1971 360 question

[neilyoung561]

1/2 the vacuum on a PV is the standard. Even Holley states that on their videos and tech support. I had a 6.5 PV in it originally and it would bog off the line, now it doesn't. The only issue now is the idle. There is a big difference in the idle between P or N and in drive.




Hmmmmm, didn't check that, but when I mash the gas peddle, it launches.

I would disconnect your gas peddle linkage and check if you are able to drop the idle down A body throttle cable doesn't have enough travel in the peddle I perches a secondary linkage to get full throttle part number 20-122 and a throttle lever extension modal number 4014412 to also get more travel if i didn't get these 2 things i would lot have full travel in my carb im just thinking your cable is too tight and keeping your throttle open a little bit and the RPM drop is because of the trany load RPM should be able to set as low enough to stall out the engine

 

[AJ/FormS]

IMO, your idle-timing is your problem.
Coupled with, most likely, a partially open Secondary throttle, and Primaries that are too far closed.
There is NO GOOD REASON to run big idle-timing numbers, NONE. People do it to get the stall-timing right, cuz they don't know how to properly modify the distributor. Or, they're just lazy.

BTW, I run a 12.5PV on my 360 with a 276/286/110 cam (230/237@050), and she will idle down to 550 in First Gear(A833) and pulling herself around the parking lot at 5* timing advance. She'll go down to 500 if I ride the brake a hair.

Fix your timing

 

[neilyoung561]

IMO, your idle-timing is your problem.
Coupled with, most likely, a partially open Secondary throttle, and Primaries that are too far closed.
There is NO GOOD REASON to run big idle-timing numbers, NONE. People do it to get the stall-timing right, cuz they don't know how to properly modify the distributor. Or, they're just lazy.

BTW, I run a 12.5PV on my 360 with a 276/286/110 cam (230/237@050), and she will idle down to 550 in First Gear(A833) and pulling herself around the parking lot at 5* timing advance. She'll go down to 500 if I ride the brake a hair.

Fix your timing

Exactly what i was thinking. Throttle is partially open

 

[72bluNblu]

The timing is off the scale, not sure where, but I would guess around 20 degs.

Not knowing the idle timing is a problem. You have to know where you’re at, exactly, regardless of who you’re gonna listen to for the rest of your troubleshooting and tuning.

Holley has been wrong about power valve selection going on 50 years, at least as long as I've been involved with cars.

You have a tune up problem with 400-500 rpm drop from p/n to in gear. Hillbilly timing tape the balancer so you know where you sit for initial and total timing.

C’mon man. They’re not wrong about PV tuning. Lots of different tuning philosophies out there, and they don’t all apply to every build.

Their method of selecting a PV is a recommendation of where to START tuning, not where to end it.

And it’s meant for the average user. Hell some builders will put a plug in the PV and do all the carb tuning with the jets. Good way to lose gas mileage and run rich at cruise on a street car, but for a car that’s always wide open on the drag strip that’s not a problem.

IMO, your idle-timing is your problem.

lol, the PO doesn’t even know what it is.

Coupled with, most likely, a partially open Secondary throttle, and Primaries that are too far closed.

Could be, still not enough info to determine that for sure though

There is NO GOOD REASON to run big idle-timing numbers, NONE. People do it to get the stall-timing right, cuz they don't know how to properly modify the distributor. Or, they're just

Complete bullshit. There are absolutely reasons to run higher idle timing than what the factory recommends, and that can be very build specific.

Sure, not every build should have 20° advance at idle, but not every build is gonna be right at 10° either.

BTW, I run a 12.5PV on my 360

A perfect example that one size does not fit all. On my 340, that would mean the PV would be open at idle.

And hey, maybe your 360 has a vacuum of 20 inHG at idle and that works for you, but it absolutely wouldn’t work for everyone.

 

[RustyRatRod]

1/2 the vacuum on a PV is the standard. Even Holley states that on their videos and tech support. I had a 6.5 PV in it originally and it would bog off the line, now it doesn't. The only issue now is the idle. There is a big difference in the idle between P or N and in drive.




Hmmmmm, didn't check that, but when I mash the gas peddle, it launches.

Holley has gotten it wrong for over fifty years.

 

[RustyRatRod]

C’mon man. They’re not wrong about PV tuning. Lots of different tuning philosophies out there, and they don’t all apply to every build.

Their method of selecting a PV is a recommendation of where to START tuning, not where to end it.

And it’s meant for the average user. Hell some builders will put a plug in the PV and do all the carb tuning with the jets. Good way to lose gas mileage and run rich at cruise on a street car, but for a car that’s always wide open on the drag strip that’s not a problem.

I'll go along with that. I think most scoff at Holley acting like there's only one way to skin a cat. Although what you say makes sense and I agree 100%, nowhere at any time have I seen anything in print from Holley that backs up the idle vacuum method as being a "starting point", though it does make perfect sense. There's many ways to skin this dog. I like cats, so.......

 

[72bluNblu]

I'll go along with that. I think most scoff at Holley acting like there's only one way to skin a cat. Although what you say makes sense and I agree 100%, nowhere at any time have I seen anything in print from Holley that backs up the idle vacuum method as being a "starting point", though it does make perfect sense. There's many ways to skin this dog. I like cats, so.......

They should definitely say something along the lines that they’re making a basic recommendation for the average user, not an “end all be all” declaration that all PV’s must be half the idle vacuum.

But they’re a big company. Lots of people that buy their carbs open the box and install the carb and if they get as far as changing the jets up or down a size or two they’re doing good.

To me, any of those basic manufacturer recommendations are a starting point for your tune, nothing more. Your car idles at 20 inHG? Well that off the shelf 6.5 PV your carb came with may not be right. Your car idles at 10 inHG? Again, probably gonna have to look at that PV. That’s all it is.

It’s not “wrong”, but it won’t be right for every application either. Just like some of the advice here isn’t right for every application.

 

[RustyRatRod]

They should definitely say something along the lines that they’re making a basic recommendation for the average user, not an “end all be all” declaration that all PV’s must be half the idle vacuum.

But they’re a big company. Lots of people that buy their carbs open the box and install the carb and if they get as far as changing the jets up or down a size or two they’re doing good.

To me, any of those basic manufacturer recommendations are a starting point for your tune, nothing more. Your car idles at 20 inHG? Well that off the shelf 6.5 PV your carb came with may not be right. Your car idles at 10 inHG? Again, probably gonna have to look at that PV. That’s all it is.

It’s not “wrong”, but it won’t be right for every application either. Just like some of the advice here isn’t right for every application.

Puzackly.

 

[Newbomb Turk]

They should definitely say something along the lines that they’re making a basic recommendation for the average user, not an “end all be all” declaration that all PV’s must be half the idle vacuum.

But they’re a big company. Lots of people that buy their carbs open the box and install the carb and if they get as far as changing the jets up or down a size or two they’re doing good.

To me, any of those basic manufacturer recommendations are a starting point for your tune, nothing more. Your car idles at 20 inHG? Well that off the shelf 6.5 PV your carb came with may not be right. Your car idles at 10 inHG? Again, probably gonna have to look at that PV. That’s all it is.

It’s not “wrong”, but it won’t be right for every application either. Just like some of the advice here isn’t right for every application.

Once again, the power valve can open at idle but it won’t flow any fuel.

No one should be setting power valve opening from idle vacuum. That makes ZERO sense because it doesn’t have any affect on the idle and it’s not influenced by idle vacuum or anything else.

To CORRECTLY set the STARTING POINT for a Holley power valve you need to know IDLE CRUISE VACUUM because that’s where the power valve functions.

You should start power valve opening 3-4 inches below CRUISE VACUUM.

Holley knows it’s wrong to use idle vacuum as a starting point.

That’s one reason why guys have fucked up, sorry running Holley carbs.

They get the power valve opening late, so they use a massive pump shot and every time they touch the throttle it slobbers fuel into the engine when it doesn’t need it.

I can’t believe it’s 2025 and guys still say there’s more than one way to skin a cat when it comes to this. It’s crazy to perpetuate the wrong.

Idle vacuum and power valve opening have no connection whatsoever.

Do what you want but doing what you say is wrong.

 

[mopowers]

I've never owned a car where choosing a PV based on half of idle vacuum was a good starting point. They're better off just choosing an arbitrary number as a "starting point," which is pretty much what Holley does since most new Holley's come with 6.5 PV's.

Anyone willing to tune the PV should know that the opening event is based on vacuum when actually driving, not sitting at idle. Using idle vacuum as a starting point is just ignorant.

 

[67Dart273]

If that 71 360 still sports the original ONE YEAR ONLY pistons, then it has higher compression than ANY other year LA 360 and probably DANG CLOSE to the 5.9 Magnum, because they had one year only pistons with a taller compression height and that gave those engines around a true 9:1 "or so" compression. So yeah, it'll suck that 750 down and blow it out the tail pipes.

I had a 71 hi compression. My understanding, perhaps not correct, is that some were high and some low. Maybe early/ late, etc.