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L28 w/ SU Carb intake manifold vacuum leak


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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!

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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 by Zed Head
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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.

image.png

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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 by Awk34
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  • 2 months later...

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.

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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.

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  • 1 month later...
Posted (edited)

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?

PXL_20210405_015155550.jpg

 

Edited by Awk34
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Can you confirmed there is no debris that is holding the float open? There are two in line filters within the banjo fittings on the float chamber. Mine were full of crud, kept causing overflows on the rear carb. I would check that. The pump you have seems to have an OK range of PSI for SUs.

2000 idle is a lot, shoot for 800-900. You modified cam might want to make it go a bit higher.

get your floats adjusted correctly. One you have the floats set then you can use the guide, otherwise the guide won’t work!

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I don't think so. When I manually move the float up while blowing on the fuel tube with my mouth, I can feel the valve close.

I cleaned out the banjo and put new fuel filters in recently.

The modified cam might make it want to idle higher, but not 1000RPM higher (I'd hope).

"get your floats adjusted correctly" - any recommendations vs what I'm already doing?

PXL_20210225_013429329.jpg

PXL_20210225_013100300.jpg

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Okay, so i figured out why the sync tool was shooting fuel: the carb tuning guide says to plug all vacuum lines which were attached to the air box. I removed the plugs which i had put on the carb fuel bowl vents and it doesn't shoot fuel anymore.

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The comment about getting the float adjust correctly was in comment to the sync tool showing  too much fuel. Now that the tool can be used the level can be confirmed.

What needles are being used in the carb? SM?

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https://youtu.be/5KwKMwHtFMI

I'm going so nuts with this thing. I tried spraying carb cleaner all around the intake area again and it didn't seem to make a difference in RPM, but there has to be an intake leak somewhere, right? The throttle is all the way closed.

At this point I'm thinking it has to be something to do with the cam in there. It definitely seems like the previous owner put some sort of performance cam in there, but I have not idea what kind. In the video you can see the rear 3 exhaust headers getting red hot (timing, combustion in exhaust?).

I tried moving the distributor in both directions (retarded, advanced), but it didn't seem to make much difference. At the end you can see me jump from a huge bang. I assume it was fuel igniting in the exhaust.

At this point I'm so frustrated with this thing I'm considering just replacing the cam with a new one from MSA, or taking it into a shop so I can get some professional help..

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 I can't imagine an intake leak large enough to idle an engine at 4,000 rpms. When you say the throttle is all the way closed, are you referring to the throttle pedal or the carb butterflies? I think your butterflies are partially open which would indicate something is wrong in the linkage. Mine did the same thing one time when I forgot to hook up the heat shield springs.

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It's hard to tell from the video, but I believe they are raised a bit. I'll get a better video or some pictures tomorrow. Last time I had the carbs apart It seemed like the butterflies were closing fine, but maybe I'll take them apart again to check. What should they look like when 'closed'? Should the butterfly be straight up-and-down, 90° to the intake? I remember them being at a slight angle to the intake.

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There should be a very small gap (just enough air for idle) between the butterfly and the carb housing if the linkage is correct and the choke linkage isn't opening the butterfly a bit for cold start. The only way the pistons will lift is by the engine drawing air past the butterflies.

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