240z Vacuum Advance

[240zcv]

I am a new first time owner of a 72 240z.  The car has a partial acceleration problem where the car will surge and stumble hard enough to chirp the tires in second gear.  This only happens with partial throttle acceleration with rpm's below 3000.  Flooring the accelerator causes the problem to go away.  I have set the ignition timing at idle to 17 degrees BTDC.  I have pulled and checked all of the spark plugs and their color indicates that the fuel mixture is correct.  Compression is good and within 10% across all cylinders.  I have balanced the carbs which were already very closely balanced.  The spark advances with engine speed and the mechanical advance seems to operate freely.  I have taken the vacuum advance off of the distributor and it appears to differ significantly from the specs in the FSM.  My vacuum advance does not even begin to move until between 11 and 12 inches of vacuum.  The specs in the FSM say that advance should begin at 4 inches of vacuum and be at maximum at just under 10 inches of vacuum.  Could this be causing my partial acceleration problem?  The vacuum diaphragm does not leak, it just appears that the spring is really strong.  I would appreciate any suggestions on other things to look at for my acceleration problem and would like to know if anyone else has run into this issue with the vacuum advance before.

[Zed Head]

12 hours ago, 240zcv said:

The car has a partial acceleration problem where the car will surge and stumble hard enough to chirp the tires in second gear.  This only happens with partial throttle acceleration with rpm's below 3000.  Flooring the accelerator causes the problem to go away

This sounds more like a carburetor problem.  Flooring the accelerator removes vacuum advance from the equation.  A tight spring in the vacuum advance canister would reduce timing advance.  Low RPM would also reduce timing advance.

I don't know much about the SU carburetors but it sounds like a jetting or needle placement or piston problem.  Maybe one of the carb guys has some ideas.

Edit - the behavior is what engines do when the fuel-air mixture is lean.

Edited January 18, 2022 by Zed Head

[SteveJ]

What ignition does the car have? Points? Pertronix? ZX? MSD? Crane?

Have you verified there is no wobble in the distributor shaft?

Edited January 18, 2022 by SteveJ

[240zcv]

The car has a Pertronix ignition.  I am focused on the vacuum advance because this problem occurs during light to moderate acceleration and my understanding is that this is when ported vacuum from the front carb should be causing vacuum advance to occur.  The throttle opening exposes the port to vacuum and manifold vacuum is not too low because the throttle is not wide open.  The vacuum advance does not even start to move until the vacuum is higher than where the FSM says it should be at maximum advance. Everything in the distributor moves freely and I am testing it out of the distributor so I am reasonably sure it is not a mechanical problem in the distributor itself.

[Zed Head]

A timing light should give you an idea of how the timing changes with RPM and throttle position.

[JDziark]

OK, I have the same concern as '240zcv'. I have a 72 240z and the vacuum advance canister barely moves. If I physically pull vacuum on it, I can barely get it to move; I don't think there is any way this thing will move at all when the engine is running. If you pull vacuum by sucking on the vacuum hose, how easy should it be to see movement of the vacuum canister arm? Thanks.

[Zed Head]

It won't move far but you should be able to see it move.  I posted a picture years ago showing one that I took apart.  The diaphragm inside had grown brittle and cracked.  I would guess that on the way to cracking it had stopped moving correctly.

[siteunseen]

I found an old thread from @Jarvo2 that maybe helpful? Couldn't find your taken apart thread Zed Head. 

https://www.google.com/search?q=vacuum+advance+Zed+Head+classiczcars.com&bih=688&biw=1455&rlz=1C1BOFA_enUS494US494&hl=en&sxsrf=AJOqlzVK5ONJsSCqMcJz2sHkT2mwvGxntA%3A1676663907335&ei=Y9zvY6qBFKGmqtsP5P68uAk&ved=0ahUKEwjqvurkq539AhUhk2oFHWQ_D5cQ4dUDCBA&uact=5&oq=vacuum+advance+Zed+Head+classiczcars.com&gs_lcp=Cgxnd3Mtd2l6LXNlcnAQAzIICCEQoAEQwwQyCAghEKABEMMEOggIABCGAxCwAzoFCAAQogQ6CggAEPEEEB4QogRKBAhBGAFQ6QZYtxlgoSdoAnAAeACAAZQBiAHBA5IBAzEuM5gBAKABAcgBAsABAQ&sclient=gws-wiz-serp

Edited February 17, 2023 by siteunseen

[JDziark]

I have the same issue. Vacuum advance is not working. When testing with a vacuum pump, it moves only at full vacuum; FSM indicates it should start moving at about 100 mm Hg. Does anyone know if these can be repaired and if so, who does it? Thanks.