Carb selection

[Rut]

My 1275 currently has a Weber downdraft and I would like to change over to SU HS2s. I have 2 sets, AUD136 from a 64-66 1098 or 67-68 1275 and a newer set, AUD549 from a 72-74 1275. The main diferences I see are the floating needle attachment and the provision for vacuum lines on the AUD549s. Since I've had rear oil leak issues that have been somewhat successfully fixed with a vacuum connection added to the Weber, would the later SUs be better? Is there any appreciable difference between the 2 in terms of performance or reliability?
Thanks, Rut

 

[Rut]

BTT, someone's got to know the difference between them. Any thoughts on ease of tuning, built in vac connections, etc.
Thanks, Rut

 

[Guest]

I've had rear oil leak issues that have been somewhat successfully fixed with a vacuum connection added to the Weber,

Huh? Please explain that to me.

 

[RickB]

I'd heavily weigh the set that has less issues.
Does one set have worn shafts?
The condition of the carbs would be my most important deciding factor.

All else being equal it seems like a no brainer to go with the later set, already has the vac connection and it sounds like you need it.

 

[Rut]

Kellysguy, I tapped the boss on the Weber manifold for a vac source and it slowed the rear oil leak quite a bit so I know I'll need a vac source on the SUs.

RickB, I rebuilt the earlier SU carbs with new shafts and kits and they look good. After a good soak in the carb cleaner bucket and a quick trip to the polish wheel they are very presentable. I did the same to the later SUs, but I have not put in new shafts yet. They have the larger vac ports for the Y connector and I thought that would be a good way to go. The later carbs have the spring loaded needles and that's new to me. It looks like it would be simpler to center the needles and keep them there. Since they are both the same size I didn't know if there would be a difference in performance or reliability due to the changes in the later carbs. Thoughts?
Thanks, Rut

 

[nomad]

I've had trouble tuning the later sets and, if it were me, I would go with the early set. The manifold has vacuum ports.


Kurt.

 

[Rut]

Kurt, that's the kind of info I'm looking for and I have 3 different styles of intake manifolds so that should not be a problem. I must admit that the newer carbs look like they have a few 'improvements', but the 'valves' on the butterflies threw me off. The casting of the newer carbs is a lot rougher on the inside than the earlier ones and required some cleaning up. It's nice to hear that click when you let the pistons drop and that's easier to make happen vs centering the needle on the older style. Additional thoughts are welcome.
Thanks again, Rut

 

[Hap Waldrop]

Rut I would go for the early carbs, they are set up for ported vacuum advance, but with none of the PCV crap, also should have solid needle set up and solid butterflies.

 

[Rut]

Hap, thanks for your input. The earlier carbs have the solid butterflies and the vac advance port and are ready to go. Since I'll be changing the manifold as well I think I'll need a PCV to go between the manifold and timing cover separator. My engine REALLY needs negative pressure to keep the rear leak from being a flood.
Thanks, Rut