Stumbling 240Z with Arizona Z car intake and Holly 390

[Zaspen]

I just picked up my first 240Z in 20 years and this one has a Arizona Z Car intake with the Holley 390 CFM 4 barrel carb.  The car has a noticeable stumble off idle and a lack of low end.  It seems to do OK at higher RPMs but it seems sluggish compared to what I remember my old Z to be.  I've read a lot on this site about the AZ/Holley combo and it seems that some love it and some hate it.  Are there any tuning tips for this set up?  I have no experience with the holley carb so I don't know where to start.  Previous owner said it has been performing that way since install.  I would like to put SUs back on but its financially not in the cards now.  Thanks for any help!

[Zed Head]

Not really enough to go on.  Unknown engine, Holley 390, AZC manifold.  Is it the stock L24 engine?  Is the cam stock?  Cam timing,  ignition timing, etc. all can affect performance.  Get as much technical info together about your parts as you can, and check the settings against the specifications.  It might just need a thorough tune-up.

Here's a link to some service manuals.  Links are on the right - https://www.classiczcars.com/files/

Welcome.

 

[Zaspen]

Stock L24 engine.  Per the service records / previous owner it had plugs/cap/wires and a valve adjust in the last 5K miles and it has an electronic ignition installed.  I still need to verify the timing and also compression.

Edited August 9, 2018 by Zaspen

[Mark Maras]

 IMO, the first thing to check is the accelerator pump in the Holley. With the engine off and the air filter removed look down into the carb. and operate the throttle linkage. There should be a visible squirt of fuel. I'd also verify the float level is correct. 

[Zed Head]

Accelerator pump seems reasonable.  I was going to ask also if it was new Holley 390 tuned for the L series or if it was a used 390 from some other vehicle.

https://www.summitracing.com/parts/hly-0-80507-1/media/instructions

 

[Zaspen]

The receipt says its the 8007 Holley that is recommended by Arizona Z Car.  The accelerator pump looks to be spraying fuel fine.  I did pull the plugs to take a look and found some odd stuff.  The cylinder 1 plug looks fine, 2 and 3 are extremely carbon covered and 4, 5 and 6 have moderate carbon.  Not sure what this is telling me quite yet.  

[Mark Maras]

12 hours ago, Zaspen said:

The receipt says its the 8007 Holley that is recommended by Arizona Z Car.  The accelerator pump looks to be spraying fuel fine.  I did pull the plugs to take a look and found some odd stuff.  The cylinder 1 plug looks fine, 2 and 3 are extremely carbon covered and 4, 5 and 6 have moderate carbon.  Not sure what this is telling me quite yet.  

 The fact that #1 looks good and others are carboned up makes me think the problem is electrical. Probably somewhere in the cap, rotor, plug wires, plug wire connections. New plugs, resistance check on the plug wires, double check the plug wire connections and a good quality cap and rotor should clear it up.

 Jalex in Panama had a problem similar to this. #4 plug was always a little darker than the rest. He had good NGK plug wires, at the time. He replaced the plugs, and bought a Bosch cap and rotor. His #4 gremlin disappeared with those changes.

[Zaspen]

I was thinking something similar so I plan to replace the cap/rotor, plugs and wires with good stuff.  In addition I found the air mixture screws on the carb were not set equally- one at a 0.5 turns and the other at 2.5 turns so it makes me think I need to make sure the carb is set up ok.  Thanks to everyone for all the help!

[Zaspen]

Quick update.  The Holley carb was way out of tune and the secondaries weren't even opening.  Its much better now but still has a slight stumble when you first touch the gas but I'm not sure if its lean or rich.  And this was supposed to be easier to tune then the SUs????

[Patcon]

3 minutes ago, Zaspen said:

Quick update.  The Holley carb was way out of tune and the secondaries weren't even opening.  Its much better now but still has a slight stumble when you first touch the gas but I'm not sure if its lean or rich.  And this was supposed to be easier to tune then the SUs????

I have messed with some Holley's definitely not easier than SU's. SU's are super simple with only one circuit and a float level. Holley's have multiple circuits and sometimes two float level. More variables...

It may be stumbling from the pump jet, overly rich. See if you can figure out if the pump jet is getting actuated when you are getting the stumble. There is a kit that has different pump cams and allows you to change how the fuel gets added by the pump jet

[Zaspen]

When you immediately press the gas pedal just a tiny bit (when it dies) the accelerator pump isn't really doing anything yet.  All the pump cams have a very similar initial rate so I'm not sure if that will help but its on my list anyway. I read something about immediately off idle is the point when the carb transfers from the idle circuit to the transition circuit and that can be lean if the butterflies are too open at idle.  I just wish I has a stock set up!

[madkaw]

1 hour ago, Zaspen said:

When you immediately press the gas pedal just a tiny bit (when it dies) the accelerator pump isn't really doing anything yet.  All the pump cams have a very similar initial rate so I'm not sure if that will help but its on my list anyway. I read something about immediately off idle is the point when the carb transfers from the idle circuit to the transition circuit and that can be lean if the butterflies are too open at idle.  I just wish I has a stock set up!

All the down drafts I’ve worked on - Holley , etc... the accelerator pump squirted with ANY movement of the throttle . It should be very sensitive to throttle input . I’d start there since it’s the easiest and cheapest