SU Needle info

[Michael Oritt]

I have OA7 needles (1.6" long) in my HS6 carbs--measuring at the tip, in the middle and at the shoulder I came up with .064", .080" and .100". I would like to get needles that are just a bit leaner, perhaps about .002 throughout the scale.

I can't find the OA7's on any of the online needle charts and wonder if anyone can make a needle recommendation.

 

[DART]

Michael- came across this eBay site with a lot of needles, including an 007. I tried for 0A7 with the "o" both alpha & numeric, no luck.
eBay seller British-Parts-Source-Store https://myworld.ebay.com/ebaymotors/britpartssource/

fwiw

 

[ChrisS]

This is a plot of the OA7 verses the OA8, the OA8 gets progressively leaner with throttle position.

 

[Hap Waldrop]

You could always just call Joe Curto, I'm sure he could fix you up. Nice to finally meet you face to face last weekend.

 

[Michael Oritt]

Chris--

I found this program online at:
https://www.mintylamb.co.uk/suneedle/

but it did not have info about the OA needles or several others that I knew were close to it for that matter.

I'd appreciate knowing how and where I can access the program that you used to send me this info.

 

[ChrisS]

It is WinSU . I can give you whatever data you want or setup a go-to-meeting session and give you control over my PC so you can look yourself.

 

[Michael Oritt]

Chris--

Thanks for the reference and I have downloaded the program though it looks like I need to register it for a fee in order to get the full functionality which includes the needle comparison program. I will investigate furthe and thanks for the kind offer.

Over the winter I put on a new exhaust manifold and alsop installed a Maniflow intake manifold in place of the cast aluminum model. (I know Hap had some negative thoughts about the Maniflow but perhaps foolishly I had already bought it before soliciting opinions!)

I raced last weekend at Kershaw SC where it was quite cold ane with these new pieces in place and using last year's needles (KD) the car was running quite rich. I had a set of (leaner) OA7's and they helped a lot, though I still had to raise the jets a bit from baseline, and since it was so cold down there I figured that some leaner needles would be in order.

I just called Joe Curto who verified that yes the OA8 would be the next step leaner, so I ordered a set of those. I currently have a set of the red (4.5 oz)springs installed and am not clear on how going to a heavier or lighter spring affects mixture. Do I understand it correctly that a heavier spring slows the rise of the piston and therefore lets in less air and makes the mixture richer, and vica versa for lighter?
Thanks to all for any input.

BTW I ran into Hap down at Kershaw and he is as nice and helpful a fellow in person as he is on this forum. it's always good to be able to put a face to a familiar name.

 

[ChrisS]

.

 

[red57]

Sorry Chris, but I've got to disagree. If all other factors remain the same, a stiffer spring will cause the piston to run lower in the dashpot for any given steady state, short of full throttle, and a weaker spring will do the oposite. The oil on the other hand will slow the rise but allow the piston to go to whatever height the spring allows. Think of it like a suspension system -the spring resists the piston rising for any given vacuum (load) applied and the oil in the dampler acts just like a shock absorber, slowing the action but not controlling the height.
I highly recommend Des Hammill's book "How to Build & Power Tune SU Carburettors" He gives the best explanation of how they work that I have seen and includes lots of pictures & drawings along with step-by-step instructions for performance tuning including how to polish needles to suit your application. The book was a real eye opener for me when I got it a few years ago.

Good Luck
Dave Phillips

 

[Michael Oritt]

Sorry Chris, but I've got to disagree.

I hate it when mommie and daddy argue!

 

[Hap Waldrop]

Michael, We had some carb issues with Bubba's car (grey GT), mixture was good, EGTs spot on, but it had dead spot right off throttle then picked up, I used to immediate response. he had alot of issues, he still had crankcase evacuation tubes hooked, I'll have him get rid of that crap, and he using biased spring needles, I at some time will convert the carbs to solid needle, Joe makes a simple bushing for this. If none of that works, then I may use oil in the dampners, something I normally don't use in race carbs, I wanted to try oil at CMP, but his cas didn't have dampner rods on them, so oil wouldn't have done any good. ON the last set I built, I installed SL needles, at Davids Headley's recmmnedation, it will will be spring be fore the custoer gets out to race, so the verdict is stll out there. It's a 1800 13.0 to 1, so I guess your needle requirements may be alot different than mine, but there they are FWIW. Kent prather would be a good source of info as well. DO you have SGTs gauges, if so what numbers did you see at WOT.

 

[Michael Oritt]

DO you have SGTs gauges, if so what numbers did you see at WOT.

Hap--

I have the dual EGT guage--one probe in the front down tube and the other in the rear (middle siamesed is not monitored). They read about 1200 (just above middle of scale) at WOT. I am experiencing a bit of waivering in the front one and do not know whether it is sender error or not--I will switch them to find out.

 

[Michael Oritt]

Chris--

I sent you a PM.

 

[Michael Oritt]

I'm waiting for a leaner set of needles from Joe Curto but in the meantime can we pursue this discussion a bit further?

I understand that a stronger spring will tend to keep the piston from ascending as fast as would a lighter spring (and vica versa), but I thought that the WOT mixture is strictly a function of the needle profile at any given piston height AND that the amount of carb piston lift is strictly a function of vaccuum and air passing through the carbs as demanded by the engine.

Put differently does the stronger/weaker carb spring have an effect on the ultimate mixture at WOT or simply mid-range?

Any input will be appreciated.