SU carburetors - reservoirs for oil? Why?

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Jul 14, 2020
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Motorcycle CV carbs have an idle circuit, a main jet, and an emulsifying chamber in the discharge tube, on an SU it is all done by the needle. This gives poor atomization and makes them a dirty carb as far as emissions go...and why we don't see them anymore. SU wouldn't allow Triumph to use the SU on their Triumph 2000 (don't know why, the TR's had SU's, so maybe just a supply issue) So they got Stromberg to make them a copy of the SU. The series of carbs by Red Edmonton, the Lectron being the most famous, are like the SU, with the needle doing everything, no idle circuit, no main jet, and no emulsifying chamber.
 
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Instead of increasing the fuel level, what SU did was to lower the needle jet which amounted to the same thing. So when you pulled the choke control on an old car with an SU carb, what you were actually doing is actuating a linkage to physically lowering the needle jet closer to the float bowl fuel level.

Except on the HIF series carbs where they made it even simpler.

HIFs have a second passage from the float bowl to the throat drilled, and it is normally closed off with a stopcock-like screw. Pulling the choke cable opens this second passage.

The HS and early jet drop works well, but you're moving a lot of stuff around and against some decently strong springs. Especially on a dual(or more) set-up, this translates into a very heavy pull on the cable to engage the "enrichment circuit." Given how crummy modern parts are, it can be difficult to even find a reproduction cable that will lock on. I managed to find an NOS cable for mine that works and will actually lock, but it's not the right style handle and is not the easiest to grab with the style dash on my car.

Some of the fancy set-ups(Jags, Bentleys, RRs) used a small "cold start" carb that could be turned on automatically and as I understand it was meant to just run very rich and then get shut off completely once the engine was warm.
 
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Some of the fancy set-ups(Jags, Bentleys, RRs) used a small "cold start" carb that could be turned on automatically and as I understand it was meant to just run very rich and then get shut off completely once the engine was warm.
the aux carb was a jag thing.

rolls royce used a conventional butterfly with a solenoid
 
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Jan 2, 2004
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IIRC, the Japanese, specifically Hitachi, licensed the rights from SU or whoever held the IP then for their automotive carbs. Ironically the Zenith Stromberg design was more of a 'copy', but utilized a diaphragm to seal the vacuum chamber and some other differences to operate outside of the SU patents.

I thought the slide as throttle design may have been developed by one of the Japanese makers, but Nthach is probably correct.
AFAIK, Hitachi licensed SU’s design, while Aisin licensed out Solex for their use for Toyota. Not sure who Keihin copied, I’m willing to think GM Rochester or Carter for their Honda car carbs.
 
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