1978 Datsun 280z No Acceleration Above 4000 RPMs

[Kad77]

I have a 1978 Datsun 280z. I rebuilt the engine last year. Removed the fuel injection and installed 240z SU carburetors.

The car starts and runs great, I typically use it as a daily driver.

The only problem I have, is that I hardly ever rev it above 4000 rpms. I watch videos of people driving Zs and they always take it up to 5000 or even 6000 rpms. 

But for my Z there seems to be no additional power or acceleration above 4000rpms.

There doesn't seem to be any stumbles or misfires, it just seems like the power is flat above 4000 rpms.

I thought Zs had power all the way to the yellow line. I've never driven another Z besides mine, so I do not know for sure.

Any advice is appreciated!

Thanks in advance!

[A to Z]

IGNITION.  Check your timing.  Disconnect and plug the vacuum advance.  Shoot for 18-20 degrees advance at idle.  Have you sync'd the carbs?   are oyu getting any popping if you rev to 4000 and let off the gas and down shift?  If it is a lean condition, you may need some "SM" carb needles.

[Kad77]

Thanks for the reply!

 

So I just set the timing yesterday. Since I currently have a tank of 87 octane in the car, I set the timing to stock 10* BTDC. Once I'm through this tank of gas, I'll fill up with 91 octane and advance the timing. 

 

There was some backfire and popping on deceleration. So yesterday I pulled the plugs and there were a little white. So it was in fact running a little lean. 

 

I richened the carburetors a little by turning the adjustment on the bottom of the carburetors. This seemed to help with the popping. It now decelerates without popping, or at least significantly less popping. 

 

I synced the carbs when I first rebuild the engine. I've checked them once since then and there were still synced. 

 

Any further advice is greatly appreciated!

[bunkhouse]

 Will the engine rev to 6000 rpm when there's no load on it? Years ago I was driving my 71 on I-5 when I found the engine wouldn't go any faster than 4000 rpm. I kicked in the clutch and found it would rev to 6000 but under a load it wouldn't go past 4000. The problem turned out to be a partially plugged fuel filter.

[Kad77]

Thanks for the reply!

 

Yes, my engine will rev to 6000 when not underload. But it still feels like flat power after 4000. 

 

While driving, it will I've revved it to 5500, but above 4000, there is just no acceleration.  

 

I've got brand new fuel filters on mine as well. The old ones were definitely clogged.

 

Any advice is appreciated!

[A to Z]

what elevation are you at?

 

The stock SU needles are a compromise between power and emissions to a point.  "SM" needles are a performance needle set.  they give more fuel at higher RPM's.  Being a L28 engine, it IS larger than the factory L24 engines these carbs were intended for and that may be at least some of the cause if not THE cause.  When installing the needles there is a process to make sure they are installed just right.  I would recommend some SM needles.  Try "Z Car Depot".

 

Did your setup ever pull to 6K or has it always been like this?

 

 

[Kad77]

I'm in Salt Lake City, Utah. So around 4000-5000ft elevation. 

 

The car has always been like this since I rebuilt the engine. Prior to the rebuild, the car did not run. I bought the car non running, and had to rebuild the engine after realizing it was toast. 

 

Thanks! Any advice is greatly appreciated!

[NewZed]

Probably worth your time to study up on how the SU's work.  They're not your Dad's Buick's carburetor.  Air flow is controlled by vacuum.  Seems like your "suction pistons" might not be rising fully, choking off air flow.

 

This from the 1973 Engine Fuel chapter -

 

[bunkhouse]

Check the banjo filters where the fuel enters the float bowls. Then I'd do a fuel pressure and volume test. The symptoms all relate to a lack of fuel.

[NewZed]

Clogged exhaust system comes to mind also.  Especially if it has a catalytic converter.

[seattlejester]

Would suggest checking fuel too, engine uses more fuel under load. Depending on how you converted the fuel setup over could be running the bowls dry under load. My problem with my SU conversion was my bad wiring with my electric fuel pump, on cruise would be fine, but on load it would intermittently pulse and not feed enough fuel and the bowls would run dry

[Kad77]

Thanks for the reply!

 

How could the wiring be screwed up for the fuel pump?

 

As far as I understood it, it was simply two wires.

 

When I first got the car running, I didn't have a fuel pump with high enough gallons per minute and the car would shut off under load. 

 

I upgraded the fuel pump, and it seems to do the job. It is a 4-7psi 30gpm pump.

 

Do you think this is enough?

 

Thanks!!

[bunkhouse]

4 psi is enough but the fuel supply can meet pressure requirements and not meet volume requirements for an engine under a load.

[A to Z]

do you have an adjustable fuel regulator?  If so richen it up a tad and take it out and see if it helps.

[A to Z]

With the SU's, I would go with SM needles, turn the mix screws about 2 turns down from the top, re-sync the carbs, and verify ignition.  should have 18 degrees advance at idle with the vacuum disconnected and plugged.

 

~Joe

 

[Miles]

Go to ZTherapy and buy this video: 

 

“JUST SU’S”

British HS6 and Hitachi “SU”

https://ztherapy.com/just-sus/

Watch the video until you name every part and understand how the SU works.

You are experiencing a lean condition.  Note that the throttle shaft on the SU tends to wear out allowing air to leak into the carb causing a lean condition. This air leak makes proper mixture adjustment impossible.  Contact Ztherapy for the fix.

[toolman]

If your SU carbs were not modified to allow for the larger displacement motor, you might have a fuel problem.  Meaning the carb needs to provide a richer

mixture to run properly.   You can try smaller SU metering rods which would provide a richer fuel mixture.   But doing this may still not provide a rich enough mixture as the SU bore size maybe too small.   

[Datsunpowers]

I know this could sound a little out there, but this problem happened on our race car where it would not rev above 4500. The valve timing was one tooth advance, because the harmonic balancer had spun and the TDC zero mark was not at TDC after a rebuild. When we took the head off for the second time that's when we found the harmonic balancer mark did not line up with the TDC mark. 

 

Make sure your Harmonic balancer reads zero when the piston is at TDC and the top cam sprocket lines up with the cam bearing mark shown in the FSM for the correct valve timing.

 

Hope this helps.