L28 w/ SU Carb intake manifold vacuum leak

[Awk34]

So I posted a while ago about some high idle issues I'd been grappling with. I still have a bit of an issue, but I think I've found where it's happening.

I have a 240z with an L28 Engine block, a P90A cylinder head, and SU round-top carburetors.

I took some pictures and videos of what's going on: https://photos.app.goo.gl/xft1yprkDfB7egaW6

I still have to dial the timing in. I think I might have a slight vacuum leak between the intake manifold and the cylinder head. I sprayed some carb cleaner around that area and the RPMs increased a bit. Is it okay that I have this intake manifold with the SU carbs on the P90A head? I've also removed the fuel rail and the mechanical fuel pump, and opted for an electronic one. It's currently a cheap K&N dead-heading pump. I intend to eventually change that out.

In the photos you can see some studs/bolt holes, and how the intake lines up to the head. Is this head & intake compatible, or is the combo likely causing my leak?

Other work I've done:

• swapped exhaust manifold for headers
  • installed new MSA performance exhaust
• cleaned carbs, intake
• new MLS head gasket
  • timed chain & cam according to Tom Monroe manual
• new oil pan gasket
• new valve cover gasket
• new oil pump, timed according to Tom Monroe manual
• Kameari distributor installed
• cleaned some carbon buildup on cylinder head
• Installed internally-regulated alternator
• new chain cover & water pump gaskets
• some more stuff I'm probably forgetting about

I don't know what cam is in there from the previous owner, but it seems like it might be slightly aggressive.

Would a cylinder leakdown test be a good idea? Would valve seals / seats cause these sorts of issues?

Thanks in advance for any help!

[siteunseen]

The p79 is most common.

That p90 head needs early valves and a pretty deep cut, .40 I think?

 

https://datsunzgarage.us/p90/

[Zed Head]

3 hours ago, Awk34 said:

So I posted a while ago about some high idle issues I'd been grappling with.

. I think I might have a slight vacuum leak between the intake manifold and the cylinder head. I sprayed some carb cleaner around that area and the RPMs increased a bit. Is it okay that I have this intake manifold with the SU carbs on the P90A head?

Should have posted the link to your high idle thread.  Don't remember the details.

It sounds like you might have a gasket mismatch problem.  The P90 has injector notches.  You didn't mention if you used an L24 intake gasket or an L28 gasket.

Found it - 

 

 

Edited December 5, 2020 by Zed Head

[Awk34]

You're right, thanks for linking to it.

Hmm, I'm not sure exactly which gasket I got. It was provided when I bought my headers: https://www.thezstore.com/page/TZS/PROD/classic19d/15-6003CH

[Zed Head]

If you gave them the P90 head code they would have sent you an L28 EFI gasket.  If you told them you needed 240Z headers, maybe not.  Maybe you have order details, or maybe they do.  Good to know.  I don't know, really, which mismatch might cause a leak.

Anyway, you should never have intake vacuum leaks.  So, you should focus on that first, because if you tune around it, you'll have to start over when you fix the leak later.

[Awk34]

I gave them the P90A head casting number

[siteunseen]

The p90 has the square exhaust like a proper 240 head but the notches for efi on the intakes.

Get a Nissan from your local dealership if possible. They're the best.

 

[Awk34]

I have the one mentioned in that thread on the way. If the gasket doesn't fix it, any other ideas? I need to double check that there was a leak around that area just to make sure I'm not going insane.

Edited December 8, 2020 by Awk34

[Awk34]

Here's that new manifold gasket installed.

[siteunseen]

I can't tell from the photo if you have it bolted down yet but I have always heard to start in the middle and tighten them down. Working your way to the ends as you go. Maybe the others could correct me but that's what I've always done.

[Awk34]

Yep, that's what I did to bolt it back on. I follow a similar sort of pattern as head bolts, alternated back-and-forth, starting from center.

[Awk34]

So got to working on the Datsun again today. I'm still having the same issues. It will only start with the choke on a little bit, and it idles above 2000 RPM (it was 70℉ today).

 

I followed this guide again (minus letting the car warm up first obviously): https://zcarguide.com/tuning-adjusting-datsun-240z-su-carburetors/. I backed the idle screws all the way off and then tightened them 1.5 turns, and put the carb jet adjust all the way up and backed it off 2 turns.

 

After letting it "idle" there for a bit, I started noticing my rear few exhaust headers getting red hot, so I advanced the timing on the distributor, but this caused the RPMs to continue spiking up higher.

After some more testing and shutting the engine down for a bit, I noticed fuel leaking out of the rear carb intake. My next step was to check the fuel float. The valves on both seem to close fine with the float up (blowing on the fuel tube). I recently bought a Float Sync on Ebay and decided to try that out. Watch the attached video to see what happened there.. (fuel shooting out)

 

I'm running a K&N 81-0402 electric fuel pump. Other's have advertised it as being a drop-in replacement for the mechanical fuel pump (I think Motorsport Auto sells it as their electric replacement).

 

I tried bending the tab on the fuel float up a bit, but fuel is still shooting up out of the sight glass and out the carb now. Any ideas?

 

Edited April 5, 2021 by Awk34