SU Carb Tuning re-visit

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Geistfahrar
Roadster Nut
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Joined: Tue Sep 11, 2018 1:50 pm
Location: S G Valley, So Cal

SU Carb Tuning re-visit

Post by Geistfahrar » Sat Apr 06, 2019 4:31 pm

While waiting for more parts & tools to arrive ( to finish the cooling, alt-swap & dizzy upgrades ), I decided to do some searches, to see how little I knew regarding SU tuning. As I noted in previous posts, I have balanced the carbs many times, however my "procedure" used only the Uni-Syn, a rubber hose ( to listen to the sounds at the carb throats ) & the "feel" & drop-rate of piston-lifts.

I have learned from the SU Gurus on this forum, that to cover the bases, I should also use the Color-tune tool & probably & O2 sniffer at the exhaust. I also learned that I should give up the search for 20 Wt. oil for the dashpots, & go for a mix ( proportions depending on climate & driving habits ) of ATF & Marvel Mystery oil. Nismou20 in Pasadena ( my closest neighbor ) said he used a 50/50 mix.

The post on the subject by nismou20 in 2017, got me thinking again about needle selection & matching parts. I went down in my shop & pulled out a couple of old ( and carbon-caked ) needles, to see if I could find any identifying marks on the mounting surfaces. With reading glasses & library-strength magnifying glass, the only marks I could see, were the old set-screw marks. Do I just need a stronger magnifier, or can needles only be identified by mike-ing down the length of the needle at prescribed intervals? My needles were only replaced once ( years ago with replacements from Dean A. ).

Nismou20 also remarked that the domes & bodies of the SUs should have matching numbers. Being lazy I didn't bother tryin to clean up one of the bodies on my carbs, but I did scrub-down and wire-brush the dome of the front carb, & again could find no numbers or other identifying marks on the outer surface. Are numbers on some inner surface, or am I really half-blind?

Any new info on needles, numbers or SU tuning in general will be appreciated.

Garry S.
Location Southern Cal
69' 2000 Roadster, being repaired by O/O
2001 Mercedes ML430; Was Wife's ride, now Grocery-hauler
2005 Jeep Wrangler X; Was Motor-home Toad, now local shopper
2005 33' Dolphin motor-home; Was vacation home, now guest house

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nismou20
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Re: SU Carb Tuning re-visit

Post by nismou20 » Sat Apr 06, 2019 8:03 pm

Hi Garry, Most identifier letter/numbers are either stamped or printed on the short shank that is held in the piston. Many printed ones will be illegible after time. The stock Nissan for our U20 was N17 but Dean sells the Hi Performance one as well,N25 I believe. I found the N25 to perform just Ok but if I tried to richen the top end it would idle rich in the low end. I ended up with RA which seems to work for me.

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Lorna c
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Re: SU Carb Tuning re-visit

Post by Lorna c » Sat Apr 13, 2019 11:52 am

are you feeling hesitation or any symptoms or just gaining info ? do you have a balancer for them ? looks like a snail . they run about $30-40 online but will make a huge difference in your tune . good luck on the learning of the carbs . I like the SU carbs
:wink:
"Is it me, or does everyone want to race ?"

late 68 1600 3 main motor .

Geistfahrar
Roadster Nut
Posts: 60
Joined: Tue Sep 11, 2018 1:50 pm
Location: S G Valley, So Cal

Re: SU Carb Tuning re-visit

Post by Geistfahrar » Sat Apr 13, 2019 2:10 pm

Thanks, Lorna --

Like I said, at this point, I am just collecting more info while I wait for more parts. The tool that looks like a snail, is the Uni-Syn, and it has always done the job pretty well over the years, for visual balancing. My understanding, from posts on the subject, is that the Color-tune "puts the finishing touch" on getting the individual cylinders balanced.

BTW, I have finished re-working my old smog supports, & my reman 10SI Delco alt is due in tomorrow. I decided to dump the old radiator bolts, & put in new hardware, keep the existing temp-sender until it breaks, & just replace to top dome of the water tower. No word back yet from Tom on the EI dizzy. He said he has quite a backlog.

Happy Roaderstering,

Garry
Location Southern Cal
69' 2000 Roadster, being repaired by O/O
2001 Mercedes ML430; Was Wife's ride, now Grocery-hauler
2005 Jeep Wrangler X; Was Motor-home Toad, now local shopper
2005 33' Dolphin motor-home; Was vacation home, now guest house

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Lorna c
Monthly Subscriber-Roadster Enthusiast
Posts: 2433
Joined: Sat Aug 20, 2016 12:47 am
Location: central coast , ,ca

Re: SU Carb Tuning re-visit

Post by Lorna c » Mon Apr 15, 2019 12:57 pm

sorry . I'm awake now and see
:wink:
"Is it me, or does everyone want to race ?"

late 68 1600 3 main motor .