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Violent Idle When PCV Connected
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Author:  morsim [ Thu Jul 31, 2025 8:48 pm ]
Post subject:  Violent Idle When PCV Connected
Hey all,
Long-time lurker, finally posting because I’ve hit a snag.

Running a 225 with a Holley 1920. It idles fine only when I plug the PCV port on the carb. As soon as I reconnect the PCV, it runs rough and shakes violently.

I don’t have a vacuum gauge, but when I removed the oil cap with the PCV hooked up and cover it w/ my palm, I got some serious suction that it's hard to remove my hand. Seemed like excess vacuum is leaning out the mix, perhaps due to a bad valve?
So, I replaced the PCV valve (with a new OEM-spec one per Dan’s recommendation), and now the suction is much less - just enough to hold a piece of paper - but same issue; engine sounds like it's knocking and idles very very rough.

I’ve seen advice to adjust the idle mixture screw and I tried 2.5 turns out from seated position, but as long as the PCV was connected, it just wouldn't/barely idle. Same with other mixture screw positions: with the engine running and throttle engaged enough to prevent stall, I slowly rotated the mixture screw many 1/4ths both clockwise and counter, to no avail.

I imagine it's a mixture-related problem, but per this thread, mixture tuning should wait until the PCV system is stable (which I think it now is) and engine is warm (which I won't be able to achieve unless I plug PCV on carb side). Catch 22.

What should be my next step here?
Appreciate any input.
Author:  eric22t [ Fri Aug 01, 2025 5:05 am ]
Post subject:  Re: Violent Idle When PCV Connected
go ahead and plug the pcv to run the engine up to temperature.
once it's fully warmed up the unblock the pcv and go about your adjustments as per fsm.
also consider...
1) pre broken in the new box.
yes it can be defective new
2) misboxed part
Author:  Dart270 [ Fri Aug 01, 2025 5:54 am ]
Post subject:  Re: Violent Idle When PCV Connected
Sounds to me like the PCV is open and not functioning so that it is presenting an open hose to the carb base. That is like opening your throttle a little with no extra fuel. Could be the hose has a break in it instead, but not likely.

Lou
Author:  SlantSixDan [ Sat Aug 02, 2025 12:49 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
engine sounds like it's knocking and idles very very rough.
That sounds like it could be two different problems. Clearly you have a lean-mixture condition at idle with the PCV hooked up, and that'll take some diagnosing, but a lean mixture (or a vacuum leak) does not cause engine knocking at idle.
Quote:
I’ve seen advice to adjust the idle mixture screw and I tried 2.5 turns out from seated position
That's not how it's done. There is no magic/set number of turns to use. Sounds like you've already found the how-to-adjust-the-idle thread on here.
Quote:
with the engine running and throttle engaged enough to prevent stall, I slowly rotated the mixture screw many 1/4ths both clockwise and counter, to no avail.
The idle mixture can only be adjusted productively with the throttle in the idle position (very nearly closed, just barely cracked open by the idle speed screw).
Quote:
What should be my next step here?
What's the history and condition of the carburetor? You might have trash in the idle circuit which is leaning out the mixture such that the only way the engine will idle is by cutting down the idle airflow, which you're doing by plugging the PCV port. So see if some cleanout will help; do as suggested here.

What exact, specific PCV valve did you buy and install? It's likely not the problem, and if you're getting suction at the oil filler cap hole with the PCV valve hooked up, you probably don't have a leaky hose, but let's check.
Author:  morsim [ Sat Aug 02, 2025 8:46 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Thanks all for replying, here's to address some of the questions.

Quote:
That sounds like it could be two different problems.
Figured so at first, but since it was running like a top with just the PCV plugged, I figured less.
Quote:
That's not how it's done. There is no magic/set number of turns to use. Sounds like you've already found the how-to-adjust-the-idle thread on here.
I tried using that as a baseline just to get the engine running at all at idle, to no avail.
Quote:
The idle mixture can only be adjusted productively with the throttle in the idle position (very nearly closed, just barely cracked open by the idle speed screw).
Hmm, so what comes first the chicken or the egg? :o
If I can't get it to idle, then how do I adjust the idle mixture?
Quote:
What's the history and condition of the carburetor?
Condition can perhaps be inferred from its history. That is, sitting connected to the car sitting in a garage for around 35 years.
I'll have to try that clean-out - and it's okay to let it run rough at that high rpm while doing so? I should mention, with the PCV plugged, as with idle, just running at all is quite rough. It obviously won't stall at high rpm, but things are shaky.
Quote:
What exact, specific PCV valve did you buy and install?
OEM MOPAR 5175351AA (linked in post)
Author:  SlantSixDan [ Sun Aug 03, 2025 10:32 am ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
it was running like a top with just the PCV plugged
Quote:
with the PCV plugged, as with idle, just running at all is quite rough. It obviously won't stall at high rpm, but things are shaky.
These two statements seem to contradict each other.
Quote:
The idle mixture can only be adjusted productively with the throttle in the idle position (very nearly closed, just barely cracked open by the idle speed screw).
Hmm, so what comes first the chicken or the egg? :o If I can't get it to idle, then how do I adjust the idle mixture?[/quote]

If a satisfactory idle can't be obtained using the mixture needle, then the other problem(s) will have to be solved first.
Quote:
Quote:
What's the history and condition of the carburetor?
Condition can perhaps be inferred from its history. That is, sitting connected to the car sitting in a garage for around 35 years. with the PCV plugged, as with idle, just running at all is quite rough. It obviously won't stall at high rpm, but things are shaky.
Oh-HO! This is new and important information.

Cars and their parts degrade from sitting around unused. The fuel in the tank, lines, and carburetor will have turned into varnish over those years; everything will have to be thoroughly cleaned. A quicky spray-job on the carburetor might serve as a very temporary get-by, but it will need to be removed, thoroughly cleaned, and rebuilt with a good-quality kit (not parts store junk, and do not let yourself into the hell that results from getting a "remanufactured" carb). You likely have at least one sticky engine valve. Et cetera.

Let's go back to letter "A" and start from the start: please list everything you've done on this car in the process of waking it up.
Quote:
I'll have to try that clean-out - and it's okay to let it run rough at that high rpm while doing so?
Yes.
Author:  morsim [ Sun Aug 03, 2025 12:05 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
These two statements seem to contradict each other.
Apologies - what I meant was with the PCV connected properly, as with idle in that state, running at all is quite rough.
Quote:
Let's go back to letter "A" and start from the start: please list everything you've done on this car in the process of waking it up.
Here's it all, hopefully I didn't forget anything. This is my first Mopar!

Replaced the following:
- Distributor cap
- Ignition wires
- Ignition coil
- Ignition condenser (was an initial, naive suspect of lack of spark)
- Balast resistor
- Spark plugs (removed rings)
- Fuel filter
- Oil change w/ STP filter
- New battery

Was burning oil out the exaust. Compression was bad. Valves were indeed stuck. Pushrods kept bending (likely from stuck valves). Top-end wasn't getting oil.
So, replaced head gasket with Fel-Pro HS7918 set.
Along with the following:
- Replaced all intake valves and 2 exhaust valves
- New valve seals
- Rocker arm cleanup
- Rattle can paint job
- Replaced gaskets (with that included in the set) and manifold reinstalled as per this article.

This fixed all of the issues mentioned above. No more oil burning, compression consistent across all cylinders (100-110, a bit low, but battery was also a bit weak if that's a factor), oil flowing across rocker arms.

Misc.:
- New brakes and lines (+ MC)
- New diff cover gasket
- New front and rear shocks
- Fresh fuel

Carb had little to no work done, yet.
Since between plugging and connecting the PCV either made or broke the situation, it appears like purely a carb related issue to my unsuspecting gut... And certainly, I've seen the anti-reman warnings and will most definitely heed.
Author:  SlantSixDan [ Sun Aug 03, 2025 2:29 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Good list; we can narrow in on the carburetor (incl. PCV) and the manifold installation (even despite the new gaskets).

Next: what year car + engine is this?
Author:  morsim [ Sun Aug 03, 2025 4:28 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
Next: what year car + engine is this?
1969 Dart Custom, 225

I sprayed carb cleaner around where the manifold and head meet while it was running and there was no change in RPM.
I also used a smoke tester to test for any other air leaks I may have been missing, one thing that leaked was the anti-ice line from carb to exhaust manifold; now that I can remember. It was cracked from when I started with the car.
Hmm, could that be our culprit?
Author:  SlantSixDan [ Sun Aug 03, 2025 4:35 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
1969 Dart Custom, 225
Okeh, a 1969 model has an interesting quirk that will merit some attention, in the form of another passage admitting extra air at the base of the carb, just under the throttle plate. That system is described here.
Quote:
one thing that leaked was the anti-ice line from carb to exhaust manifold; now that I can remember. It was cracked from when I started with the car.
That is exactly the system I'm pointing you at. It is far on the other side of useless with today's gasolines, but the '69 carbs were calibrated for that extra air coming in. Best thing to do for a permanent fix is put a hose barb-to-thread fitting in the carb port and run rubber hose directly to the nipple on the underside of the air cleaner, and remove the steel tube. But it would be interesting to try capping the carb port (and plugging the air cleaner nipple, which is on the clean side of the filter so you don't want dirty air getting in) and see what effect that might have on your idle situation.

Quote:
I sprayed carb cleaner around where the manifold and head meet while it was running and there was no change in RPM.
Good. Means we can probably now focus just on the carb and what's attached to it.

It's sounding more and more likely that a thorough carb rebuild (kit from Jon Hargrove at www.thecarburetorshop.com ) will address the problem.
Author:  morsim [ Sun Aug 03, 2025 5:13 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Excellent, I appreciate your dedication to helping out. :D

Quote:
It's sounding more and more likely that a thorough carb rebuild (kit from Jon Hargrove at www.thecarburetorshop.com ) will address the problem.
Will give 'em a call on the morrow.
Any tips and tricks on completing a first-time carb rebuild without breaking much yet ensuring a thorough job?
And, once rebuilt, how should I configure it by default to get it running + fine-tuning once that's accomplished? this/this?
Author:  SlantSixDan [ Sun Aug 03, 2025 6:01 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
Excellent, I appreciate your dedication to helping out. :D
Shirley!

Quote:
Any tips and tricks on completing a first-time carb rebuild without breaking much yet ensuring a thorough job?
Read, read, read. Get the factory service manual if you don't already have it, and read the fuel system chapter of the Petersen book mentioned and linked in this thread. You'll need a means of properly cleaning the carburetor and its parts. Decades ago I kept and used one of these in my folks' garage, and I fervently wish I hadn't. It was effective, yes, but it came with long term "buy now/pay later" costs I wrote about here.

There are now dip-type carburetor/parts cleaning brews that are somewhat less hideously toxic and injurious, but there are also cheap ultrasonic cleaners, and these magical boxes allow you to use much, much less noxious cleaning agents—detergents rather than solvents—and still wind up with a cleaner part (and oh yeah, you won't be dealing with hazmat-level toxic waste afterward). This is the one I have; I got it used for $0.00 (friend gave it to me). It's a cheap Chinese item, so it's a bit on the loud side, but it works great and it doesn't leak.

You'll need a float gauge, so check with Jon if he still sends one with his carb kits; see here. You might need a float; if yours weighs other than 7.5 or 12.5 grams, it's absorbed fuel and gone heavy and will need to be replaced (Jon might have a replacement for you).

Check to make sure your choke pull-off holds vacuum; if not, you'll need one of those, too.

Add some stretch to your accelerator pump drive spring — grab it firmly by both ends with needlenose pliers and pull. Don't be a gorilla about it, just add about 1/2" to 3/4" overall at-rest length to it before reinstalling it.

Check your choke thermostat to see if past owners and mechanics have bent it out of shape. For the matter of that, actually…what exactly did you replace your exhaust manifold with? Did you find another '69 manifold with the anti-ice tube provisions, or did you buy a new exhaust manifold? Answer to this question has implications for your choke's operation.
Quote:
And, once rebuilt, how should I configure it by default to get it running + fine-tuning once that's accomplished? this/this?
Yes. Start with the mixture needle about two turns clockwise from a light seating (never reef on this screw against its seat; you'll wreck the carb and the needle).
Author:  morsim [ Sun Aug 03, 2025 7:02 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
This is the one I have
:shock: was hoping not to drop the wallet on this one -- hate to ask, but can I just use carb cleaner spray all over the disassembled parts? If not, any alternative to skinny dipping or is that really the only way to go?
Perhaps I can buy a Carter BBS for that price - that is, if anybody has one available for me.
Quote:
what exactly did you replace your exhaust manifold with?
I didn't replace any manifolds themselves, just the gaskets. Re-attached the stock manifolds. Not sure where I may've accidentally implied otherwise...

I bent the choke thermostat "stick" (that hooks into the throttle plate lever) as well as the fast idle cam link according to this video and FSM specs to ensure proper choke positions.
Think I got it right.

Choke pull-off is operational, too.

Quote:
You'll need a float gauge, so check with Jon if he still sends one with his carb kits; see here.
This link brings me back to this thread.
Author:  SlantSixDan [ Sun Aug 03, 2025 7:43 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Quote:
can I just use carb cleaner spray
No. It's not nearly up to the task at hand.
Quote:
Perhaps I can buy a Carter BBS for that price
Only if you have a time machine.
Quote:
I didn't replace any manifolds themselves, just the gaskets. Re-attached the stock manifolds. Not sure where I may've accidentally implied otherwise...
No, my fault, I see now you mean the anti-ice tube was cracked, not the manifold.
Quote:
I bent the choke thermostat "stick" (that hooks into the throttle plate lever)
Urg. Bending the pushrod is a good recipe for spoiling the choke thermostat. There is an adjustment (which should almost never need to be touched) accessable by removing the choke thermostat from the exhaust manifold and loosening a nut. If you think you have to bend the pushrod, it's because something is wrong. Incorrect (too-thin) carburetor-to-intake gasket, for example.

If you can't unbend the pushrod to restore its original geometry, put in a № 1231 electric choke kit .
Quote:
This link brings me back to this thread.
Fixed now.
Author:  morsim [ Sun Aug 03, 2025 10:21 pm ]
Post subject:  Re: Violent Idle When PCV Connected
Thank you, I hope to find time for the carb rebuild in the near future.
For now, I'll try the clean-out method as a temporary bandaid...
I will also bypass the anti-ice system as described and report back.

Just to recap the problem for my own understanding:
The carb is clogged and so there isn't enough fuel mixing with the current amount of air going through.
By plugged the PCV, the air supply is lessened and the fuel/air mixture becomes just right (kind of) to run nice.
And adjusting the mixture screw to allow more fuel to pass isn't actually making it run richer because of the clog.
Sounds about right?

And with that said, technically if I close the choke more it would have the same effect as plugging the PCV?

--

Just looked up the ultrasonic cleaner... that thing really is a magic box huh. Ordered.
What kind of cleaner do you use in it and at what ratio (if diluted)?

UPDATE: On a bit of a time crunch, and Jon's working with a 8-10 week backlog.
I did see a recommendation for the kits over at DaytonaParts back in 2007 - would you say they're still a good alternative?
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