Here's one for you all to ponder..

Sometimes, (usually when warmed up), my '72 U code Supreme idles erratically. Not bad,, just a transition from 800 rpm then down to 600 then back up to 800. The cycle occurs about once every 10 seconds and only when in gear at idle. I don't think it's a vacuum issue because it runs fine otherwise. Also, no issues under normal cruising or WOT. Carb is a Holley 770 Street Avenger and was installed brand new 2 1/2 years ago, along with a new fuel filter. Brand new mechanical fuel pump installed about 2 years ago. Again, no issues under normal or WOT.

Mind you, I'm not overly concerned about this, but if it's an easy troubleshoot/remedy, then I'll look into it. Otherwise, just looking for ideas.

 

One of the symptoms of a bad vacuum - is an erratic idle. There is no vacuum advance @ WOT; albeit, no chance to influence ignition parameters via vacuum. However, @ idle, a weak, leaking vacuum can significantly affect timing parameters; in particular, your vacuum advance since vacuum advance operates @ idle not WOT. Might check your real vacuum in Hg" to confirm you have no vacuum leaks. A number of other possibilities, but the fact it runs fine on the open road under normal cruising or WOT does not mean the vacuum is not suspect because it is, since it only operates at idle.

 

Disconnect the vacuum advance and check the vacuum with that tubing connected directly to your gauge. If you have the vacuum advance connected to a ported source on the carb, the idle could be adjusted so you are getting partial vacuum strength to the advance canister on the distributor. A cheap and dirty test would be to connect it to a direct manifold source and see if it changes. Or vice-versa if it is already connected direct.

Another possibility is an intermittent spark plug at idle that only effects the idle speed while the pressure of the torque converter is against it.

 

Seems like a simple enough check...but why would it only occur when warmed up?

 

Vacuum advance is hooked up to the carb...and if it were a plug issue, wouldn't I have a miss?

 

Because "warmed up" is the normal optimal operating temperature of your engine in which it spends >95% of its normal operating/running time.

 

Also, when not warmed up, the choke is closed to some extent, which dramatically changes the air/fuel ratio. The richer mixture can offset a lean condition caused by a vacuum leak.

 

If you have access to a vacuum gage, connect it to a manifold vacuum port with engine at operating temperature, choke fully open, distributor vacuum line plugged, and watch the reading at idle.

Steady 17-20"Hg vacuum reading at idle is great. Cam specs can affect idle vacuum.

If the vacuum reads steady low, 12-15"Hg, good chance the timing chain has stretched causing valve operation to be off.

Steady low 15-17"Hg indicates an ignition timing problem. Adjust distributor timing for highest vacuum reading at idle, taking detonation into consideration.

Steady low below 10"Hg indicates a significant vacuum leak. Usually intake manifold or brake booster.

If vacuum gage is drifting or fluttering it often indicates burnt or sticky valves or a plug misfire. A smooth drift from low to high reading can indicate a carb or mixture problem.

If everything checks normal, reconnect the distributor vacuum advance and see what engine does. Possible the advance is coming in due to small fluctuations in vacuum and changing idle speed on you.

As you see a good vacuum gage is one of the most useful diagnostic tools you can own. It can help with ignition timing, idle mixture and troubleshooting why vacuum operated stuff doesn't work.

 

 

While explaining operation of the DTVS to a new user in another thread, occurred to me that if your car still has the DTVS it could be switching distributor vacuum advance between ported and manifold vacuum at idle. This would definitely change idle speed.

Someone else on here was having that issue because his coolant temp was running borderline of the point the valve switches from port to manifold vacuum, 235° or so. If you have a temp gage see if coolant temperature has any effect on when this happens.

 

Not sure what DTVS is, but she runs a steady 195-200 on any given day.

 

Distributor Vacuum Control Switch = DVCS

 

Every time I see this thread, I misread the title as "Eric Idle Question" and immediately imagine that the question is "WHAT is the airspeed velocity of an unladen swallow?" Sorry. Can't help it.

 

At 195-200 DTVS shouldn't come into play but it's one of those things that helps to know what it does and how it works when you're chasing rabbits on an emissions controlled engine.

Distributor Thermal Vacuum Switch. It's at front passenger side of the intake in the coolant passage the thermostat rides in.

It will have 3 or 4 vacuum lines depending on year. Each port is marked what it goes to.

C for ported vacuum source on the carb
M for manifold vacuum source on intake
D for distributor vacuum advance

Around 235-240° it will switch distributor vacuum D port from C (ported vacuum from carb) to M (manifold vacuum) source. Idea behind this is if you're stuck in traffic or other extended idle period, manifold vacuum can advance the distributor to help cool the engine.

Some years have V for vent to the back of the carb air horn, right next to the choke hot air tube fresh air source. Don't think 72 had that but you don't have the factory carb so makes no difference.

 

I don't have one of those....probably because it's been converted to HEI.

 

😂

So do I, and watching "MP and the Holy Grail" over the weekend hasn't helped! Airboddy needs a little British lunacy once in a while. The lunatic is in the hall, and mad dogs and Englishmen go out in the midday sun...

 

HEI or points would not matter as both use a vacuum advance canister, so the operation would be identical.

 

That can be ruled out of the equation then.
How old is your carb? Could the throttle shaft(s) be worn and loose? That could also cause an intermittent vacuum leak and exhibit the erratic idle symptoms.

 

So here's what I got...when I tap directly into the manifold vacuum and disconnect (and cap) the distributor advance, it fluctuates between 10 and 17 Hg, and when I snap the throttle it pulls to a steady 20.

When I tap into the carb vacuum with the distributor advance disconnected from the carb and capped, this is what I get..

And, when I disconnect from the vacuum canister and connect the hose to the carb, this is what I get...



Thoughts?

 

Ported vacuum (distributor hose) looks great.

That rapid fluttering concerns me. That usually indicates worn valve guides, burnt/sticky valves or weak valve springs. How long since the heads were done?

Quick and easy check is pull the valve covers and make sure all the rocker arm bridges are tight. Then see if there's any loose motion between rocker arm and pushrod, and rocker arm and valve stem.

Past that you may be looking at head work.

 

Dave - You did well to post the vacuum gauge reading to a video to share. You have more than a small vacuum leak. Two things directly:
(1) You should see a steady needle (nearly always) if it were strictly related to a vacuum leak involving your vacuum hoses; and,
(2) You have a significantly lower than normal vacuum reading - below 10"Hg. You should see a minimum of 15"Hg if you had a mild to moderate cam, but the reading should be steady; and, optimally between 17"Hg - 21"Hg with a steady needle (always).

The fact you're below normal vacuum is one thing, but more importantly your needle is swinging violently while it should always read steady. Snapping the throttle should always lower the vacuum immediately to ~5"Hg but it should then return to a steady normal needle w/in the range of 15"Hg - 20"Hg. The fact yours returns to a much higher vacuum is peculiar. You stated it returns to a steady 20"Hg. You might post a video of you snapping the throttle. You have significant departure from a steady vacuum.

What is your current idle speed in Park?
What is your current idle speed in Drive?
Where are you on timing?
You have Pertonix Ignition I believe?

 

I would perform one additional test. With the vacuum gauge hooked up in the exact same configuration as in your video, slowly increase the acceleration until you reach roughly 2500RPM. The vacuum should decrease steadily to ~7"Hg and then return once you return to idle. I'd be more concerned/interested in knowing what the needle demonstrates as you increase acceleration (to ~2500RPM). Is it a wildly fluctuating needle which lowers to ~7"Hg (as indicated in you video) or is it a steady needle which lowers to ~7"Hg?

I'm betting (although you didn't ask me to bet) you need new valve springs.

 

Engine was worked about 26,,000 miles ago..bored .030 over with a W30 cam (unknown specs).

 

Norm, when I snap the throttle, it climbs to 20, then goes back the reading on the video.

Idle in park is around 900, and in drive its about 800, and I have a Performance Distributors HEI Street/Strip HEI, and here's my timing :

Idling at about 800 with vacuum advance connected: 34-36
Disconnect vacuum advance at 800 rpm: 10-12
Vacuum advance connected and increase rpm to 2800: Around 40-42...

 

That test will have to wait till Saturday...getting dark and my car has cooled off. But I'll post the findings then.

 

In case you may be looking for a decent valve spring symptom write-up (regardless if the article is not about a classic V8 engine), the manner in which the information in this article is written is very good. You may not have weak valve springs (I suspect you may); generally, an engine with significantly weak valve springs will misfire especially at higher RPMs. In any case, the manner in which the author describes valve float, valve spring compression, valve keepers, vacuum etc. are spot-on - not a bad read.

Diagnose Weak Valve springs

 

Lot of people will say it's snake oil, but dump some Marvel Mystery Oil in the gas tank and see if things calm down any. That or what's sold as "top cylinder lubricant". If a valve is sticking any, those solvents might free it up.

 

I don't have any issues with it misfiring at any rpm, but I'll give it a read..knowledge is power!😀

 

I thought about trying some of that, but I'll wait to see what happens after I run the test Norm suggested.

 

This likely has nothing to do with your current issue but is just an observation about overall tuning.

Your vacuum canister is giving 24* advance, which is a lot, especially for a non-smog, non-low compression engine. My 77-79 403 HEI distributor had a 24* canister and I made an adjustable limiter to set it to 10* and the engine runs well with less chance of preignition / detonation.

 

I dunno if it would make a difference, but when I checked the manifold vacuum, I was connected to a port at the back of the intake manifold.

 

Ok Norm, when I ran this test, here's what happened. As soon as I started increasing the throttle, vacuum went to 20, and stayed there all the way to 2500. As soon as let off, it went back to what was indicated in my video above.

 

Dave - What does the needle demonstrate with regards to erratic swinging and steady? Was the needle erratically swinging as seen in your video at the beginning of this test? As you accelerated did the needle continue to swing erratically up through 2500 RPM and reach a "steady" needle at 20"Hg or did the needle remain erratic through that test? As you let off the RPMs did the needle remain erratic or did the need remain steady as the RPMs were decreased? When the test was over and back to idle, was the needle erratically swinging at the idle position again as demonstrated in your video?

 

As soon as I started to increase the throttle, the needle immediately went to 20 and stayed there until I backed off. It remained at a steady 20 with no erratic swinging or movement. Once I backed off, it went back to what you see in my video above.

 

The erratic swinging of the needle is perplexing/troubling - you shouldn't have any erratic swinging of the vacuum needle at idle, it should be steady. The fact you have no misfire throughout the engine operating range does not lend itself to weak/broken valve springs, but the fact the needle swings violently at idle still remains suspicious.

Based upon results of this last test - immediately climbs to 20"Hg upon acceleration and remains there throughout the acceleration range until you go back to idle with erratic vacuum needle swings at idle, I'd like to see if you can isolate (find) a steady vacuum needle reading at idle. It's possible you have a vacuum leak at idle in at least one vacuum hose at idle. If you're able to demonstrate a steady needle @ 20"Hg upon beginning that test and throughout the RPM range, it sounds like you're pulling great vacuum. I noted you stated you have no misfiring within any RPM range, so it isn't pointing to weak valve springs.

Two things I'd look for:
(1) Check and recheck each of your vacuum hoses for any hose which is loose and any hose you suspect demonstrates cracking/wear. I think you may simply be losing vacuum at idle with no effect once you begin to accelerate; and,
(2) Try the brake booster intake manifold vacuum port as your vacuum source port. That manifold port should be rock steady on the vacuum gauge. If you can't get a steady vacuum needle on that port, I'd once again review each of your vacuum hoses for integrity of cracks and loose fittings.

 

Norm, my brake booster vacuum is connected to the carb but below the throttle plates...will that make a difference?

 

In "theory" (ONLY) it shouldn't necessarily matter. However, the intake manifold port for the brake booster is located in a position on the intake manifold which is the "sourced" vacuum for a brake booster hose; and, this "could" be why you're experiencing erratic swings of the vacuum needle. You might have a slight issue with that carburetor port, the seating of the carburetor to the intake manifold. I don't know. Preference and history dictates the best place for the brake booster vacuum hose is a direct intake manifold feed sourced for the brake booster line. In a nutshell to answer your question, yes - it could make a difference. Can you move the brake booster hose to an intake manifold vacuum source for this test?

 

I could move the hose for the booster, but not without changing the fitting on the intake. So here's what I'm gonna do...I'm going to check vacuum where the brake vacuum hose attaches to the carb, check vacuum where the pcv hose attaches to the carb (if necessary) , check vacuum at the hose for the vacuum canister at the firewall, check the vacuum at the hose coming up from the transmission modulator. If any of these are not necessary, let me know 😀

And just to confirm, all tests are done with the vacuum advance disconnected, correct?

 

Not being there and not really knowing anything about the motor it's hard to diagnose because alot of things can cause this condition. You did say that you had a 770 Holley. The meeting blocks have the idle restriction high on the well. Also from the video you posted it seems to run rough. This position can cause an erratic idle especially if the fuel level is to high in the bowl. Add the fact it runs rough can make a high fuel level more sensitive. I would look at you float adjustment first. You can have a bad adjustment or a bad needle and seat if it's too high. Your bouncing vacuum needle can also be from connecting it to 1 port (runner) and not the plenum and adding a rough idle makes it worse. Out of curiosity what cam is in the motor and what CR?

 

I'm following your statements but maybe the manner of your written statements differ slightly than what you are thinking maybe. At any rate, you want to check the vacuum of the "sourced" vacuum ports. Example, you don't want to check vacuum at the hose coming up "from" the transmission modulator, you want to instead check the vacuum from the port which is "sourced" from the intake manifold "to" the transmission modulator since the transmission modulator is dependent on the "sourced" vacuum port and not visa versa. You're basically checking all the "sourced" vacuum ports and of course the vacuum hoses for each of those sourced ports. I think you know what I'm talking about. You can also use a "T" in the line at various places with your vacuum gauge attached to the "T", which is what I most frequently do, but then again I don't have the erratic swinging vacuum needle as you do. So, check the sourced vacuum ports and those vacuum hoses in the manner you've proposed. Well, any time you remove a vacuum hose (example vacuum advance canister) you have to cap the line from the device (vacuum advance canister) and the source (if you're not measuring that specific source). You're measuring one vacuum source at a time, all other items must be capped.