intake manifold vacuum measurement

[kenward1000]

Did a vacuum test yesterday on my 72 240z.  plugged the gauge into a port on one carbs intake manifold.  Reads about 14" at idle, goes up to 20" when revved above 3000 rpm.

Are these values "normal" or do they indicate a vacuum system leak somewhere?

If the reading is low, how to figure out where the leak might be?

Thanks

[AK260]

I think it varies engine to engine depending on displacement, cam, compression etc.

If it at all helps, here is what mine was doing ...

Mine is 10.5:1 compression with a lumpy cam as you can see at idle.

Here is a good guide ...




Tell us more about your engine and it’s spec.

[AK260]

Pa. this is an excellent resource to review also ...


http://www.atlanticz.ca/zclub/techtips/vacuum/

[Racer X]

14 minutes ago, AK260 said:

I think it varies engine to engine depending on displacement, cam, compression etc.

If it at all helps, here is what mine was doing ...
 

 

Mine is 10.5:1 compression with a lumpy cam as you can see at idle.

Here is a good guide ...




Tell us more about your engine and it’s spec.

 

Your Z sounds nice and healthy.

[Racer X]

38 minutes ago, kenward1000 said:

Did a vacuum test yesterday on my 72 240z.  plugged the gauge into a port on one carbs intake manifold.  Reads about 14" at idle, goes up to 20" when revved above 3000 rpm.

Are these values "normal" or do they indicate a vacuum system leak somewhere?

If the reading is low, how to figure out where the leak might be?

Thanks

Connect the gauge to a port on the balance bar. Your readings should then look more like those in the video AK260 posted.

[kenward1000]

5 minutes ago, AK260 said:

Pa. this is an excellent resource to review also ...


http://www.atlanticz.ca/zclub/techtips/vacuum/

Found that site yesterday, thanks.

Engine is stock 72 240z, Pertronix ignition, 32 degrees total advance at 3000+ rpm, dwell 30 degrees, upgraded coil.  Compression is 160psi per cylinder, +/- 5. 

I've rebuilt the carbs, synced with a TwinMax.  Mixture screws 1 turn out from the stop, which makes the plugs burn clean.  

Starts readily with full choke, 2000rpm.  Engine almost immediately begins to run rough at idle, rpm slowly reduces until it's about to die.  Revving the engine above 3000rpm a few times improves the idle.  Idles at 700-800 when fully warm.  Really seems cold blooded.

Just wondering if the 14" vacuum at idle is low, contributing to poor idle.

Thanks for all the replies!

[Racer X]

37 minutes ago, kenward1000 said:

 

Starts readily with full choke, 2000rpm.  Engine almost immediately begins to run rough at idle, rpm slowly reduces until it's about to die.  Revving the engine above 3000rpm a few times improves the idle.  Idles at 700-800 when fully warm.  Really seems cold blooded.

 

Carburetor ice?

[AK260]

Your Z sounds nice and healthy.

Thanks Racer - it’s good to have an experienced opinion on it. I also worry I have a slight leak still somewhere!

Found that site yesterday, thanks.
Engine is stock 72 240z, Pertronix ignition, 32 degrees total advance at 3000+ rpm, dwell 30 degrees, upgraded coil.  Compression is 160psi per cylinder, +/- 5. 
I've rebuilt the carbs, synced with a TwinMax.  Mixture screws 1 turn out from the stop, which makes the plugs burn clean.  
Starts readily with full choke, 2000rpm.  Engine almost immediately begins to run rough at idle, rpm slowly reduces until it's about to die.  Revving the engine above 3000rpm a few times improves the idle.  Idles at 700-800 when fully warm.  Really seems cold blooded.
Just wondering if the 14" vacuum at idle is low, contributing to poor idle.
Thanks for all the replies!

You’re most welcome.

Your compression readings are in the ball park for a stock engine and it’s good that she starts without issue.

Assuming you are using the stock needles, I can’t imagine 1 turn down would be rich, if anything it’s a little too lean.

I had a similar issue twice: first time, the idle was too rich and fouling plugs.

Second time, the carb to manifold nuts were loose, causing an big vac leak. Oh and then I discovered that the diaphragm in my vac advance canister was completely dead, sucking air into the manifold!

Have you checked the 4 nuts on each carb and all the manifold nuts for torque?

Also take off vac advance pipe from the manifold side and suck on it to make sure you don’t have the same issue as I did.

Finally, check the brake booster vac hoses to ensure: 1. You don’t have a split somewhere and 2 they are sealing with the jubilee clips.

[Racer X]

17 minutes ago, AK260 said:

 

 I also worry I have a slight leak still somewhere!

 

 

Headers have that pinging sound that can make one think there is a leak. Steel tubing doesn't absorb the shock of the exhaust pulses like cast manifolds do.

Or, they may just be leaking some. Your car is pretty clean, so finding evidence of leaks should be easy, just look for the streaks of carbon where the exhaust gasses are squirting out.

Edited November 27, 2020 by Racer X

[siteunseen]

56 minutes ago, Racer X said:

Headers have that pinging sound that can make one think there is a leak. Steel tubing doesn't absorb the shock of the exhaust pulses like cast manifolds do.

Or, they may just be leaking some. Your car is pretty clean, so finding evidence of leaks should be easy, just look for the streaks of carbon where the exhaust gasses are squirting out.

I agree with mr Racer 100% on this. I have had so many tell me I had an exhaust gasket leak but I do not. You can hear things so much better with header runners it sounds like a leak but isn't. Do the short section of hose as a stethoscope and you'll hear all 6 sound the same.

[kenward1000]

On 11/27/2020 at 1:19 PM, AK260 said:

Have you checked the 4 nuts on each carb and all the manifold nuts for torque?

Also take off vac advance pipe from the manifold side and suck on it to make sure you don’t have the same issue as I did.

Finally, check the brake booster vac hoses to ensure: 1. You don’t have a split somewhere and 2 they are sealing with the jubilee clips.

 

Checked all the carb to manifold nuts, they weren't loose, but I tightened them up significantly anyway.  Also tightened the hose clamps on the brake booster hoses significantly.  Measured vacuum again, this time at the manifold rail, seems much better.

Unclear on your suggestion about vacuum advance; suck on what?  Bought the car last year and it's been disconnected all this time.  Do I need to add a fitting to the manifold rail to accommodate the disconnected fabric braid hose that runs from the block?

Thanks for your suggestions!

[AK260]

Great job!

OK, first up the disconnected hose you are showing is a Positive Crankcase Vent (PCV) tube and when connected to the balance bar (the bit that is plugged) blow by gasses and hot oil fumes from the crankcase are sucked into the intake. You must connect that into a catch tank otherwise 1) it will eventually make a mess of your engine bay 2) it really is anti-social! ;) It’s not supposed to vent directly to the atmosphere My 0.3L catch tank was £20ish off ebay and works perfectly.

It’s not this but it’s similar with a view tube so I can see how full it is.



On this topic - trust NOTHING a PO has done, we all have different standards! That plug / grub screw may also not be fully sealed, so I would personally take it off, then use some plumber’s PTFE tape around the threads to re-seal it.



Next up, you will find there to be a take off on the front carb that should connect to the distributor vac canister in your photo. It looks like this ...



If that isn’t plugged you will be causing a massive lean condition at part throttle or idle - it may also be whistling loudly - reducing manifold vacuum by quite a margin.

I was suggesting that you suck on the tube that normally would have been connected to the canister on your dizzy, but you don’t appear to have one.

Finally - cue[mention=19495]Captain[/mention] Obvious to tell me I’m at the wrong end of the car or saying something that is “obviously” wrong! :p


Edit: now this I’ve never seen before - is this little pipe some kind of USDM emissions type thing!?