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78 280Z need help tuning

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Hello everyone. 


I'm going to jump right in. I just installed a new motor. Fully stripped of emissions except charcoal canister. Cold start delete. Bcddd deleted and plugged. EGR delete. All new vacuum lines. New plugs E3's /wires MSD 7mm. New fuel pump (95psi) regulated to 35psi on rail, and filters before and after. Clean grounds. New battery, starter, alternator. Clean connections on injectors. All injectors were bench tested and contacts cleaned w/ new gaskets. Vacuum tested @ 17. New Fidanza flywheel installed with exedy clutch. 


The issue: 

Car starts right up strong. Will rev to 3k decent. If I hammer throttle it backfires out the exhaust. I have played with the AFM because both units I have already been played with. I have calibrated one using the guide on Atlanticz.ca and everything went well. On reinstall, the car would not start. I had to tighten the clock spring on afm for it to run again. Now it is soo tight I am afraid to go any further. If I hold extra tension on the AFM it runs beautiful and smooth to 6.5k easy. 


What else can/should I do to correct this issue? Open to all suggestions. 

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Have you adjusted valve lash and checked cam timing (notch and groove)?  Is the distributor in good shape, including the mechanical and vacuum advance mechanisms?  Is ignition timing set correctly?


The other way to increase fuel is through the coolant temperature sensor, another Atlanticz tool, linked below.  Or just raise the fuel pressure.  And if you're using old injectors it might be that one or two are not flowing at full rate.  That will give you lean cylinders.  You can't fix that without installing a set of balanced injectors.



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My apologies, I think I  didn't explain enough. It acts like it's getting too much fuel. When adding more tension to afm (making it leaner) it runs beautiful. 


All valves are adjusted correctly. I did a cold adjustment during the build. Exhaust set @.010 - intake @.008. I would like to adjust again when warm. The distributor looks to be in decent condition. I have cleaned all points on cap and rotor. Spark is strong. I'm not too sure about the vacuum advice. I'll look for a way to test this. And ignition timing seems dead on. I'll put a timing light on it again tomorrow to confirm where it's at. I will also be installing a permanent fuel pressure gauge tomorrow if amazon arrives early 🙏  

I have a feeling my fpr is not working correctly causing high pressure in the rail and a over fueling situation. I'll report back in a day or two with results. 


Thank you!


Edited by Mike33Stig
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On 9/6/2021 at 8:28 PM, Mike33Stig said:

I have calibrated one using the guide on Atlanticz.ca and everything went well. On reinstall, the car would not start.


When you say would not start do you mean that it would not stay running or that it would not start at all?  If it just won't stay running, you might still consider the fuel tweak, or increasing pressure with your FPR.  The AFM needs to have the correct rate of movement of the vane with air flow and that's what the atlanticz procedure is supposed to do.  If the vane spring is too loose it will open too quick and run rich as soon as RPM increase, if it's too tight it will open too slowly and run lean.  Get the rate right and use fuel pressure or the sensor mod to change the overall fuel mixture.


If you think that it's running rich make sure that your FPR is not leaking through the diaphragm.  They can do that and still show correct pressure.  Not clear what type of FPR you're using.  Make sure it has the vacuum hose attached also.  It's important for the 280Z EFI system.


Good luck.

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Ok. Update time. 

New fuel pressure gauge is installed and it's telling me what I was suspicious of. Immediately showed around 93psi 😬

I disconnected the return line and placed the return hose into a gas can, fuel pressure then dropped to 40psi. When re connected it system will hold around 25psi at rest reliably. 


So now I'm thinking I have 2 issues to resolve.

1 - Return line is restrictive.

2 - FPR is not sufficient enough. 


I'm running a stock regulator. It doesn't have any leaks and works well, but it seems it can not keep up with this 95psi pump. 




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Are you sure that you have the return hose connected to the proper line?  There are vent lines over there too.  It's not the pressure that would cause problems but the volume.  The 280Z has lines designed for 30 GPH.   The typical aftermarket pump will hit 90 psi.  Like the Airtex E8312.  I had that on my 76, with no problems.  People run the Walbro 255 with no problems except noise.  That's 67 GPH.


It's unlikely that you'd have a restriction due to the design of the return line.  More likely something got stuck in the hose if the connections are right.  I'd doublecheck connections all the way back to the tank and in to the tank.  Looks like you're on the right path.

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I just checked the return lines from the rail to the tank. Used an air compressor @120psi to blow through. Everything checked good until I got to the tank itself. I'm positive I'm connected to the line for return. It would be the lower line. The line right above it would be the feed line. 


As much as I don't want to, I think I will have to drop the tank. I tried a thin piece of steel to fish through and unblock the line, but I can get about 5-6" deep and it seems im hitting a wall or hard bend. 


I restored this tank about 12 years ago using a por15 restor kit and I'm afraid the liner might have blocked the return line. 

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If you have some stiff small diameter cable you can put it in a drill and use it to bore holes and clear passages.  Like snaking a sewer pipe.  It will pass through some bends.  It does look like a bendy path to the opening inside the tank, here are some good pictures.  At least you know what you're dealing with now.



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Tank is fixed and fuel pressure is proper! I read online about using a speedometer cable cut to length and just like you said, run it through the holed with a drill just like a snake. I had to do this a few times with slightly longer lengths each time. Finally made it through the blockage and fuel started flowing out. 


Fuel ressure is now stable around 36-38psi


Now I need to calibrate my AFM again and I should be golden. Thank you for your help! Happy motoring!

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