70 240Z revs hang when shifting

[East LA Mike]

My car has the original engine and it runs beautifully except when accelerating thru the gears the revs hang up for a second and I have to delay releasing the clutch. When the cars in neutral and I rev the engine the revs drop like they should. Only while driving do I have this aggravation. Any advice would be helpful to this nagging problem.

[datzenmike]

There's a big difference giving it a quick rev in neutral and a long pull under load even if the same top RPM is reached. 

 

Rev hang is a recent phenomenon on cars with electronic throttles. Your engine revs not dropping when the gas is released is caused by something else. Normally it would never do this so something is slowing down the throttle closing. 

 

Is the throttle linkage dry or dirty and in need of cleaning and lubricating?

Return spring missing on one carb?

Carb linkages need lubricating or cleaning?   

[wayno]

I had this exact thing happening to me for a long time, a few days ago I was having the engine rev again, I entered the freeway and headed on home, at one point I was almost floored, I let off because of a reason I forget right now but the damned thing stayed floored, I was tapping on the pedal trying to get it to release to no avail, I was on the freeway and the reason I was initially floored was because I was having trouble accelerating, so I just went with it and used the brakes to slow me down a little if I needed to, I took the off ramp(long off ramp) a couple miles later and when I got close to the signal I turned the ignition off and pulled over, I lifted the hood and pushed on the linkage and it finally released, I pushed down on it and it stuck again, so I drove home without getting deep into the pedal as I was on main city streets and there was no need to floor it and arrived home without any more issues.

I lifted the hood and pushed on the linkage till it jammed again and sprayed all the linkage parts with PB Blaster and then started cycling the linkage and it quit sticking, dirt gets into the linkage parts and stuff starts getting dry and it starts revving in between gears, eventually it will stick if your ignore it long enough like I did, I have dual SUs.

Edited September 6, 2020 by wayno

[East LA Mike]

Thanks Wayno but there’s no sticky linkage, it’s well oiled and never had a problem with it not returning to the closed position. Initially I thought the choke wasn’t closing but the choke lever and the choke linkage also returns to the closed position. The carbs were taken apart and cleaned too so I’m bummed on what’s the problem.

[East LA Mike]

Yes, the original SU’s are still on the engine. My car is one of the 37 that was restored by the factory in 1996. Other than the spark plug wires and coil it’s original.

[datzenmike]

Well I don't have a lot of experience with SUs only motorcycle slide carbs.

 

Check that the mounting bolts are tight and the dash pot oil is filled up. Inspect all hoses that see intake vacuum are secure including the brake vacuum booster hose.

 

 

As I mentioned, full throttle in gear can't be simulated sitting still. Engine torque will try to twist the engine to the right under load and this may briefly jam the linkage on the firewall.

[East LA Mike]

Carbs oiled and tight. I’ll check the vacuum hose to the brake booster. As I mentioned, I have to hold the clutch in a second longer to shift into the next gear to allow the revs to drop. This is all under normal driving and not pedal to the metal.

[Stoffregen Motorsports]

Though I am not certain of the years, some SU linkages included a vacuum damper that held the revs. I believe this was an emissions item back then, but the reason new cars have it is to aid in shifting. I always assumed Nissan installed this as an emissions device, but maybe it was, in fact, to aid in shifting...?

 

Does your manifold have that vacuum device attached? If so, it may be out of adjustment.

 

Other than that, vacuum leaks or sticky linkage or throttle shafts would be the most obvious culprits. Distributor timing with vacuum advance can also keep the revs up. Have you messed with the timing lately?

[Stoffregen Motorsports]

This diagram shows the vacuum diaphragm.

 

[East LA Mike]

Yes it does have the vacuum control valve shown on the diagram as 1,2 and 3. Haven’t touched it because I’ve found no info on servicing it or adjustments.

The car runs so strong so I haven’t adjusted the timing. The car starts up perfectly and shuts off perfectly with no run on or pinging. Someone suggested to unplug the hose and cap the control valve, which I tried but no change. Anybody have any info on adjusting the control valve before I check the timing. Hate to create a new problem being that it runs so strong other than taking my time when shifting up thru the gears.

[Stoffregen Motorsports]

#6 in the diagram is the part that connects to the linkage, and I believe there is an adjustment there. So even with the vacuum to the control valve disconnected, the idle may still be high.

 

I am no SU expert, but I have seen enough of them in my years to know that your problem must be related to the servo. I am not 100% certain about the adjustment.

 

EDIT - assuming all other parts are properly adjusted and in good condition.

Edited September 6, 2020 by Stoffregen Motorsports

[East LA Mike]

I’ve oiled the rod that goes into the bottom of the servo several times already. Thanks!!

[jagman]

I run SU's on my jag although they have no vacuum ctl valve I have set up a stout return spring on my throttle linkage to drop the revs quickly.

[East LA Mike]

Interesting!! I have a Jag as well. I’ll try that spring on the linkage, wouldn’t that be something if that solved the problem. Thanks!

[Stoffregen Motorsports]

As I was saying, all other things being equal and in tip top shape, which is never the case here on Ratsun...

 

If everything else is in good shape, and you said you already verified the linkage is working properly, the vacuum solenoid is one of the three things I could think of that would cause your problem. The stock spring should return the throttle just fine. Unless there is another problem.

 

I guess I need to ask, has the car always acted like this? Is there anything that precipitated this problem? Was some work performed just prior to this problem occurred?

[East LA Mike]

Yes, had a head gasket leak. Turned out the original head was cracked. A shop I used that specialized in Z’s sent the head out and they were able to weld it. Picked up the car and ran perfectly with no overheating issues since but since then this rev hang up problem. Took it back to the shop after 1000 miles to re-torque the head and explained to them the problem and was told the cars 50 years old and no telling what it could be. They disconnected the hose and plugged the control valve and basically said if that doesn’t work they’ve got no idea why the revs are hanging. So I’m doing my own research and going thru a process of elimination. This week I’ll add a spring to the linkage, check the vacuum hose at the brake booster and then the timing.

 

 

[datzenmike]

So if the head was off then everything above the block was messed with and is suspect. No telling what they may have done.

[wayno]

I would try stronger return springs for the SUs first since you already tried other stuff, if the pedal gets too stiff then go from there.

[Stoffregen Motorsports]

Are you good at adjusting the carbs? I know a lot of people struggle with the various adjustments.

 

Try this, physically remove the linkage rod that goes between the servo diaphragm and see if that fixes it. If not, then you just ruled out that entire system and can move on.

[East LA Mike]

Problem solved for now. Jerry rigged a spring to the linkage by the firewall and revs now drop like they should. After all that oiling and messing around I guess the throttle linkage had a slight hang up. 
Thanks everybody for your help, this old guy that’s losing his patience is grateful,

Mike