Chasing engine issue in my 73

[Geoff's 240z]

Hi team!

I have an on/off (random, every couple weeks recently) hesitation under load through all gears. Fine to idol and get off the line, but as I add more gas, it stumbles and feels like it is choking. I heard detonation under this while driving last night. I thought I'd limp home but it mysteriously fixed itself and full performance came back.

New fuel filter in the engine bay, checked plugs and they are fine. 280 block, head was redone about a year and a half ago, same time added new Ztherapy carbs. 280zx dizzy and standard coil. Any thoughts?

Thanks in advance, pic below for inspiration.

Edited February 22, 2021 by Geoff's 240z

[SteveJ]

Check all fuel lines from the tank to the nozzles in the SU to make sure there are no kinks that could restrict fuel flow. Check float levels. Make sure the vent line from the engine bay back to the tank is in good shape and will allow air into the tank as a vacuum will reduce fuel flow. 

Make sure the carburetors are balanced at idle and at 2000 to 3000 RPM.

What block/head? What is the compression on the engine? The detonation could be a separate issue. It could even be crappy, low octane CA gas.

[Mark Maras]

 Gradually pull the choke on when it's stumbling. If it improves, the stumbling is from a lean mixture. Was the detonation a backfire out of the carbs? I'd also pull a spark plug to check the color.

[Geoff's 240z]

30 minutes ago, SteveJ said:

Check all fuel lines from the tank to the nozzles in the SU to make sure there are no kinks that could restrict fuel flow. Check float levels. Make sure the vent line from the engine bay back to the tank is in good shape and will allow air into the tank as a vacuum will reduce fuel flow. 

Make sure the carburetors are balanced at idle and at 2000 to 3000 RPM.

What block/head? What is the compression on the engine? The detonation could be a separate issue. It could even be crappy, low octane CA gas.

Thanks @SteveJ

Carbs balanced and set to correct mixture. Shaved E88 to get to 10:1 compression on an N42 block (pinging is a new issue). 

Plugs look good. 

[Zed Head]

12 hours ago, Geoff's 240z said:

Fine to idol and get off the line, but as I add more gas, it stumbles and feels like it is choking. I heard detonation under this while driving last night. I thought I'd limp home but it mysteriously fixed itself and full performance came back.

Silver 240Z's are popping up everywhere.  Weird.  Looks good.

One thing that might fit some of your symptoms is a sticking vacuum advance breaker plate, if you have vacuum advance connected.  You'd have high advance at idle and low throttle, then it should retard as you open throttle.  The detonation might be from high advance because the plate is sticking.  Pull the cap and see if the breaker plate moves freely.

Mark Maras's suggestion will tell you if it's a fuel/air ratio problem.  You should try what he suggested.  Pull the choke.

[Geoff's 240z]

Carbs balanced. But take a look at this...does it look like my fuel level is low in my filter while at idle? Sucks all the way down to the bottom of the paper filter when rpms are increased.

[SteveJ]

Do you still have the electric pump at the fuel tank? Is it running?

[Geoff's 240z]

10 minutes ago, SteveJ said:

Do you still have the electric pump at the fuel tank? Is it running?

No, I do not believe so. Only fuel pump is mechanical at the block.

Edited February 24, 2021 by Geoff's 240z

[SteveJ]

How old are the fuel hoses? Cracked hoses can let air in.

[Mark Maras]

It's time to test the mechanical pump for pressure (psi) and volume (gpm). The symptoms have the feel of a dying mechanical fuel pump. I remember a similar problem with my 71 back in the day.

[siteunseen]

3 hours ago, Geoff's 240z said:

Carbs balanced. But take a look at this...does it look like my fuel level is low in my filter while at idle? Sucks all the way down to the bottom of the paper filter when rpms are increased.

That's what mine looked like too. Turned out the floats were keeping the fuel level low and starving the carbs. It was very clear once I drove up a steep mountain. The front carb went dry and I was running on the back 3 cylinders.

 

[gundee]

I am pretty positive that the 1973 had a TSB that included installing an electric fuel pump in the rear.

If yours has that mod then that is where the blockage is, if it is fuel issue. 

You may need to pull the filter from the pump and replace. Very common on those.