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

Rusty Muffler

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I've had the distributor apart and everything looks great, just checked the wires and plugs, compression is 100psi across the board, carb is rebuilt, looked closely at the distributor cap and it looks new, but still has an erratic miss at cruise or still at 15-2k rpm. Only other clue is the vacuum needle shakes rapidly in one spot but reads good around 18. 301 V8 Engine has about 600 miles on it. Also re adjusted the valves at 500 miles and changed the oil. Now I'm back to thinking fuel and maybe should mess with the metering rods?
 
Generally, no adjustments on the BBD metering rods. A carb mixture issue will result in a slowly-wandering vac gauge needle. Something which is happening on one cyl will result in specific movements when the cyl fires.

When you adjusted the valves, did one need more adjustment than the others? Were all of the adjustment studs the same length when done, with the same number of threads showing?

How is the generator output? Has it been checked? Are all of the connections clean and tight? With the lower output of the earlier generators, compared to the later alternators, things which might not matter in newer situations might become more "iffy" on the earlier lower output systems. BUT they worked when the cars were new.

Just sine thoughts,
CBODY67
 
You can try using a reducer in the vac line to stop the shake. See if it improves the reading. You still have a miss though. Is the miss intermittent? Stromberg 2 bbl carb?
 
Generally, no adjustments on the BBD metering rods. A carb mixture issue will result in a slowly-wandering vac gauge needle. Something which is happening on one cyl will result in specific movements when the cyl fires.

When you adjusted the valves, did one need more adjustment than the others? Were all of the adjustment studs the same length when done, with the same number of threads showing?

How is the generator output? Has it been checked? Are all of the connections clean and tight? With the lower output of the earlier generators, compared to the later alternators, things which might not matter in newer situations might become more "iffy" on the earlier lower output systems. BUT they worked when the cars were new.

Just sine thoughts,
CBODY67

You can try using a reducer in the vac line to stop the shake. See if it improves the reading. You still have a miss though. Is the miss intermittent? Stromberg 2 bbl carb?
Intermittent and above idle. Carter 4 barrel original.
 
Generally, no adjustments on the BBD metering rods. A carb mixture issue will result in a slowly-wandering vac gauge needle. Something which is happening on one cyl will result in specific movements when the cyl fires.

When you adjusted the valves, did one need more adjustment than the others? Were all of the adjustment studs the same length when done, with the same number of threads showing?

How is the generator output? Has it been checked? Are all of the connections clean and tight? With the lower output of the earlier generators, compared to the later alternators, things which might not matter in newer situations might become more "iffy" on the earlier lower output systems. BUT they worked when the cars were new.

Just sine thoughts,
CBODY67
Haven't checked but the idiot light works. Also the battery is always charged and it starts no problem. I usually run my lights but wouldn't the battery show weakness if that were the case? Funny, when I do any driving or acceleration I don't feel a misfire, just that cyclical vibration I've felt before. When sitting still in neutral and bring the rpm's up, I can hear the erratic misfire.
 
is this miss steady? What Im getting at is when cold if you go start it and immediately hold the throttle at 1500-2000 will it miss? If so you can squirt water on the exhaust manifold by the exhaust exits to see which one takes the longest to heat and boil off. That should point to the misfiring cyl. If you really think its fuel related you can get an adapter and an edelbrock carb.. yours should be a wcfb..
 
Running Mopar electronic ignition? I once had pretty much the exact problem. Turns out the distributor shaft had a slight bend in it, which caused the vac needle to shake, and would kind of mis fire around 1500-2000 rpm range when in park, it wasn't really noticable when driving. And that was a brand new distributor.
 
I've had the distributor apart and everything looks great, just checked the wires and plugs, compression is 100psi across the board, carb is rebuilt, looked closely at the distributor cap and it looks new, but still has an erratic miss at cruise or still at 15-2k rpm. Only other clue is the vacuum needle shakes rapidly in one spot but reads good around 18. 301 V8 Engine has about 600 miles on it. Also re adjusted the valves at 500 miles and changed the oil. Now I'm back to thinking fuel and maybe should mess with the metering rods?
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.
 
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
Honestly, if it IS a fresh rebuild, a compression reading like that is indicative of cylinder rings that haven't seated yet. It takes anywhere from 500-2000 miles for rings to fully seat, depending on ring thickness, materials, and the degree of cylinder hone. On top of THAT, that excessive ring blow-by could be responsible for the erratic vacuum signal as well.

When I rebuilt my 400 back in 2020, initially I saw 105-115 compression readings on my vacuum gauge, and this was with flat top pistons too. I called the machine shop that did the work to complain and was reassured that once the rings seated, it would go up. 4 years later with a few thousand miles, I see 130 PSI on the comp gauge. That equates to nearly 10:1.

On a side note, concerning compression gauge readings: divide the # by 14.7 & add 1 to get your static compression ratio. Atmospheric pressure is 14.7 at sea level, yet the gauge reads zero. At 2:1 compression ratio, the gauge would only read 14.7 (or 15 to the naked eye if using a analog needle gauge). 100/14.7 yields 6.8, so then you add 1 and get 7.8, almost 8:1 ratio. If this was a smog era motor rebuilt to factory specs it would be perfect for a new rebuilt engine.

Dr. Dave's recommendation: drive it, take 2 tankfuls of non-ethanol gasoline and baby it, but still occasionally blast it full throttle for no more than 10 seconds each day when fully warmed up (or 2 full throttle runs at 5 seconds each depending on road & traffic conditions). This will help seat the rings. Then change the oil & retest the compression with a gauge and see if it doesn't improve.

On a fresh motor, I like to jet the carb a little rich to make sure it doesn't overheat.

And if you are using synthetic motor oil, break-in mileage is double the length of time/miles versus conventional oil. A magnetic oil plug and/or a bar magnet strapped to the oil filter isn't a bad idea either.
 
In addition to the above ideas, run it at night or in closed garage quick to see if you are sparks anywhere for misfire
 
LOL, two tanks full of non-ethanol gas is some kind of magic potion?
No, but ethanol blended fuel wasn't widely available when our cars were new, therefore they weren't "hardened" against the effects & damage ethanol can cause.
Such as water condensing in the fuel, dry rotting all the rubber lines from the inside out, and leaking rubber & neoprene seals in the carb or the mechanical fuel pump diaphragm.

Ethanol can and will absorb moisture out of the air and water doesn't burn very well, and may result in foul tuning & drivability issues. Water collecting in the bottom of the carb float bowl will also result in difficult starting.

Plus, gasoline vehicles built prior to the advent of ethanol fortified gas just run better and get better mileage on pure gasoline.

If the original poster already has a miss, shudder or stutter to begin with, let's make sure all the controllable variables are correct, such as running the proper fuel. It's not a magic pill, but it does remove a cause of potential problem from the equation. And if he already has some E15 in the tank, it could take up to 2 whole tankfuls a of real gas to flush it all out. That, and by the time he has burnt thru 2 full tanks SHOULD give it time to seat the rings.
 
I wouldn't go beating the piss out of it... but definitely don't be afraid to drive it at varying speeds and rpms, that's the best thing for it. Don't let 'er sit either. After the break in, the first oil change you should be running something like diesel oil or something high with zinc. If you don't know exactly how and what was done to the heads i.e. hardened valve seats and guides installed consider a lead additive too.
 
No, but ethanol blended fuel wasn't widely available when our cars were new, therefore they weren't "hardened" against the effects & damage ethanol can cause.
Such as water condensing in the fuel, dry rotting all the rubber lines from the inside out, and leaking rubber & neoprene seals in the carb or the mechanical fuel pump diaphragm.

Ethanol can and will absorb moisture out of the air and water doesn't burn very well, and may result in foul tuning & drivability issues. Water collecting in the bottom of the carb float bowl will also result in difficult starting.

Plus, gasoline vehicles built prior to the advent of ethanol fortified gas just run better and get better mileage on pure gasoline.

If the original poster already has a miss, shudder or stutter to begin with, let's make sure all the controllable variables are correct, such as running the proper fuel. It's not a magic pill, but it does remove a cause of potential problem from the equation. And if he already has some E15 in the tank, it could take up to 2 whole tankfuls a of real gas to flush it all out. That, and by the time he has burnt thru 2 full tanks SHOULD give it time to seat the rings.

Been running ethanol blended fuel in vehicles as old as 1938 with several rebuilt engines in the bunch as well as small and medium equipment and only had two instances where the fuel might have been a problem.

1) An Echo string trimmer that after using it for 20 years the grommet going into the tank the fuel lines go through got mushy.

2) Fueled up the 58 and headed out of town for the weekend. Car started to run a little funny and continued to feel not right the rest of the weekend. The morning we're leaving the car barely started and had to nurse it home about 160 miles.
Got home, let it sit a couple days put an electric fuel pump on the fuel line coming from the tank and pumped the gas into large glass jars to see what it looked like.

Had definite separation of the fuel. Could have been phase separation, could have been water seeing how I pumped from the fuel pick up in the tank, don't know but that was the only fuel related problem I ever had over many old vehicles over many years.

While I agree about using ethanol compliant hoses and gaskets, your suggestion to run two tanks of non-ethanol fuel now won't reverse anything the ethanol fuel "may" have caused.

That is the meaning of my post.
 
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Put a timing light on the distributor to coil high tension wire and place the light where you can see it through the windshield and watch it while you drive at night at the miss fire rpm. Then repeat on each individual plug wire.
Put a timing advance tape on the harmonic balancer and look for erratic timing at the miss fire rpm in the driveway.
Aim a timing light on the valve rocker arms. Move it from plug wire to plug wire as you check each and every rocker to identify sticking valves while the engine is missing in the driveway.
You don't have a systemic compression problem, and probably not a fuel problem. You have an ignition problem or a sticking valve.
 
Put a timing light on the distributor to coil high tension wire and place the light where you can see it through the windshield and watch it while you drive at night at the miss fire rpm. Then repeat on each individual plug wire.
Put a timing advance tape on the harmonic balancer and look for erratic timing at the miss fire rpm in the driveway.
Aim a timing light on the valve rocker arms. Move it from plug wire to plug wire as you check each and every rocker to identify sticking valves while the engine is missing in the driveway.
You don't have a systemic compression problem, and probably not a fuel problem. You have an ignition problem or a sticking valve.
I'm trying your tests as we speak. Timing light on the drivers bank hooked near the spark plug shows solid flashes even when I can feel it miss randomly. However, hooking the light to the coil shows occasional flicker but doesn't seem to match the missing necessarily. Next the passenger bank. If you've read my thread completely it's a random miss that causes an in and out vibration. Thanks for your help!
 
Put a timing light on the distributor to coil high tension wire and place the light where you can see it through the windshield and watch it while you drive at night at the miss fire rpm. Then repeat on each individual plug wire.
Put a timing advance tape on the harmonic balancer and look for erratic timing at the miss fire rpm in the driveway.
Aim a timing light on the valve rocker arms. Move it from plug wire to plug wire as you check each and every rocker to identify sticking valves while the engine is missing in the driveway.
You don't have a systemic compression problem, and probably not a fuel problem. You have an ignition problem or a sticking valve.
The other bank is good too, no signs of a miss when it's missing quite randomly. While running at higher rpm, the coil reads 13.5 volts and on the other side of the ballast is only about 1 volt less (12.5). With engine off, key on the voltage reads 12volt on one side and about 9.5 on the dist side.
At this point only the high tension wire is showing an unsteady light. All spark plugs are uniform light in color.
 
Put a timing light on the distributor to coil high tension wire and place the light where you can see it through the windshield and watch it while you drive at night at the miss fire rpm. Then repeat on each individual plug wire.
Put a timing advance tape on the harmonic balancer and look for erratic timing at the miss fire rpm in the driveway.
Aim a timing light on the valve rocker arms. Move it from plug wire to plug wire as you check each and every rocker to identify sticking valves while the engine is missing in the driveway.
You don't have a systemic com
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pression problem, and probably not a fuel problem. You have an ignition problem or a sticking valve.
I tried closing the choke, no change in missing.
 
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