SB Needles to Resolve Lean WOT with SUs

[Duffman]

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]

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]

FYI:

[Duffman]

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

[240260280]

Old hoover bag never fails.

[Randalla]

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]

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, 2019 by german240

[Duffman]

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]

RH needle profile

[Duffman]

[Jeff G 78]

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]

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.