Vacuum reading help.

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sfvaliant

sfvaliant

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I've been developed an erratic idling problem over the past few weeks, I'm hoping for some insight as I do not think it is a vacuum leak. I'll try to provide as much info as I can...

This is a stock /6 with Carter 1BBL.

I first assumed it was a leak, I tried multiple times to find a leak by spraying WD-40 all around the intake where it meets the head, carb base, throttle body, throttle shaft - no change in RPM.

Read some other threads about vacuum leak symptoms that were actually related to a dirty carb, so I pulled it apart, cleaned it thoroughly, new gaskets etc; issue still remains.

Put a vacuum gauge on the carb at the choke pull off inlet (with engine warm) and reads a shaky 20-21in but can sometimes bump down to 18 or even 15in. I took a video of the bouncy needle but can't get it to upload here.

For in town driving, the issue is bad enough to stall frequently at red lights..
What are some opinions?
 
Certainly informative - thanks for sharing.

It's behaving most like #3 but with higher vacuum, and somewhat like #5.
Thinking I should probably remove the valve cover and take a look... might be running a little too lean.

Hoping I don't need to replace the a valve or valve guides..
 
You may need to set your valve lash. Not too hard to do, and works well to use the vacuum gauge while you do the lash.
 
You might want want to check to see is the vacuum advance is blown. Take off the vacuum hose at the carb and suck, if the vacuum does not hold it's bad.
 
A leaking vacuum advance would be a steady leak once throttle is opened enough for it to see vacuum because it is operated with ported vacuum giving a wee bit lower than normal vacuum reading. This would not cause gage needle to quiver. Quivering is caused by unequal intake manifold pressure between valve events due to a stuck, burnt, or maladjusted lash allowing gasses to backtrack into intake.

Check your valve lash first, one valve not fully seating would cause the fluttering readings you are getting, inconsistent lash settings will also change overall vacuum readings, poor idling, and bad performance.
 
When a engine is misfiring and/or not running smoothly any test instrument used will twitch. Years ago I tryed everything imaginable to correct a poor idle/run slant 6.
In the end a remand distributer was like dipping that car in the fountain of youth.
I must add that its poor run wasn't consistant. Sometimes it ran better or worse than other times. My guess is the point dwell was changing with distributer temperature.
Good luck with it.
 
wjajr said:
A leaking vacuum advance would be a steady leak once throttle is opened enough for it to see vacuum because it is operated with ported vacuum giving a wee bit lower than normal vacuum reading. This would not cause gage needle to quiver. Quivering is caused by unequal intake manifold pressure between valve events due to a stuck, burnt, or maladjusted lash allowing gasses to backtrack into intake.

Check your valve lash first, one valve not fully seating would cause the fluttering readings you are getting, inconsistent lash settings will also change overall vacuum readings, poor idling, and bad performance.
Click to expand...

This seems very likely, and I need to check it. I haven't done a valve adjustment myself on this car so it'll be a learning experience. I've looked over the procedure on this forum a couple times so I'm going to check it when I get the chance... which may be a few days away or maybe even next week.
 
RedFish said:
When a engine is misfiring and/or not running smoothly any test instrument used will twitch. Years ago I tryed everything imaginable to correct a poor idle/run slant 6.
In the end a remand distributer was like dipping that car in the fountain of youth.
I must add that its poor run wasn't consistant. Sometimes it ran better or worse than other times. My guess is the point dwell was changing with distributer temperature.
Good luck with it.
Click to expand...

I'm converting to HEI soon - parts are in transit. So if it's not the valve lash than I should be able to see if the distributor is a factor...
 
Before you change out points with electronic ignition thus adding a second variable to the mix, try to find and fix the problem first. The elephant in the room with poor idle most often is maladjusted valve lash. Losing lash adjustment is gradual under normal operation and is recommended to be checked and adjusted every 20,000 miles or so.

Also it would be a good idea to perform a compression test.
 
I agree that setting the valves is the first logical step, followed immediately by a compression test. If you have a burned valve a compression test will show this real quick. That being the case, the cylinder head needs to come off and go to the machine shop to be rebuilt ($500 to do it right). An engine must be mechanically sound, meaning 12 sealing valves and even compression before you can ever hope to get it to run right.

A clogged or sticky PCV valve can cause rough idle. The early metal style can be cleaned by soaking them in lacquer thinner overnight, or blasting them out with carb cleaner spray. The later plastic ones are easily replaced

A worn out distributor can cause no end to poor running issues, remember that 90% of carburetor problems are ignition related. Electronic ignition is a great upgrade over points, highly recommended. If by HEI you mean adapting some sort of GM based distributor, well, it's your car...
 
I always start with the Basics. A good tune up first. Plugs, Cap, Rotor, Points, PCV, Filters, Maybe Wires. All the stuff that you're supposed to do as maintenance. Then talk about pulling the Head off. JM2C
 
I agree with Redfish that most likely the vacuum pulsation is due to mis-fire. My 69 Slant was like that for years, and mechanics said "bad valves", "vaccum leak" and such. Turned out it was the carb and a ~4th one made it run perfect. That was a Holley 1920. Your Carter BBS looks easier to diagnose (have one on my 64 Valiant). If an ignition problem, the HEI may help. Also lookup posts about bad new distributor caps by SlantSixDan and others. The posts are ground off-center, which can cause strange mis-fires.
 
Carb replacement expensive, feeler gage set & perhaps a new valve cover gasket cheap. Do the low or no cost stuff first.
 
I'll definitely be checking the valve lash first - may be a few days away right now though.

C130 Chief - The electronic ignition upgrade is not intended to be a fix to this problem, just something I had planned on doing at some point. And I'll be using a Mopar distributor from a later slant, using the HEI kit here.
The reason I mention it is in case point dwell being is being effected by temperature - that possibility will soon be eliminated.

Thanks everyone for all the feedback, I'll check back once I set the valve lash.....
 
BillGrissom said:
I agree with Redfish that most likely the vacuum pulsation is due to mis-fire. My 69 Slant was like that for years, and mechanics said "bad valves", "vaccum leak" and such. Turned out it was the carb and a ~4th one made it run perfect. That was a Holley 1920. Your Carter BBS looks easier to diagnose (have one on my 64 Valiant).
Click to expand...

Were you ever able to figure out exactly what was the carb issue?
I read some other threads regarding what you are describing so I had initially tore apart the carb, cleaned, and rebuilt.

I've heard the BBD's were prone to worn throttle shaft bushings which caused vacuum leaks, but haven't heard this about about the BBS.

I'm leaning toward some form of valve-related issue but curious to know what exactly was the carb issues you had were.
 
sfvaliant said:
Were you ever able to figure out exactly what was the carb issue?
Click to expand...
Previously, I had tried rebuilding my carb(s), and bought at least 3 rebuilt carbs from auto parts, that I recall. This was over ~18 years. When it still idled rough after installing a rebuilt long block, I was upset. 2 years prior I had installed a different intake manifold, plus checked for vacuum leaks many times w/ propane. I also had a Crane XR700 electronic ignition. Broke down and bought another Holley 1920 and it idled perfectly with no more stumbling off the line - vroom - vroom like my Honda motorcycle used to run. The Holley 1920 has a sealed metering block, so I don't think any rebuild house can check those. The carb that ran great had a strange small hole drilled thru the side into the bore, like a CNC machine went crazy. I put a hollow Q-tip in there so it wouldn't suck in dirty air. Was that a fix? I have read the same about Holley BBD shaft wear, but that is a problem for most any carburetor.

sfvaliant,

I used the same GM 8-pin module that TrailBeast sells but got my parts at the junkyard for ~$15. If you do that, be sure you get the cable that goes from the module to the coil, which is easier than wiring your own as in TrailBeast's kit. Also snip off the GM pickup connector. A great time to also get the knock sensor and module. It screws in the block drain. For a slant, you just need a 1/4 NPT to 3/8 NPT bushing from Ace or Home Depot.
 
So I was able to check and adjust the valve lash tonight. Nothing was too majorly out of wack, some were a little tight but feel good now. However, the rough idle and stalling problem remains.

Here's my notes/comments/questions/thoughts:

At idle, my exhaust is a little white. I've adjusted the idle mixture numerous times and saw no change in color, and I've yet to get any black smoke. Something is causing it to run too rich...

With the valve cover off, the white exhaust color went away.
While adjusting the valves, the adjuster screw on the exhaust valve for cylinder #6 felt way more difficult to turn than the others.
I also noticed that there seemed to be a little more blow-by fumes around cylinder #6.

Are these indicators of a sticking exhaust valve on that cylinder?
Faulty PCV system?

Also, idle is much smoother in neutral, and when the engine is cold. And it's most rough and stalls frequently in reverse... Am I correct to assume that with these factors it is NOT an ignition problem?

Thanks!
 
At idle, my exhaust is a little white.
Click to expand...
That is water vapor, a component of burning fuel, humid air, and once engine is warmed, moisture from crank case. Crankcase moisture build up is caused by short duty cycle never getting engine up to temperature long enough to evaporate it.




I've yet to get any black smoke
Click to expand...
Good that would indicate pig rich condition.



With the valve cover off, the white exhaust color went away.
Click to expand...
With valve cover removed, carburetor can’t be feed moisture laden vapor from crankcase via PCV system. Also as engine warms exhaust system that vapor remains hot enough to be invisible once out of tail pipe.



While adjusting the valves, the adjuster screw on the exhaust valve for cylinder #6 felt way more difficult to turn than the others.
Click to expand...
Not uncomon, it could be starved for lubrication. When adjusting lash was oil dribbling from that rocker? It oil passages could have some crud in them.

If top of engine is sludged up, it would be a good idea to remove rocker shaft, and keeping all rockers in order so they return to the valve they came from, clean shaft, and each rocker’s oil passages with carburetor cleaner or such.



I also noticed that there seemed to be a little more blow-by fumes around cylinder #6.
Click to expand...
Number six is furthest form fan, less breeze.

All blow-by by ends up in sump, a large air fill space occupied by six pistons churning its environment, so if one cylinder is contributing more than its sisters it would be imposable to discern from a puff wafting out of push rod openings.
A compression and leak-down test can pinpoint ring or blow-by issues to a particular cylinder or general engine condition.



Are these indicators of a sticking exhaust valve on that cylinder?
Click to expand...
Probably not.

Sticking exhaust valve would cause a weak power stroke, accompanied possibly by a skip, fuff, fufff, fuffff sound at tail pipe, a compression test can determine if there are valve problems. Also a vacuum gage will show valve problems as well with a waging needle.



Faulty PCV system?
Click to expand...
Is PCV valve stuck, dose it rattle when shaken? PCV valve could be wrong for your engine allowing too much air to be introduced to intake mimicking a vacuum leak. PCV valve generally works or not.



Also, idle is much smoother in neutral, and when the engine is cold
Click to expand...
Ah a clue…
In neutral engine has higher rpm than when in gear, in other words more air & fuel is passing through engine so a vacuum leak’s effect on air fuel ratio is reduced, and as engine speed increases become insignificant. Also I’m assuming that during cold start choke is engaged which enriches A/F ratio which will mask any vacuum leaks.



And it's most rough and stalls frequently in reverse
Click to expand...
Perhaps you are gentler with throttle feathering to keep her running when reversing. Theoretically R & D would cause a similar drop in rpm from park.
 
wjajr, thanks for the detailed replies - I do have an audible skip at the tail pipe, it's very erratic, and I also get an unstable needle with a vacuum gauge hooked up, it's usually within a couple inches as noted in the initial post. So with these symptoms do you think a sticking valve is still unlikely?? The rockers all appeared to be receiving oil, I don't notice cylinder #6 receiving less but I can check again.

If not a valve problem, what are some other possibilities?
So far I'm hearing carb but with not much other specific details - as my jets seem clear...
What are the odds of a dying fuel pump, maybe just fuel starved when engine is warm and choke is open all the way?
 
OwdKasd said:
I always start with the Basics. A good tune up first. Plugs, Cap, Rotor, Points, PCV, Filters, Maybe Wires. All the stuff that you're supposed to do as maintenance. Then talk about pulling the Head off. JM2C
Click to expand...

x2
 
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