Rusty Muffler said:
No smoke ever, starts right up cold, idles perfect and the dual exhaust at that time sounds even and crisp. After warm up the sound isn't as distinctive. Before the very first start after installing the engine, I replaced the gas tank with a big M dry junk yard tank that leaked so I replaced the tank with a new one. I doubt a dry tank would have absorbed residual bad gas from a dry tank? I know what bad gas can do, glue the valves. I also know carb cleaner will loosen them up. I just adjusted the valves hot a second time, no major differences and the valves look perfectly clean. I do see blow by coming through the valve cover cap though, thought that was normal for the engine only having 700 miles on it.
Click to expand...

The varnish goo stuff that is stuck to the sides of an old, contaminated tank will dissolve and find its way to the intake valve stems. But I have never heard of a metal tank leaching nonvisible goo.
 
Just Carbs said:
The varnish goo stuff that is stuck to the sides of an old, contaminated tank will dissolve and find its way to the intake valve stems. But I have never heard of a metal tank leaching nonvisible goo.
Click to expand...
I took the carb apart just to double check and blew out passages. No change. Swapped coils, no change. I wet the cap and wires in the dark I found a little leakage at the base of the high tension wire at the coil and on the cap near it. The cap looks brand new but I might as well change it before looking into the valves. I still think the valves would be intermittently noisy if they were sticking or spit through the carb. Remember, these are solid lifters.
 
Hogie said:
100 psi sounds a little on the low side to me for a rebuilt motor… the erratic vac reading sounds suspiciously like an intake valve that is leaking slightly, you may not not notice it under light load conditions.. under normal conditions if the valves are seating properly you really shouldn’t see much movement of the vac gauge. I would highly recommend doing a cylinder leak down test before you chase your tail around too much. Just a suggestion.
Click to expand...
I tried applying 100psi to #1 cylinder and the leaking was easy to hear. After that the engine kept rotating when I applied pressure. I guess I'll have to buy a tester, Darn it!
 
Hogie said:
100 psi sounds a little on the low side to me for a rebuilt motor… the erratic vac reading sounds suspiciously like an intake valve that is leaking slightly, you may not not notice it under light load conditions.. under normal conditions if the valves are seating properly you really shouldn’t see much movement of the vac gauge. I would highly recommend doing a cylinder leak down test before you chase your tail around too much. Just a suggestion.
Click to expand...
To be clearer, #1 on it's compression stroke TDC. The next cylinders the motor wouldn't sit still.
 
Sounds like the machinist might be cutting or lapping the valves and seats again.. check the valve spring tension too while you're at it. After this issue gets fixed though.. gonna be a honey for driving!
 
I haven't done a leak down test because I didn't want to spend the $ yet. I did another compression test DRY then WET and here are the results. Can someone tell me what the numbers mean as this engine has about 1000 miles on it? It looks like a leak down test is next. The vibration is really throwing me. Can carbueration cause that?
IMG_20240615_190106222.jpg
 
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