Topic: Holley Idle Circuit Adjustment

[67gtasanjose]

OK, so are you suggesting that IF the engine increases in RPM with your hand held of the choke plate, then the carb is set too LEAN?

Yes, this pretty much holds up to that fact, everything is relevant to what is actually wrong in the first place. I learned basics by means of driving old farm tractors, old pickup trucks and the likes BEFORE things became automated so I guess it could be said that "I get it" easier than others might.

You need to kinda regulate the amount of "choke" you do with your hand, again it is best on a warmed up engine. (You always get the carb "correct" as best possible on a WARM engine before you attempt any actual choke adjustments, choke adjustments are last) See if things improve or at least change by adding fuel, this is done by using the method of "choking", literally by using your hand over the carburetor and watching for a smoothing out of the idle or maybe even a noticeable increase of idle speed. If you notice ANY improvement, this proves it is running lean...THEN by knowing indeed it is actually LEAN can you start eliminating any would-be causes.

I have right now an old Dodge that has a similar problem, it runs almost exactly like what you are describing you have. I KNOW beyond even a shadow of a doubt, it is running lean at idle... and just like your 390, it runs 100% perfect at any speeds away from idle...I just haven't yet run the very tests you need to do to solve the "why" of my problem. My guess for MY problem is that the EGR valve is leaking...Early 390's (in 1967) do not have an EGR valve so you have at least one less possible cause than I have on my old Dodge.

Contrarily, if it is running RICH, the choking method will result in NO CHANGE or a worsening of your idle condition. RICH conditions usually accompany a bit of black exhaust (smoke) or chalking buildup in the tailpipe and/or the smell of raw gas in the exhaust.

[Texas Swede]

Angela,
When I have had a similar problem I have always started with a vacuum test.
Your car should have a minimum of 17 inches of mercury at 550 rpm idle speed.
Does the car react on the idle screws? If not, it could be a faulty (leaking) power valve.
Also remember that the power valve number should be at least 1.5 inches of mercury lower than the vacuum at the recommended idle speed.
Do you have an original type cam shaft in the car. If not, and it's hotter the symptoms
you have could happen.
Texas Swede

[Angela]

OK, I finally attempted the experiments suggested in this thread, concerning (a) cupping one's hand over the carb primary and (b) re-checking vacuum at idle.

You guys are amazing... the experiment actually did provide me with some additional information. I have no idea what the following results mean, but maybe someone here can help me further???

* I started the 390 and let it warm up until it would idle.
* Once idling on it's own and the choke plate had fully opened, I cupped my hands over the primary throttle plate. I tried this repeatedly yet couldn't really convince myself doing so had any effect. *If* the engine responded to covering the carb with my hands, the effect was so small that I can't say definitively that it was repeatable.
* However, while idling and warmed up, I pushed forward on the secondary linkage (see photo for details) and noticed a HUGE difference in the way the engine idled!!! I can't say exactly, but I'd estimate the engine RPM increased as much as 100rpm and smoothed out noticeably when I pushed forward on the linkage shown in the photo below. Note that the linkage probably moves forward less than 1/16" (more likely 1/32") yet has the significant effect described above.

What does the above described observation mean? Would what I described cause the previously mentioned stalling when I shift into any gear? What do I do now?

Oh, and the vacuum gauge read 17 inHg at idle. Need on the vacuum gauge was rock steady....never moved.
 

[WT8095]

The linkage is designed to force the vacuum secondary throttle plates closed when the primary plates close, i.e. when you take your foot off the throttle. There's a little bit of play in your linkage apparently, so when you push on that lever, you're opening the secondary throttle plates slightly, allowing more air into the engine (as they open further, they expose the transfer slots for the secondary idle system, which would then start allowing additional fuel into the engine as well).

The results you describe are consistent with Richard's description of a rich condition at idle.

One question based on your photo:

Is there clearance under the rubber fuel bowl vent valve when the throttle is closed or at idle? There should be, but it's hard to tell from the angle of the photo.

[Angela]

"Is there clearance under the rubber fuel bowl vent valve when the throttle is closed or at idle?"   --> YES, there the rubber stopper raises up above the fuel bowl vent hole when at idle.

Also, I forgot to mention that I checked the idle mixture screws (one on each side of the primary); they're both set to 1.25 turns out. The engine does react when I turn these screws.

*If* the observation I described means my 390 is running rich at idle, (a) what do I do about it? and (b) does that explain why my car stalls when I put it into any gear? I find it difficult to see how the rich mixture explains the stalling...

[Angela]

Well, after additional reading I suspect that at 1.25 turns out (from fully seated) my idle mixture screws are open too much (rich). I will try turning them in 1/4 turn and restart the car.
However, I'm looking for comments as to whether or not a rich idle mixture could cause an engine to stall when shifting into any gear. I fail to see how this rich mixture could be my issue.

[Angela]

Am I crazy to suspect my torque converter, instead of the carburetor, as root cause of the car stalling when shifted into any gear? Seems (to me) the torque converter is supposed to allow the engine to continue idling, with very little loss in RPM, while in gear with one's foot on the brakes. In my case, the moment I shift into any gear, the car behaves as though the engine is immediately locked to that gear and stalls. *If* I am able to "feather" the throttle enough to keep the engine running while shifting into gear, I can feel and hear (hearing the rear end locking up?) the drivetrain engaging and usually one or both rear wheels will very briefly chirp.

Could my troubles be a bad torque converter? I cringe at the thought...... because this would mean complete removal of the drivetrain (again)...... and anyone having down this with a 390 knows what a pain this is. On a "daily driver" it wouldn't be so bad I suppose. But in this case I sweat bullets every time I have to remove a bolt, wondering how much time I'll spend trying to make it look like said bolt had never been removed.

[Hipo giddyup]

Hi Angela, this all seems very frustrating and i hope you find a solution soon. I have one thing to throw out but not sure it could be the culprit.. There is a secondary throttle stop adjustment under the base plate and I believe your carb has that. This can allow the secondary throttles to stay closed or adjusted to stay slightly open. Unsure if this could be causing a rich running problem but maybe worth a look? I have included a pic of where it would be and it is only accessible from under the carb.
  Lastly, I wanted to say what a beautiful engine/carb you have !! So clean!! If you dont mind me asking, where was the carb rebuilt from? I think it looks fantastic!

[Angela]

I agree that the secondary throttle stop adjustment could play into the mixture being wrong. In my case I'm fairly confident that I have the set screw adjusted properly, merely because the engine responds positively only if I *open* the secondary plates.... allowing more air to enter the engine.... which I think means I'm running too rich.

I'm still very curious to understand more about the torque converter and whether or not that could be my problem. It just seems like something is choking the engine when I engage any gear.... yet isn't the torque converter supposed to isolate the engine from the tranny at idle?

[Angela]

I've read a number of other posts (outside of this forum) that discuss the same symptoms (stalling when put into gear) and point to the torque converter as root cause. I don't know enough about the torque converter to decide if I agree it could be the problem.

These other posts talk about two issues:
(1) something called a "torque converter clutch" (could this be the lock-up function found only on modern cars?)
(2) a plugged transmission cooler

Again, if anyone can help me rule in or out the torque converter as root cause of my engine stalling, I would appreciate the help.

[Mike_B_SVT]

I agree that the secondary throttle stop adjustment could play into the mixture being wrong. In my case I'm fairly confident that I have the set screw adjusted properly, merely because the engine responds positively only if I *open* the secondary plates.... allowing more air to enter the engine.... which I think means I'm running too rich.

I may be mistaken, but if you open the secondary plate (or even if they are already open due to the set screw), that will allow more air in... and that air will in turn draw in fuel from the secondary bowl.  Won't it?  So in my mind that would not necessarily indicate a rich condition, merely that your secondaries are drawing air + fuel as expected.

I would think you would want to cover the secondaries with your hand, which would cut the air and thus any fuel being drawn in there as well.  If you cover the secondaries and the car bogs / dies, then you know the secondary plates are not fully closed at idle.  Which should be corrected.

Please correct me if my thinking is not right.

[69GT350H]

I may be mistaken, but if you open the secondary plate (or even if they are already open due to the set screw), that will allow more air in... and that air will in turn draw in fuel from the secondary bowl.  Won't it?  So in my mind that would not necessarily indicate a rich condition, merely that your secondaries are drawing air + fuel as expected.

I would think you would want to cover the secondaries with your hand, which would cut the air and thus any fuel being drawn in there as well.  If you cover the secondaries and the car bogs / dies, then you know the secondary plates are not fully closed at idle.  Which should be corrected.

Please correct me if my thinking is not right.

Along with working on my Holley equipped Mustangs, I have almost fully read David Vizard's How to Super Tune and Modify Holley Carburetors and believe you to be correct about opening the secondary plate and possibly allowing more fuel. If there are no idle screws on the back, there is no idle circuit back there, only main jets that might get activated by opening the butterflys. There is a possibility of a power valve but I do not believe that would be on a stock carb. But there still might be an issue of it being too rich if either the front or rear float level is set too high, or the power valve is blown out. And yes, there is the possibility that the problem can be non carb related as well. What is the initial timing set at? There should be no vacuum at the distributer at this point.  Just trying to re-read the thread and see what has not been checked yet, sorry if I have overlooked something.

[rockhouse66]

A 4150 Holley idles on all four venturis, so the secondary plate must be adjusted open the proper amount to idle correctly.  Generally, with the carb linkage "closed", the secondary butterflies should be open just enough to see the idle transfer slot.  It should look like a square, meaning the amount of the slot visible should be as high as the slot is wide.

One way to know whether the secondary butterflies are open at least in the ball park is that you should be able to adjust the idle stop screw to obtain a proper idle speed and still have no vacuum at the ported vacuum opening in the primary metering block.  If you have vacuum here, then the secondary is not open enough and not contributing to the idle speed as it should.

[preaction]

A 4150 Holley idles on all four venturis, so the secondary plate must be adjusted open the proper amount to idle correctly.  Generally, with the carb linkage "closed", the secondary butterflies should be open just enough to see the idle transfer slot.  It should look like a square, meaning the amount of the slot visible should be as high as the slot is wide.

One way to know whether the secondary butterflies are open at least in the ball park is that you should be able to adjust the idle stop screw to obtain a proper idle speed and still have no vacuum at the ported vacuum opening in the primary metering block.  If you have vacuum here, then the secondary is not open enough and not contributing to the idle speed as it should.

This seems to be a key piece of information I didn't know, thanks for the post.

[Angela]

Here's what I think I heard from the last several responses:

#1 I should cover the secondary opening with my hands and see if the engine reacts in any way. I'm 90% certain I already did so and the engine did NOT react. I'll double check by repeating this test
#2 Next, I should see if I have vacuum at the carb vacuum port when idling at the proper idle speed. If the secondary stop set screw is where it should be, I should not have vacuum at idle (at the carb vac port), correct?
#3 I should attempt leaning my idle mixture screws 1/4 turn (turn them IN) and see if it helps. They are presently set at 1.25 turns out from seated.

Also, I have seen any comments concerning my question about the torque converter being root cause of the stalling. I finally got a hold of the guy who rebuilt my transmission and sold me the rebuilt fomoco torque converter. I explained the symptoms and he stated they could indeed be caused by the torque converter. I could scream. The thought of tearing apart a concours restored car makes me nauseous. If anyone has suggestions about the best way to swap a torque converter in a car you don't want to scratch anything, please let me know. Thanks.