Help Diagnosing Warm Idle Issue in '71 Skylark

Hi all,

I'm looking for some help diagnosing an issue with my '71 Skylark. When the engine is cold, it idles fine in gear, but once it warms up (180F+), it dies after a couple of seconds in gear. It idles perfectly in Park and Neutral, even when warm. The fuel pressure stays consistent at 5.5 PSI.

I've attached two videos showing how the car behaves when it's warmed up:

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Car Details:

Engine:

• '71 Buick 350
• TA intake
• Holley 750 carb
• 290-94H cam
• Holley Electric Fuel Pump
• Motor was refreshed with new cam, crank, and rod bearings, and new rings
• 16" electric fan
• Flowmaster exhaust

Transmission:

• New Street Lethal TH350 (less than 1000 miles)

Any help or advice would be greatly appreciated. Thanks in advance!

 

What fuel are you using? A guy at the local cruise was complaining about his car not idling in gear. The next time I saw him, he said he mixed a 1/4 tank of race fuel in with 93 octane ethanol free and said it idles perfectly after that. Not sure I believe it but it would be worth a shot.

How tight is your converter? Is it dragging the engine down too much? It seems it is idling quite high in neutral and it pulls it down almost 500RPM when in gear.

Also, what's your vacuum advance situation? If you are using manifold vacuum, putting it in gear may be pulling a lot of timing out. You might try switching to ported vacuum so it doesn't add so much timing initially. Then, up the idle screw to your desired neutral RPM and see if it will run in gear like that.

 

^^^ That.

Idle mixture screws adjusted properly?
Checked for vacuum leaks?
Has the engine EVER run properly at idle? Cam timing incorrect? What is the cranking compression of all eight?

 

When it dies in gear will it start right back up?

do you have to pump the gas pedal some to get it to restart, if so check for vacuum hose leaks.

to get it to restart do you have to hold the throttle open .
Also was some work or a change done to the motor or fuel system and the car was fine before that?

is the motor idling with the vacuum advance hooked up and working or is there a water temp control solenoid that the hose goes to from the Distributor?

 

Videos don't work you have to sign in to see it when they start that one?
Is the carb a mech secondary? If so you can up the idle for the back two by adjusting the screw under the carb. Then you can turn down the idle on the primary and get the 2 or 4 idle screws to get the idle where you want it. There is an upgrade for the secondary screw so it is on top of carb, this makes this adjustment easier.
It is best to adjust those screws while the car is in gear with parking brake on and the wheels blocked. You get the best set up this way and you want the engine idling as fast as possible with those 4 or 2 screws. Use a vac gauge if you have one and use it with a tach on the outside of car.

I use my old Craftsman engine meter for that as it has a large scale to work with.

 

Disconnect the vacuum advance, give ‘er 14 to 16* initial advance and see how it does.

 

Hey everyone, I appreciate all the suggestions so far, but I'm still dealing with this idle issue and could really use more help.

To give you some background, I haven’t worked on my '71 Skylark much over the last five years—just started getting back into it now. The transmission and 2300 stall converter were installed in 2018, and I think I might’ve had the same idle issue back then, but honestly, I can’t remember. I’d start the car every few months to keep everything lubricated, but no real work was done until recently.

Here’s what I tried today, based on the advice I’ve gotten from you all:

1. Set the initial timing to 16° (in Park at 1200 RPM).
   
   
2. Disconnected the vacuum advance and plugged the line.
   
   
3. Checked manifold vacuum—holds steady around 11 at 1200 RPM.
   
   
4. The converter is a 2300 stall and was new when installed.
   
   
5. Adjusted the air/fuel screws on the carb for best vacuum.
   
   
6. Did a compression test on all cylinders (results below):
   
   • CYL 1: 120
   • CYL 2: 121
   • CYL 3: 133
   • CYL 4: 131
   • CYL 5: 139
   • CYL 6: 128
   • CYL 7: 145
   • CYL 8: 150

The compression isn’t ideal, but is it bad enough to cause this kind of issue? The car dies once it's warm, and it’ll start back up easily most of the time, but it won’t idle in gear for more than a few seconds.



Thanks in advance for any additional advice or insight you all can provide!

 

16 degrees is what you have at idle. Do you plug the vacuum advance back to a port after you check it? Does the engine idle rpm change?

What is your total advance?
16 initial + ___ centrifugal + ___vacuuum….

How much centrifugal advance does the distributor advance?

Is your vacuum hooked up to a vacuum port that has that 11” at idle….. or is it vacuum advance going to a ported vacuum nipple, that, when you put a vacuum Guage on that port and slowly rev the engine in park and parking brake on, does the vacuum Guage needle go up as the throttle/ engine speed goes up?

This will help the guys diagnose and troubleshoot the idle / stall problem….

Good luck with it. You got guys pulling for you.

 

There's alot to check, everything effects the next. From what your showing it's idling way too high, and it's way too fat. Once initial timing is set correctly, you want to make sure primary transition slot just barely exposed, a small square showing, about .020, set idle screws to 1 full turn out from seated. Start there,, no engine should idle ice cold, (non choke carbs) if it does it's way too fat. What's happening is your engine is loading up. On average from idle to in gear rpm should drop around 250 rpm. That's when you know it's right. Also any and all carb tuning should be done with everything fully warmed to operating temp. I don't mean just coolant, Inake as well,, go for a 10 min drive come back, take carb off set transfer slots, idle screws , floats should be set half inch from top of bowl. Also get a external secondary idle screw for adjusting secondary blades. Once the primary is set correctly you make any idle adjustments from secondary. There is WAY more tuning but hopefully this gets you close. I tell people if it starts good, runs good, pulls good, shuts off good, leave it. People get way too involved in tuning and end up further out in left field. Make ONE adjustment at a time. One thing I might add is take the fuel pump off the key, never want a fuel pump pressuring the needle and seat on startup. Just my 02 cents. Hope this puts you in close direction.

 

This whole problem is a carb adjustment once the timing is where it belongs that does not affect stalling. Start with the idle jet screws at 1-1 1/2 open on the screws close each one until the engine starts to stall then open 1/4 turn do all the same way then go back and move 1/4 one way or another to get your highest idle.
You shouldn't be at 1200 rpm at idle in neutral. The compression test is really low got to wonder if the cam is in correctly.

Take the carb off and take a pic of the blades in the bottom of the carb to show the transfer slots so we can see if it is open too far. If the primary gets opened too far the idle jets will not work properly. This is where you can open the secondary so the primary can get closed some. This will bring back the idle from the idle jets.

 

Yeah, is that 16* initial timing ? Shoot for 32-36* using total timing with the vac line to dist. plugged.

 

Compression is weak and 11 for vacuum? Weak. So weak signal to carb. Cam is kinda large, did you do anything to up compression ratio. To match cam? 71 is actually measured out about 8.2 in most cases.

 

I wanted to give everyone an update. The issue has been resolved! The car will now idle in gear at around 850RPM and has around a 300RPM difference between Drive and Park. The main issue was I had over exposed transfer slots on the primary side of my carb. I also found a small vacuum leak between the carb and intake (fixed with a new gasket). I also didn't realize that the vacuum secondary's had there on idle adjustment screw under the carb (Holly 750cfm 4160 with Vacuum secondary). So, I set the primary and secondary transfer slots to a square. At that point the car would idle around 1100RPM in drive. I made very slight adjustments to both the primary and secondary idle screws and timing to get the idle to 850RPM. The car now idles great in gear. Thanks to everyone for the help!