There are lots of "local" sources for SU parts but i always go to the factory: Burlen Fuel systems (google it). Their on-line catalog is a pain to navigate sometimes but works OK. If all else fails they respond to e-mail pretty well. Shipping turns out to be resonable.

I suggest getting two new dampers and full rebuild kits and new needles (you have to order those seperately). The jet tends to wear quickly on these due to the biased needle and that can make tuning difficult. The wear is not always noticeable to the eye.

Only choking one carb is useful as the enriched mixture only really needs to occur for a little while and the carbs have a plenum (of sorts, a balance tube, at least) so some "mixing" of charge occurs. Also, most useful is the fast idle throttle setting and that should occur on both carbs regardless of whether or not the choke assembly is in place.

By the bye, this is not really a "choke" as the air flow is not restricted in any way. It is an "enrichment device."

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Mike!