SB Needles to Resolve Lean WOT with SUs

[duffman 32]

I am one of the many Z owners, with SUs, that has battled a lean condition on hard acceleration, or WOT.  My idle and cruise AFR readings (have AFM attached to header) are great at 14.5 or so, so have adjusted it as best as i could.  Along with SM needles, I adjusted fuel levels and tried to do anything I could think of to resolve this issue, to no avail.  I even switched to triple Webers, which allowed me to see how my car could really perform with appropriate AFR!  I decided to switch back to SUs, as I had some linkage sticking and fuel dripping that bugged me.  

So, I started looking into the different available SU needles to see if I could find something richer.  I looked at various charts and saw that the SB needle had the same configuration at the top two levels (so idle is the same), and was slightly richer heading down the needle.  I thought, let's give it a shot!  I could not find anyone locally that provides SB needles, so I went down under to Australia and ordered a set of SB needles from Competition Products for about $40 delivered (less than 2 weeks for delivery).

I was stunned at the improvement!  My idle and cruise AFR remained around 14.5, but my lean (16 to 17+ AFR) condition at WOT was gone, now hitting around 13.5 AFR!!  That is ideal!  I cannot believe the difference.  My L28 was rebuilt with flat top pistons years ago, but outside of that, there is nothing else outside of the norm.  

I would strongly recommend this solution to those with similar issues!  I strongly urge you to make sure your fuel level (and flow) is appropriate before you go this route, as that can resolve many of the lean issues.

Edited February 26 by duffman

[Zup 412]

I like stories with happy endings!

If you don't mind sharing, what was your source in Australia?

[duffman 32]

CP or Competition Products is the name on eBay, but the direct website is sumidel.com, they provided the SB needles for around $40 delivered to Arizona.

Edited February 26 by duffman

[240260280 1,432 1 Vehicles]

Great contribution to the Z community!

[7tooZ 120 1 Vehicles]

Are these the ones you ordered

NEEDLE (SB)

$12.50

Product Code: AUD1318

**Please choose your metering needle carefully as they are non-returnable. All sales are final**

 

[duffman 32]

Yes


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[240260280 1,432 1 Vehicles]

Link: http://www.sumidel.com/shop/item/needle-sb-

[duffman 32]

Thanks, Phil, I should have included the link!


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[Captain Obvious 1,642]

Good results!

So was there a thread here on the forum where you were working on fixing the WOT lean out problem with the SUs? I don't remember such a thread, but that sure doesn't mean it didn't happen.   

[7tooZ 120 1 Vehicles]

Yes


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I am going to give them a try. Have them on order


72 body and block, everything else 71, Tokico springs, Illumina, R180 CLSD, 83 close ratio, 3.90 gears, Ztherapy SUs, BRE 15X7 Libre wheels and BRE front spoiler.

[duffman 32]

No, CO, I did not create a thread about this in the past.  I read other threads about fuel levels, fuel pressure, etc.  I even learned how to rebuild SUs and bought old sets, rebuilt them and sold about 15 sets on eBay!  I learned a lot about how they worked, but I could never get my SUs to attain desired performance on my Z.  I guess after a while I just decided to live with it!  My triple Weber adventure taught me that I can get more from my Z than what I was able to get with SUs.  There had to be something I was missing, that other people were able to attain great results with SUs and there was no reason I could not either.  So, pursued a variable I thought would make a difference and solve my problem; needles that allow more fuel on acceleration or WOT!  

Edited February 27 by duffman

[Stanley 80]

How many turns down for the mix nuts with the SB's?  I tried SM's for my modified 240z engine but found I had to drop the mix nuts a couple of extra turns to get it to idle, which made it way too rich at midrange, fouling the plugs. That's because the SM's (and other British needles that fit, as far as I know) are 0.099 at station 1, compared to 0.090 for the stock Hitachi needles, like N-27's. When I installed a modified 280/280zx engine, the first needles I tried were modified (richer at mid-range and top end) N-27's. Looks like my AFR numbers are about where the engine builder wants them so I guess I lucked out.

Checked my book, it shows SB's same as SM's at the top two stations, then they start to lean out more than SM's like you said. Anyway, your AFR numbers look OK, so however it works, it works. I've read where someone planned to drill out the nozzles to use British needles. Have also read about pushing the needles up into at piston as far as they'll go which is about 1/8" or 1 station higher than FSM says to put them. But that would only get you to 0.095 at the top station (if my 1/8" measurement guess is correct), not .090.

[duffman 32]

About 2 1/4 turns down for me provides the right mixture. I tried moving my SMS up into the piston but that richened my idle and goofed up everything for me. As I said the top 2 stations, as you put it, on the SB needles are similar to the SM, then get richer as you go down the needle. This, more fuel at midrange and top end, with no change at idle.


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[Stanley 80]

I think there might be another variable or two in play here that would explain why your results with the British needles were different from mine. Not at all sure but here's a couple possibilities. About three years ago I swapped my old nozzles for a couple of brand new ones. I compared the old and new ones. The old ones were worn - bigger diameter holes than new which would allow more gas to flow (the car ran OK with the old nozzles). Also, I didn't try the British needles with the larger engine. I guess more displacement would create more vacuum, pulling the piston and needle higher and so allowing it to run at idle. If those two theories get shot down (fire away, C.O. !) then I got nothing.

[240260280 1,432 1 Vehicles]

FYI:

[duffman 32]

Thanks, Phil, was trying to source the measurements to do a side by side comparison, very helpful!

[240260280 1,432 1 Vehicles]

Old hoover bag never fails.

[Randalla 24]

I second Stanley's supposition that there's a possibility of at least one of the two things he suggested contributing to the issue. I struggled with a similar issue, only to discover that after replacing my nozzles with a set of brand new ones, the problem was gone. If you have wallowed out nozzles or they become oval from needles that are not perfectly true (not always easy to see) you will struggle with optimizing how your car runs. The $100 I spent simultaneously for new nozzles and needles was worth the headaches avoided. 

[german240 16]

I spent quite some time tuning my 3.1L stroker with SUs and found RH needles to work best (they are leaner than the SB ones). Still, cruise AFR is a little too rich. 

Ian Hamiltons needle tool is great for doing needle research!

 

 

The tool can be found here: http://mk1-performance-conversions.co.uk/technical_carb.htm

Edited February 28 by german240

[duffman 32]

Thanks, german240, will have to research the needle graphing tool to understand it.  I really like the index of carb how-to books, etc, that are listed by MK1, great to have!!

[duffman 32]

RH needle profile

[duffman 32]

[Jeff G 78 122]

Great find.  It's odd that my F54 with a shaved P79 endurance race engine has near perfect AFR with SM needles.  I have a Web #91 cam and a header, but it's otherwise a pretty simple build.  

[duffman 32]

Jeff, I agree, it is weird!  I have been running lean on WOT since I got my carbs back from Ztherapy rebuilding them, just lived with it.  I wish I had the RH needles to see how different they would be on my car.

[Jeff G 78 122]

There must be some other factor at work that we are missing.  I guess as long as you found a solution, all is good.