I think the input bore can be a different diameter to the output bore (eg on an H4 which I am very familiar with). The size of the carb and therefore the identifier is measured on the engine side / output bore.
Great Explanation, I've never heard of an H5 or HS5, could these be H/HS4 or 6? I have a feeling the 1" plus 'x' 1/8ths measurement relates to the diameter of the hole (and hence butterfly disc) on the engine facing side? Really enjoy both your video's and Brian's Oak Swamp shenanigans ! Keep up the good work!👍😎
Hmm, I think you could be right about HS5 - well that is embarrassing! Stay tuned as there are more shenanigans with Bryan happening very soon and a new (massive) project will be announced very soon!
there we hundreds of bloopers.... I either said it all wrong, or *thought* I said it right until I played it back! Deffo the hardest video I've made so far to get 'right first time'!
The 'number being the number of 8ths' doesn't work for the pre-war HV SU series carbs, only the later carbs. An HV1 is a 1" diameter, a HV2 is 1-1/8 and so on. There is supposedly a HV0 which is 7/8ths but I have never seen one. Around the same era there were also the OM and UBA carbs each being a 1" throat, same as the HV1. They all take the same rebuild kit. My Riley is supposed to have OM carbs but I could only find UBAs (off early Morris). The difference seems to just be where the mounting screws for the dashpot are.
@@thebracketfactory If you order anything from the SU company directly you can add their "SU Reference Catalogue" to your order for free. It's a very handy little book. I add one every time I do an order since their overseas shipping is so expensive I feel I need to get something extra out of them. That book shows exploded views of most of the SUs and has a massive table of what carbs were used in what cars including things like jet sizes, needles and springs used.
It appears that with the brilliant circularly adjustable float bowl system you could drive your beautiful Austin 7 special upside down indefinitely if adjusted properly, notwithstanding Miss Beatrice Shilling's ( I learned her name from kind commenter here last week) modification which allowed momentary inverted or negative G intermittently without stalling out. Nice. I believe that since the slide is indirectly moved by vacuum pressure that it maintains a good mixture even when driving at high altitudes. I live at about 3500 ft. elevation here in Mount Shasta, California, and frequently tour at much higher elevations such as 6700 ft or so and down to near sea level without needing mixture adjustments, just less power at elevation. I love my Skinner's Union carbs. Smarty pants.
Hi thanks for the very informative tutorial.Question,if it’s the engine’s vacuum that raises the piston inside the carb,what happens when you’re cruising along say at 60 mph and you lift off the throttle pedal,to slow down.Would the high vacuum created suck the piston up when you didn’t need more fuel at that moment.Apologies if I’m being really stupid,but I remember the old vacuum wipers going crazy in that situation.
Excellent tutorial! The SU system for throttle response is so much simpler than accelerator pumps. So, to know if you have the perfect size carb for the engine you could mount your camera looking in the throat and take it out on the track to see if there's full travel of the slide at fully loaded and sustained rpm/cfm.
This master class is way beyond my expectations ! 😲 Thank you from bottom of my heart for sharing your knowledge. I have double SU H4 on my 1960 Volvo amazon with B16 engine so now i can tweak my SU carbs to work properly. Greetings from Poland 😄
What a fabulous explanation, I’ve just fitted an SU HIF44 to our old Harley-Davidson Shovelhead , runs nice , it has a different feel to the previous carb . Thanks very much for a great video.
Thank you, great explanation. I've used an H2 SU for up to 65bhp on supercharged A7 engines, if you are planning an engine with more than 12psi boost and cam/valves that will get you peaking power over 6500revs then go for an H4/HS4.
Thanks Bob - very useful to know! I'm planning to start the build on a NA splash fed motor with big valves and a newman cam quite soon - will prob stick with the H2 on that.
I just took apart the SUs on a Daimler 2.5 engine I have, this has been a most interesting video explanation of what I was guessing at! Really enjoying you and Brian’s adventures on Oak Swamp too
Help I have just bought myself an Austin 7 special and it won’t accelerate? The su carb that’s on it doesn’t have a rod attached to the cap that goes into the slide? Or a spring. Is this correct? Thank you,Andy.
In my circle of seasoned souls, we always called a carburetor with a sliding piston like the SU & Mikuni to be a variable venturi carburetor. Thanks Chris. Super simple easily understood explanation.
My pleasure :-). What is ingenious about the SU (as opposed to say a Mikuni) is that it automatically 'feeds' what is needed because of the vacuum mechanism. With the Mikuni you can just open the slide wide open and the engine has to try and cope with the unnecessary supply. Not sure if there are any other carb designs that do that ... I feel a googling session coming on!
