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SU carb questions

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mperdue

mperdue

Freshman Member
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Hi guys,
I've got a couple of general questions about SU carbs. They don't apply only to MG's but there isn't a general place to post so I thought I'd try here.

First, what is meant by "float angle"? I've seen 0degree, 20degree and 30degree carbs. Is that the angle the carb has to be mounted at to keep the float chamber level with the groung? Is this adjustable?

Second, does anyone know where I can find the bolt patterns for the HS4 (1.5" or 38mm) and HS6 (1.75" or 46mm) carbs? I need the manifold mounting bolt pattern, not the air cleaner side.

Thanks for the help,
Michael

edit: oops I took a closer look and there are general forums. can the mods move it?
 
Mike, the float angle refers to the angle that the float chambers sit in relation to the carb to manifold surface. As some manifolds are cast at a different angle than others, due to clearance issues, or engine mounting angles, the float chambers are indexed to the carbs by means of the rubber bushing between the float chamber and carb body. This keeps the float chambers vertical and level, as much as possible.
As far as the bolt patterns go, the HS4's are on 3" centers, oriented at a 45 deg. angle relative to the centerline of the carb bore. I don't have any HS6's lying around, but I may have some base gaskets I can check. Do you just need the mount bolt pattern, or do you need horizontal spacing between carbs?
Jeff
 
Hi Jeff,
Thanks for the reply. I think you've told me what I need. I've also done some Google image searching have found pics of those flanges (and nope I don't need the carb spacing). And thanks for the info on the float chamber angle. That's about what I expected. So it means that if you are doing carb swaps you have to pay close attention to the angle required for the float - good to know.
Michael
 
Michael, APT sells the rubber bushings needed to change the float angles if needed. When I put a single HS4 on a 1500 Spit, I think I used the bushing from a Mini carb to get the needed angle.
Jeff
 
That's great. I've taken a look at their site - they've got lots of useful parts (filters, air horns, etc). I'm sure I'll be back there when I get everything straightened out. Thanks again Jeff,
michael
 
Thanks Alec,
I don't know if you guys checked my profile but I'm actually a Datsun enthusiest - not a British car enthusiest. In the 60's and early 70's Datsun/Nissan used carburetors made by Hitachi that were modeled after SU carbs. I'm finding that among other things the bolt patterns are the same. The 1.5" (38mm) bore Hitachi's have the same bolt pattern as the HS4's, and the 1.75" (46mm) bore Hitachi's similarly match the larger HS6's. I don't think Datsun used any 1.25" Hitachis I wonder how far the similarities go, or if they somehow used the same parts? They look the same from the outside anyway. I'd like to get my hands on a set of HS6's to compare them to my 46mm 240Z carbs...

Well anyway, thanks again for the info. Who knows, maybe this info could help someone on this site, though I doubt any of you would be caught dead with a set of Japanese carbs in your cars /ubbthreads/images/graemlins/laugh.gif. On the other hand, I wouldn't mind running a REAL set of SU's in my dat! One plus is that I think there are more companies supplying parts for the real SU's compared to the Hitachi's. But if it turns out parts are interchangable (besides the fact that the carbs bolt up) that would open up a large number of parts sources to Datsun owners.

Michael
 
Mike, I race with a guy running a 1600 Roadster, with the 38mm Hitachi's, and know that he uses a lot of SU parts. I'll email him, and see how far the similarities go. I think that the Hitachi's were built under license based on the SU's. The 1600 is very close to an MGB engine.
(And, I ran 240Z front brake pads in the GT6 brakes I had on the Spitfire I raced.)
Jeff
 
I've been doing more searching and have come up with a couple more questions.

Anyone know the basic differences between the HIF4 and the HS4 SU carbs? I know the HIF4's were only used in the early 70's, so did they have emissions changes that affect performance or anything like that? Are the HS4's preferred over the HIF4's?

Also, possibly a related question is what is the diffrence between fixed and biased needle mounting and how do they affect performance or tuning? I'm not quite sure what it means. I may be well off to pick up a book on SU tuning I suppose.

Thanks again for the help
Michael
 
Michael -

Basically, it's a design difference that incorporated the fuel bowl below the throat of the carb for a more compact package. It did away with the "remote" fuel bowl off to the side. It also changed the choke operation to incorporate a temperature sensitive device to increase the delivery of fuel in cold weather. Others with more technical knowledge than I have can fill in the details about potential performance or economy gains that came with the design.

Mickey
 
Thanks Mickey,
It turns out I need a set of carbs with the 20 degree float bowl angle. Do you know if the HIF-4's were mounted at 20 degrees or 0 degrees? You can't tell by simply looking at them because the float bowls aren't visible anymore.

Also, were the HS4's manual choke carbs or did they also have an auto choke but that was activated in a different way? Thanks again.
Michael
 
Michael -

Not sure about the angle - I know the carbs are canted a certain amount, but wouldn't want to guess. As I'm at work (at least SUPPOSED to be!), I can't run out and look, but if you don't get an answer before I can, I'll check and see if I can determine for you.

As for the chokes, both the HS-4's and HIF-4's had manual chokes. The HIF has an internal temperature compensator that assists the operation in some mystical way (!), basically by raising or lowering the jet to alter fuel delivery. Again, there are some more techies on the Forum who can give you a better answer.

And about the only thing I can tell you regarding the choice between HS and HIF design is the HIF seems to be favored because of the compactness and the engineering that went into them.
 
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